Sound - Class 8 Physics - Extra Questions

Define pitch of a sound :

The quality of a sound governed by the rate of vibrations producing it is called pitch


Define amplitude of a wave :

Amplitude is the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position. It is equal to one-half the length of the vibration path. 


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.......... is an unpleasant sound produced by irregular wave patterns.

Noise and musical sound (pleasant sound) , both  are the mixtures of sounds of different frequencies but frequencies in musical sound are discrete and rational but noise frequencies are continuous and irregular . So noise is an unpleasant sound produced by irregular wave patterns .
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What kind of sound - noise or music is represented by the sound wave shown here.
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A musical sound is periodic and regular, whereas noise is irregular and nonperiodic. 
In a musical sound,  wave patterns are repeating whereas a noise has an irregular wave pattern. 
As in the given figure, we have a repeating wave pattern, so it is a musical sound. 


Name the characteristic of sounds which enables a person to differentiate between two sounds with equal loudness but having a different frequency.

A person can differentiate between two sounds of equal loudness, having different frequencies by their pitch. The Pitch of a sound depends upon its frequency. if frequencies of two sounds are different then their pitch will also be different and a person can detect them.


What is the difference between frequency and pitch?

Frequency is a physical quantity which gives number of vibrations per second whereas pitch is a perceptual quantity which depends upon the listener, in fact our ears detects the pitch not the frequency.


With the help of the given clues, unscramble the letters to make a meaningful word.
RAYXLN (produces sound in humans)

 RAYXLN can be arranged as LARYNX. The larynx is responsible for the production of sound in the human body, vocal cords are found inside the larynx.


With the help of the given clues, unscramble the letters to make a meaningful word.
ETHZR (unit of frequency of sound wave)

ETHZR can be arranged as HERTZ. It is the unit of frequency and is equal to $$second^{-1}$$.


State three differences between loudness and pitch.

Loudness Pitch
  Loudness is the property of sound
by virtue of which a loud sound
can be distinguished from a faint one		 
Pitch is that characteristic of sound by which a shrill sound can be distinguished from a grave or a flat sound

  It depends on the amount of sound energy received by the ear in unit time-more the energy received louder is the sound  Pitch is independent of the amount of energy received by the ear
 
Loudness does not change with the change in frequency

 It changes with the changes in frequency Hight frequency sound produces the shrill sound


In the following statements, tick T against those which are true, and F against those which are false:
(a) Sound cannot travel in vacuum. (T/F)
(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
(c) If the amplitude of vibration is large, sound is feeble. (T/F)
(d) For human ears, the audible range is $$20 Hz$$ to $$20,000 Hz$$. (T/F)
(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
(f) Unwanted or unpleasant sound is termed as music. (T/F)
(g) Noise pollution may cause partial hearing impairment. (T/F)

(a) True
Sound requires a material medium for travelling. It can travel in solid, liquid and gases. But it can't travel in the vacuum.

(b) False
The number of oscillations per second of a vibrating object is known as its frequency.
The time period is calculated for one oscillation. I.e it is the time taken by a vibrating object to complete one oscillation.

(c) False
The loudness of a sound is proportional to the square of the amplitude of its vibration.
When the amplitude of vibration of a sound is large, the sound is very loud. The sound is feeble for small amplitude.

(d) True
Humans cannot hear sounds of all frequencies. Humans can hear a sound whose frequency falls in the range of 20 Hz−20,000 Hz. The sound having frequency out of this range is inaudible to humans.

(e) False
The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases and vice-versa. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched for a small frequency of vibration.

(f) False
Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to ear are known as music.

(g) True
Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sound continuously for a long time, then it may cause temporary hearing impairment.



What is the difference between noise and music? Can music become noise sometimes?

Pleasing sound to the ear is called music. For example, the sound produced by violins, sitar, guitar etc are music.
Unpleasing sound to the ear is called noise.
Yes, music can become noise when played at high volumes.


Explain in what way noise pollution is harmful to humans.

Noise pollution is harmful to humans because noise pollution can lead to a number of health related problems like hearing loss, hypertension, severe headache and stress.


Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called _________ .
(b) Loudness is determined by the __________ of vibration.
(c) The unit of frequency is __________ .
(d) Unwanted sound is called ___________ .
(e) Shrillness of a sound is determined by the __________ of vibration.

(a) Time taken by an object to complete one oscillation is called the time period of oscillation.
(b) Loudness is determined by the amplitude of vibration.
(c) The unit of frequency is Hertz.
     SI unit of frequency is Hertz.
     Hertz $$= \dfrac{1}{Second}$$.
(d) Unwanted sound is called noise.
(e) Shrillness of a sound is determined by the frequency of vibration.


List sources of noise pollution in your surroundings.

1) By blowing their horns and making a noise with their engines, automobiles on the road cause noise pollution.
2) Noise pollution is caused by the exploding of crackers on numerous social and religious events.
3) Manufacturing machinery produces loud noises that pollute the environment.
4) Religious and cultural functions can be noisy.


Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

Houses along the roadside will have more noise problems. The noise produced by transportation vehicles may cause trouble to the residents. The intensity of noise decreases with the distance between the source and the listener. Therefore, it is better to take the house that is three lanes away from the roadside.


The sound from a mosquito is produced when it vibrates its wings at an average rate of $$500$$ vibrations per second. What is the time period of the vibration?

  • Hint:
    The time period is the inverse of the frequency.
Explanation: 

  • The frequency of a wave or oscillation is defined as the number of cycles or completed alternations per unit time.
  • The amount of time it takes for something to complete one oscillation is called its time period.
  • Frequency and time period are inversely proportional.

Given $$Frequency=500 \ Hz$$
Time period $$= \dfrac{1}{Frequency} = \dfrac{1}{500} = 0.002\ s$$

So, the tme period is $$0.002 \ s$$


State True or False.
A tuning fork produces $$1083$$ vibrations in $$3 \ seconds$$, then its frequency is $$361 \ Hz$$.

Given, number of vibrations $$n=1083$$
time $$t=3 \ second$$

Frequency is defined as the number of vibrations per second

So, $$f=\dfrac{n}{t}=\dfrac{1083}{3}=361 \ Hz$$

Hence, the given statement is True.


Sketch larynx and explain its function in your own words :

Larynx is responsible for production of sound. It moves when we swallow something. There are two vocal cords inside the Larynx with a small gap between them. This small gap allows air to pass through. 
When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate which produces sound.

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The frequencies below 20 Hz are known as__________sounds.

The frequencies below 20 Hz are known as infrasonic sounds. This sound is not audible to the human ear.


The sounds having frequency less than $$20 Hz$$ are called ultrasonics. True or False?

$$False$$

The sounds having frequency less than 20Hz20Hz are called infrasonics.
The sounds having frequency greater than 20kHz20Hz   are called ultrasonics.
Human beings can hear only those sounds which are in the frequency range of 20Hz to 20kHz.
Thus the infrasonic and ultrasonic sounds are not audible to humans.


Extremely loud sound can make one deaf. Suggest some measures to check loud noise.

Turning off appliances like TV, music system if not required 

Shutting the door of the room while using washing machines and dishwashers 

Use of earphones or headphones while listening to loud music should be avoided.


What is meant by noise pollution? Name one source of sound which causes noise pollution.

Noise pollution is defined as the disturbing or excessive noise that may harm the activity or balance of human or animal life. It is a discontinuous sound and unpleasant to the ear.
E.g.: sound produced when a stone is thrown on a tin sheet.


An audible sound has the frequency range of _________.

For humans, an audible sound has the frequency range of $$20 \ Hz$$ to $$20,000 \ Hz$$.

Sound waves with frequencies less than $$20 \ Hz$$ are called infrasonic and those with frequencies above $$20 \ kHz$$ are called ultrasonic.


Factories should not be constructed in the residential areas. Do you agree or not? If so, why?

Factories should not be constructed in the residential areas for the following reasons:-

1. they produce pollutants.
2. they cause noise pollution.
3. they pollute the water.
4. they affect the environment seriously.


Suggest some measures to reduce noise pollution in your residential area.

Stay away from Noisy area

Noise producing industries, airports, vehicles should be far from residential areas as it very dangerous for infants and senior citizens.

Follow the Limits of Noise level

Community law should check use of loudspeakers, outdoor parties as well as political public announcements.

Control Noise level near sensitive areas

There should be control on noise level (Silent zones) near schools, hospitals. Place noise limits boards near sensitive areas.

Go Green by planning trees

We can plant more trees as they are good noise absorbents. According to studies it can reduce noise by 5 to 10 decibels Db around them.

Use Proper Lubrication and Better maintenance

We can use proper lubrication as well as better maintenance of machines to reduce noise pollution and improve efficiency. It reduces friction between movable parts and helps to reduce noise.



Define Pitch. 

The quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone is called pitch.


What is noise pollution? Write its cause and ways to prevent it?

Noise pollution means an unwanted or undesirable sound that leads to physical and mental problems. Noise pollution is dependent on the loudness and frequency of the sound. In fact, when the sound exceeds its maximum limit, it becomes fatal for human and other organisms. The noise intensity is measured in decibels or dB. A person can bear the noise up to 85 decibels, after which his hearing power can be damaged.

Some of the major causes of noise pollution are vehicles, aircrafts, industrial machines, loudspeakers, crackers etc.

Some noise pollution preventive measures are :

i) Honking near schools and hospitals should be banned.

ii) Dense tree cover is useful in preventing noise pollution.

iii) In Industrial and commercial buildings soundproofing system should be installed.



The pitch of a sound depends on its ______ .

frequency


What are infrasonics? Can you hear them?

The term "infrasonic" refers to sound waves below the frequencies audible to humans, and includes frequencies under $$20 \ Hz$$.

No we cannot hear infrasonic sounds.


What is the audible range of frequency of sound waves for human being?

It's $$20Hz \ to\  20KHz$$.


What is the range of hearing for children under the age of five ?

Children can hear the sound between Between $$20\,Hz$$ to $$ 25000\,Hz $$.


The ringing bell in a temple has frequency $$400$$ vibration/sec, then calculate the periodic time of it.

Time period of vibration $$ = \dfrac{1}{{\text{frequency}}} $$ 

Period of vibration $$ = \dfrac{1}{400}/sec $$ 


Explain following terms
Audible sound 

Audible sound :  Sound in the frequency range of 20 Hz to 20000 Hz that can produce sensation on a healthy human ear  is called audible sound.


What is the name of sound wave having frequency more than 20 kHz?

Sound waves having frequency more than 20 kHz are known as ultrasonic sounds.
Animals like bats, dogs, cats, some birds and insects are sensitive to such sounds.


How does a musician playing on a flute change the pitch of sound produced by it ?

In musical instrument like flute and clarinet , the pitch of sound is changed by changing the length of vibrating air column when different holes in it are closed. 


What do you mean by Infrasonic sound?

Sounds having a frequency less than $$20\ Hz$$ are known as infrasonic sounds. We cannot hear such sounds. It has a frequency below the audible range.


An object vibrates $$200$$ times in one second. What is its frequency?

Frequency is defined as the number of occurrences or vibrations per second.

Since, in the given case, the object vibrates $$200$$ times in a second.

Hence, its frequency is $$f=200 \ Hz$$


If a tuning fork vibrates $$480$$ times in one second, what would be its natural frequency?

Natural frequency is the count of the number of oscillations in one second.

So the frequency is  $$f=\dfrac{number \ of \ vibrations }{time}=\dfrac{480}{1}=480Hz$$


Explain the term: Ultrasonic sound 

The audible range of sound for human beings extends from about $$20 \ Hz$$ to $$20 \ kHz.$$ Frequencies higher than $$20 \ kHz$$ are called ultrasonic sound or ultrasound. Ultrasound is produced by animals such as dolphins, bats and porpoises.


How many times vibrate a body of frequency $$290Hz$$ in $$12$$ seconds.

$$n=f\times t$$
$$f=290Hz$$
$$t=12s$$
$$n=290\times 12=3480$$ times


Represent graphically by two separate diagrams in the case where two sound waves having the same frequency but different amplitudes:

Above is the diagrammatic representation of two sound waves having the same frequency but different amplitudes.
In both the wave forms the crests and troughs are at different heights indicating different amplitudes, but the number of crests and troughs per unit of time are the same indicating same frequencies.

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Represent graphically by two separate diagrams in the case where two sound waves having the same amplitude but different frequencies :

Below is the diagrammatic representation of two sound waves having the same amplitude but different frequencies. In both the wave forms the crests and troughs are at the same height indicating same amplitudes, but the number of crests and troughs per unit of time are different indicating different frequencies.

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Two identical guitars are played by two persons to give notes of the same pitch. Will they differ in quality? Give a reason for your answer.

No two musical instruments of the same pitch have the same quality because pitch depends in the frequency or wavelength and quality depends on the waveform, upon the number or distribution and relative intensity of the different harmonics and overtones. 
Hence, the two notes are not of the same quality.


The frequencies of notes given by flute, guitar and trumpet are respectively 400 Hz, 200 Hz and 500 Hz. Which one of these has the highest pitch ?

A high pitch sound corresponds to a high frequency sound wave and a low pitch sound corresponds to a low frequency sound wave. 
In the question, the trumpet has the highest pitch as the frequency is the highest among the three instruments given.


A sound wave is vibrating at frequency 50 Hz. What is its time period ? 

The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency.
Here, a sound wave is vibrating at frequency 50 Hz. Hence, the time period T =  1/f = 1/50 = 0.02 s.
Hence, the sound wave vibrating at frequency 50 Hz has a time period of 0.02 s.


How does the medium affect the amplitude of free vibrations of a body ?

Free vibrations, which are defined as natural and unforced vibrations of a body, can take place in any medium. But, when a wave passes through a medium, some portion of it is absorbed. If the medium is highly absorbing type, the amplitude of the vibrations will be very low. Hence, the vibrations depend on various factors like changes in properties of the medium, changes in the temperature, changes in the properties of the vibrating medium.


State one difference between a musical note and noise.

The sound which is produced due to a mixture of several frequencies is called a note and is pleasant to listen to. Noise is unpleasant to the ear! Music is pleasant to hear and is of rich quality.


Any sound above $$20,000\: Hz$$ is called ...........


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Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

$$\textbf{Explanation}$$

Sound can be hear if there is any medium like air is present to travel those sound waves. On the moon, there is no atmosphere is present. So, there will be not any medium from which sound waves can propagate. So, the sound of my friend will not reach to me.

Answer:

No, I will not be able to hear any sound produced by my friend. 



Pleasant sound is termed as music and is produced when vibrations produce waves in regular pattern. Why?

Noise and musical sound (pleasant sound) , both  are the mixtures of sounds of different frequencies but frequencies in musical sound are discrete and rational and wave pattern is periodic and regular but noise frequencies are continuous and irregular . So music is an pleasant sound produced by regular wave patterns .


Noise pollution may cause deafness. Explain.

Noise pollution or sound pollution, is the regular exposure to high levels of sound that may lead to adverse effects in humans or animals. Noise pollution affects both health and behavior.

Sound is measured in units called decibels. Sounds of less than $$75$$ decibels, even after long exposure, are unlikely to cause hearing loss. However, long or repeated exposure to sounds at or above $$85$$ decibels can cause hearing loss. The louder the sound, the shorter the amount of time it takes for Noise-Induced Hearing Loss (NIHL) to happen.


State whether true or false :
A medium is not necessary for the propagation of sound.

Sound is a mechanical wave and propagation of mechanical wave is possible only when there is a medium. So medium is necessary for propagation of sound.
Hence the given statement is false.


Write the definition of pitch.

Pitch is the characteristic of a sound. It basically depends on the frequency of the sound wave. Followings are the properties of the pitch -
  • This depends on the frequency of vibration of the waves.
  • If the frequency of vibration is higher, we say that the sound is shrill and has a high pitch. On the other hand, if the sound is said to have a lower pitch then it has a lower frequency of vibration.
  • A bird produces a high-pitched sound whereas the roaring of a lion is a low-pitched sound.
  • The voice of a woman has a high pitch than that of a man.


Musical sound is produced by periodic vibrations.(True/False)

True. Noise and musical sound (pleasant sound), both are the mixtures of sounds of different frequencies but frequencies in musical sound are discrete and rational and it has periodic vibrations but noise frequencies are continuous and irregular.


What measures can be taken to prevent noise pollution?

Some of the steps to prevent noise pollution are as follows:
(i) The use of loudspeakers should only be rarely allowed
(ii) Automobiles should be fitted with silencers and soft horns
(iii) We should not play radio televisions and stereo system too loudly
(iv) The factories should be relocated far away from the residential areas
(v) The machines should be designed in such a way that they produce minimum noise
(vi) Bus and train drivers and factory workers should use ear plugs


What do you understand by the term loudness w.r.t. sound waves?

The loudness of a sound wave is the intensity of the wave. The intensity of a sound wave is proportional to the square of the amplitude of the wave ($$I\propto A^2$$). The more the amplitude, the more will be the intensity, and hence more the sound will be loud. 
the intensity of a sound wave is measure in terms of $$decibel$$.


State three differences between a musical sound and noise.

Musical SoundNoise
(i) It has a pleasing effect on the earsi) It is displeasing to the ear
(ii) It is produced by regular periodic vibrations of a body(ii) It is produced by irregular vibrations in a material
(iii) The amplitude of vibration and its frequency do not change suddenly(iii) The amplitude and frequency of vibration may change suddenly


Mention a few health hazards caused by noise pollution.

The health hazards caused by noise pollution are:
$$i)$$ Lack of sleep
$$ii)$$ Hypertension (high blood pressure)
$$iii)$$ Anxiety 
$$iv)$$ Temporary or even permanent impairment of hearing


Match the column:

For different instruments, there are different parts that vibrate. 
In a dholak, the vibrating part is a stretched membrane. 
In a guitar, it is a stretched string and in a flute, it is the air column. 
These parts begin the vibration and cause the entire instrument to vibrate eventually.


Match The following :


Column-1Column-2
AAudible rangepFaster transmission of sound
BInfrasonic rangeqSlower transmission of sound
CUltrasonic rangerLess than 20 Hz
DDenser mediums20 Hz - 20 kHz
ERarer mediumtMore than 20 kHz

The frequency range of audible sound by human ear is 20 Hz to 20000 Hz. 
The infrasonic sound range is below 20 Hz.
The ultrasonic sound range is more than 20 kHz.
The velocity of sound increases by increase in density. So, the sound travels fast in denser medium and slow in rarer medium. 

$$ A-s, B-r, C-t, D-p, E-q$$ 


Identify the part which vibrates to produce sound in the following instruments:
(a) Dholak
(b) Sitar
(c) Flute

(a)  The vibrating part in a dholak is a stretched membrane. 
(b)  The vibrating part in a sitar are stretched strings.
(c)  The vibrating part in a flute is air column inside it.


A pendulum oscillates $$40$$ times in $$4$$ seconds. Find its time period and frequency.

Hint: Use the relation between time period and frequency.

Explanation:

Step 1: Given:
No. of cycles $$N=40$$
Total time take $$t=4\ s$$
Step 1: Calculating the frequency:
The frequency of a wave or oscillation is defined as the number of cycles or completed alternations per unit time.
$$f=N/t$$
$$f=40/4$$
$$f=10 \ Hz$$

Step 2: Calculating the time period:
Frequency and time period are inversely proportional.
The amount of time it takes for something to complete one oscillation is called its time period.
$$Time \ period$$ $$= \dfrac{1}{Frequency} = \dfrac{1}{10} = 0.1\ s$$

So, the time period is $$0.1 \ s$$ and frequency is $$10 \ Hz$$



What is sound and how is it produced?

Sound is defined as vibrations that travel through the air or another medium as an audible mechanical wave. It is produced from a vibrating body. The vibrating body causes the medium (water, air, etc.) around it to vibrate thus producing sound.


Cite an experiment to show that sound needs a material medium for its propagation :

Apparatus required: 
A strong jar, Electronic Bell, Cork, Vacuum pump. 
Procedure: 
Take an electronic bell and connect it to electrical supply. You should hear the bell ringing. This confirms that the bell is working. Now, take a jar and make a small hole in the bottom and insert a small pipe through it. Connect the other end of the pipe to a vacuum cleaner. Make a very small hole in the cork (just enough to insert wires) and insert wires connected to the electronic bell. Fit the cork as shown in the figure. Turn on the bell and the vacuum cleaner. You will observe no sound is heard.
Conclusion:
Since the bell was working but its sound could not be heard outside the jar when air was pumped out, it can be concluded that sound needs a medium for propagation.

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Loudness does not change with change in__________(frequency/amplitude)

The loudness of a sound does not change with change in frequency. The amplitude of the sound wave determines its loudness.


Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Why?

Speed of sound in air  $$(330$$ $$ms^{-1})$$ is very less than speed of light in air  $$(3\times 10^8  m/s)$$.
Hence although flash and thunder are produced simultaneously, light (flash) reaches the earth surface a little earlier than sound (i.e thunder) .
So thunder is heard a few seconds after the flash is seen.


Explain how the human ear works.

The outer ear is called ‘pinna’. It collects the sound from the surroundings. The collected sound passes through the auditory canal. At the end of the auditory canal there is a thin membrane called the ear drum or tympanic membrane. When a compression of the medium reaches the eardrum the pressure on the outside of the membrane increases and forces the eardrum inward. Similarly, the eardrum moves outward when a rarefaction reaches it. In this way the eardrum vibrates. The vibrations are amplified several times by three bones (the hammer, anvil and stirrup) in the middle ear. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear. In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve, and the brain interprets them as sound.


Describe an activity to show that sound is produced by vibration of materials.

Take a rubber string stretch it tightly between two supports. Pluck the string, the string begins to vibrate. Then we will be able to hear some sound, stop the movement of the string. Even the sound also stops. From this activity we can show that the sound is produced by the vibration of material.


A mosquito produces a sound of high pitch. How can we justify this statement.

Mosquitoes do not produce sound yet we perceive high pitched sound from them because as per our hearing adaptability we perceive sounds that are higher in frequency caused by high vibrations. 
In actual, it is not the mosquito whose sound we hear but it is the sound that is caused by the vibrations of their wings.
And higher the frequency of the vibration, higher the pitch of the sound emitted to us. 


What measures would you take to minimize the sound disturbance caused to others?

The steps to be taken to minimize  the sound disturbance are as follows:
1. Turn off your electronic systems when not in use and also not pay them too loudly
2. Soundproof your music playing area
3. Plant more trees
4. Regular maintenance of vehicles and machines
5. Use earplugs.


Explain the following terms:
a) amplitude  
b) wavelength  
c) frequency

Amplitude : It is defined as the maximum displacement produced in the vibrating particle on either side of the mean position. It is denoted by A. Its SI unit is metre.

 Wavelength : It is defined as the distance travelled by a wave during the time a particle of the medium completes one vibration. It is denoted by k. Its SI unit is metre.

Frequency: It is the number of occurrences of a repeating event per unit of time. Its SI unit is Hertz (Hz).


A normal human being can listen to sounds with frequency from _________to___________ cycles per sec.

For human ear, the range of audible frequency is from $$20 \ Hz$$ to $$20,000 \ Hz$$.


Vibrations in a body produce sounds. How do you prove it? 

Body vibrates and it reaches us through the vibrations of the particles of surrounding medium.The sounds requires a medium for propagation.The vibration of the body produce vibration in the particle of surrounding medium which travels in from of wave with a certain speed depending upon the density and elasticity of the medium.




Identify the parts which vibrates to produce sound in following instruments.

We know that violin has strings whose transverse vibrations produce music.
Similarly, tabla has a membrane, which on tapping produces sound.
For producing sound in flute, we have to blow in the air column.

Thus, flute comes under air column type, tabla comes under stretched membrane type and violin comes under stretched string type of musical instrument.


Veena and Rani are on the moon, Veena calls out her friend, but Rani does not hear Veena's call even though she is near. Discuss.

We cannot hear any sound on the surface of moon because there is no medium on moon. Therefore, sound needs a medium to travel through that is why we cannot hear any sound on the surface of moon.


Displacement distance graph of two sound waves A and B, travelling in a medium, are as shown in the diagram. Study the two sound waves and compare their wavelengths.
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From the diagram, we can say that the sound wave A has larger amplitude and thus larger energy as compared to sound wave B. But both the sound wave have same wavelength.


Displacement distance graph of two sound waves A and B, travelling in a medium, are as shown in the diagram. Study the two sound waves and compare their Amplitudes.
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From the graph we say that the amplitude of sound wave A is larger than that of sound wave B.
Ratio of amplitudes of both waves   $$\dfrac{S_A}{S_B} = \dfrac{20}{10} = 2$$


Find the odd one out based on their audible range.

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Elephants can detect infrasonic sound waves. While bats, dolphins, and rabbits can detect sounds in the ultrasonic range.


Which characteristic of sound will help you to:
(a) Distinguish between the sound of a boy and a girl.
(b) Distinguish between the sounds of two girls.

(a) For distinguishing between a boy and a girl's sound, the characteristic of sound which will help is pitch. Because pitch of girl is higher than the pitch of boys.Pitch denotes the frequency  of sound production.
(b)  For distinguishing between two girl's sound, the characteristic of sound which will help is timbre. Timbre is the character or quality of a musical sound or voice as distinct from its pitch and intensity.


Your parents are going to buy a new house. They have been offered one on the roadside and another two lanes away from from the roadside. Which house would you suggest your parents? List out the ways and means to reduce noise.

I would suggest my parents to buy a house which is two lanes away from the roadsides.
It is because the house situated on the roadside will have the following disadvantages in comparison to the house two lanes away from the roadsides.
(i) Lot of noise due to passing of vehicle.
(ii) Smoke and dust produced due to traffic on road.
(iii) Sound of pressure horns at the time of traffic jams or during peak hours of the traffic living in a house nearby roadside would cause noise pollution.


Sound can not travel in ________.

Sound needs a material medium for their propagation like solid, liquid or gas to travel because the molecules of solid, liquid and gases carry sound waves from one point to another. Sound cannot progress through the vacuum because the vacuum has no molecules which can vibrate and carry the sound waves.
So correct answer is vaccum.


Take a metal plate (or a pan ). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick (Fig.13.2). Do you hear a sound? Touch the plate or pan gently with your finger. Do you feel the vibrations? Again strike the plate with the stick and hold it tightly with your hands immediately after striking. Do you still hear the sound? Touch the plate after it stops producing sound. Can you feel the vibrations now?
1163127_771e8912b73a40a2addee3f952af97b8.png

Answer to every question as red in the question is "Yes"$$. After striking the pen with a stick, its surface vibrates resulting its surrounding air to also vibrate that causes sound. In initial stage of vibration the frequency of sound produced which fall in are dilde range thus sound could be heard but after sometime sound will be in are dible as the frequency gas fallen out of the acedible range but still the vibration could felt.



What are ultrasonics? Can you hear the ultrasonic sound?

The term "ultrasonic" applied to sound refers to anything above the frequencies of audible sound, and nominally includes anything over 20,000 Hz. Frequencies used for medical diagnostic ultrasound scans extend to 10 MHz and beyond.

No, we cannot hear ultrasonic sounds because it beyond our audible range. Animals like bats and dogs can hear them.


List three methods of controlling noise pollution.  

Methods of controlling noise pollution$$:$$
$$(i)$$ Installing noise-reducing insulation like glass wool
$$(ii)$$ Planting more trees
$$(iii)$$ Using earplugs instead of speakers.


Why does $$'Sa'$$ of guiter and Flute sound different?

In the case of guitar, the sound depends on the vibration of a stretched strings. But in the case of flute, the sound is produced by the vibration of the air column. 


Sound does not travel through .......................

Sound is a mechanical wave, so it does not travel through vacuum.


Do you hear the sound when bell jar is completely evacuated ?

No, we don't hear any sound when the bell jar is completely evacuated. 
$$\underline{\text{Reason}}$$
Sound is a mechanical wave. A medium is required for its propagation. When the bell jar is evacuated, there is no medium. Hence, the sound can not propagate and we can not hear the sound. 


Describe an activity to show that sound needs a medium to travel with a neat well-labeled sketch.

Sound is a mechanical wave that needs a material to travel. It can travel through the air, water, steel, etc. but cannot travel through a vacuum.

This can be demonstrated by the bell jar experiment.

  • Suspend an electric bell inside a glass bell jar bypassing the connecting wires through an air-tight cork fitted at the mouth of the jar.
  • Place the jar over a disc that has a pipe connected to a vacuum pump.
  • When we turn on the switch, we hear the sound of the bell.
  • Now, with the help of a vacuum pump, we pump out the air from the jar. The sound becomes fainter.
  • When most of the air has been removed, we hear a very feeble sound. When there is air inside the jar, sound travels through it to the wall of the jar. This makes the wall vibrate which in turn, sends sound to us.
  • When air is removed, sound from the bell cannot travel to the wall of the jar.

Therefore, we can see that the sound waves need a material medium for propagation.

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Differentiate between audible and inaudible sounds.

 Audible Sound Inaudible Sound
 1) Audible sounds are waves which are audible to human ear. 1) Inaudible sounds are waves which are not audible to human ear.
 2) The frequency of audible sound lies in range $$20 \ Hz - 20 \ Khz$$ 2) All waves which lie below $$20 \ Hz$$ and above $$20 \ Khz$$ are inaudible to human ear.



Sound is produced when your school bell in struck with hammer. Why ?

When an object vibrates, it forces the neighbouring particles of the medium to vibrate. These vibrating particles then forces the particles adjacent to them to vibrate. In this way, vibrations produced by an object are transferred from one particle to another till it reaches the ear.


Define frequency ? 


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Explain how sound is produced by your school bell.

The bell produces the sound when it is struck by air hammer when the bell is struck by a hammer it starts vibrating. Since the vibrating objects produce sound. So the bell produces sound.


Fill in the blanks :
The total number of compressions and rarefactions produced per second in a sound wave is $$1000 .$$ The frequency of the sound wave is .............

The total number of compressions and rarefactions produced per second in a sound wave is $$1000 .$$ The frequency of the sound wave is $$ 500\,Hz$$ (One compression and rarefaction together makes a cycle of sound wave).


Describe an experiment to show that sound can travel in water.

Take a tub filled with water. Hold a bell in one hand and dip it in water. Keep one of your ears gently on the surface of water without letting water into the ear. Now ring the bell inside water. You will be able to hear the sound clearly. This shows that sound can travel through liquids.


What should an object do to produce sound ?

An object produces sound there is a creation of vibrations.


The pulse rate of a man is 80 beats in one minute. Calculate its frequency.

The pulse rate of a man is $$80$$ beats in one minute.
Number of beats = Number of cycles = $$80$$
Time,t = $$1\ minute$$ = $$60\ s$$
Frequency, $$f=$$ number of cycles per unit time
$$f = \dfrac{80}{60} = 1.33 $$ Hz.


Mr. Gupta was the President of a society which was constructed near a very busy main road. After many complaints from the residents Mr. Gupta planted some fast growing trees along the society's perimeter:

What values did Mr. Gupta shown while deciding to plant trees?

Trees act as noise reducing objects.The leaves ,stems and trunk all these absorb sound and noise pollution is reduced due to plantation of trees.
Hence the values shown by Mr. Gupta are:
  • Awareness of environment
  • Futuristic
  • Environment sensitive
  • Correct decision making.


There was a party at Aman's house. Loud music was being played at Night. His neighbor came and requested him to lower the sound of the music system as his grandfather was not well and was resting. Aman lowered the volume of the music system at one.

Do you think Aman did the right thing? Why or why not?

Yes, he did the right thing understanding the problems of his neighbor and showed that he is a good citizen.


Mr. Gupta was the President of a society which was constructed near a very busy main road. After many complaints from the residents Mr. Gupta planted some fast growing trees along the society's perimeter:

How trees help in reducing noise pollution?

Trees act as noise barriers and reduce pollution through a phenomenon called sound attenuation, which is the damping of sound.
Trees attenuate noise by absorption, deflection, refraction, and masking. Noise absorption by plants: Tree parts such as stems, leaves, branches, and wood absorb sound waves.


There was a party at Aman's house. Loud music was being played at Night. His neighbour came and requested him to lower the sound of the music system as his grandfather was not well and was resting. Aman lowered the volume of the music system at once.

What value do you learn from Aman.

We know music is soothing to human beings.While noise is disturbing to human beings.But when music is played loud it turns into noise as it disturbs people nearby and especially elder people can not sleep at night because of such loud music nearby.
Hence Aman lowered the volume.
We learn following values from Aman:
  • Sensitivity towards neighbours.
  • Awareness of noise pollution.


Mr. Gupta was the President of a society which was constructed near a very busy main road. After many complaints from the residents Mr. Gupta planted some fast-growing trees along the society's perimeter:

What was the residents' complaint about?

The residents complained about the high noise level during the day and night due to the traffic on the main road.


There was a party at Aman's house. Loud music was being played at Night. His neighbor came and requested him to lower the sound of the music system as his grandfather was not well and was resting. Aman lowered the volume of the music system at one.

If you were at Aman's place would you have done the same thing?

Yes, it is good to be obident and think about others living in our society. 


 Explain the term amplitude of a wave. Draw the diagram of a wave and mark its amplitude on it.

Amplitude is defined as the maximum displacement of the particle from its original undisturbed position.
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A tuning fork has a number $$256$$ marked on it. What does this number signify?

The number $$256$$ on the tuning fork signifies the frequency of the tuning fork.


What is the infrasonic vibrations and ultrasonic vibrations?

Infrasonic vibrations: Their frequency is below  $$ 20\,Hz $$ . 

Ultrasonic vibrations : Their frequency is above $$ 20\,KHz $$ . 


Which object is vibrating when the following sounds are produced?

a) the sound of a sitar

b) the sound of a table

c) the sound of a tuning fork

d) the buzzing of a bee or mosquito

e) the sound of a flute

a) Vibrations in the stretched strings of sitar

b) Vibrations from the stretched membrane

c) Vibrations from the prongs

d) Vibrations from the wings

e) Vibrations of the air in column



Calculate the frequency of a wave whose time-period is $$0.02\,s.$$

$$Frequency = 1 / time \,period$$
$$ = 1 / 0.02$$
$$ = 50\,Hz$$


What is the time-period of a tuning fork whose frequency is $$200\, Hz$$?

The given data,
Frequency$$(f) = 200\,Hz$$
Web know that-
$$\text{Frequency} = \dfrac{1}{\text{ time period }}$$

$$\Rightarrow \text{time period(t)} = \dfrac{1}{\text{ Frequency }}$$

$$\Rightarrow t = \dfrac{1}{200}$$

$$\Rightarrow t = 0.005\ s$$


Explain why a ringing bell suspended in a vacuum chamber cannot be heard outside.

When the ringing bell is suspended in a vacuum chamber it cannot be heard outside because there are no air molecules that vibrate to carry the sound waves.


If $$20$$ waves are produced per second, what is the frequency in hertz?

The frequency in hertz is $$20$$ hertz as the hertz is the representation of number of waves produced per second.


What is the difference between loudness and intensity of sound .

The amount of sound energy passing each second through the unit area is called the intensity of sound. We sometimes use the terms “loudness” and “intensity” interchangeably, but they are not the same. Loudness is a measure of the response of the ear to the sound. Even when two sounds are of equal intensity, we may hear one as louder than the other simply because our ears detect it better. So, loudness is the subjective quantity but intensity is the objective quantity.


Can elephants hear frequencies less than $$ 20\,Hz $$ ? 

Elephants are able to hear the infrasound range. This range lies below $$ 20\,Hz $$ ,  So, they are able to hear sound below $$ 20\,Hz $$ 


Define loudness of sound.

The loudness of sound refers to the intensity of the sound. The intensity of sound is proportional to the amplitude of the sound wave ($$I\propto A^2$$).
If the loudness of a sound is more, it is loud. If the loudness of sound is less, it is a soft sound.


We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us? 

Every frequency cannot be heard with the human ear.Frequencies between $$20$$hz and $$2000$$hz only can be heard.Therefore we cannot hear every sound of few vibrations. 


Why do we can hear the sound produced by the humming bees while the sound of vibrations of pendulum is not heard?

Humming bees produce sound by vibrating their wings. The sound produced is in the audible range $$(20\ Hz\ to\ 20,000\ Hz)$$. In case of pendulum the frequency is below $$20\ Hz$$ which does not come in the audible range. Our ear is not able to perceive such low frequency vibrations.


An avalanche of sand along some rare desert sand dunes can produce a booming that is loud enough to be heard $$ 10\ \mathrm{km} $$ away. The booming apparently results from a periodic oscillation of the sliding layer of sand- the layer's thickness expands and contracts. If the emitted frequency is $$ 90\ \mathrm{Hz} $$, what is the period of the thickness oscillation?

Given,
Frequency emitted, $$f=90\ Hz$$
We know,
$$f=\dfrac 1T$$

where $$T=$$ time period
$$\implies T=\dfrac 1f$$

$$T=\dfrac{1}{90}$$

$$T=0.01\ s$$


Illustrate with the help of an experiment that sound requires a medium to propagate . 

● Take an electric bell and an airtight bell jar.
● Suspend the bell inside the bell jar.
● Connect a vacuum pump to the bell jar as shown in the figure.
● Press the switch to hear the bell.
● Now, start the vacuum pump.
● When the air in the jar is pumped out gradually, the sound becomes fainter even though the same current is passing through the bell.
● As the amount of air decreases inside the bell jar, the sound becomes fainter.
● When there is no air inside the jar, there is no sound of the bell.
This shows that sound requires a medium to propagate.

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Define the following terms : 
Time period 

 Definition of Time period:

The time taken by a particle of medium to complete its one vibration is called the time period of the wave.

or

time period is the time taken by a vibrating body to complete one oscillation

time period is represented by the letter T. 

  • second (s) is the SI unit of time period.



Fill in the blanks 
The number of times a body vibrates in one second is called its ______ .

The number of times a body vibrates in one second is called its frequency. The SI unit of frequency is $$Hertz$$.


Define the following terms : 
Amplitude 

Part (a): Definition of Amplitude:

the maximum extent of a vibration or oscillation , measured from its mean position is called the amplitude of a wave.

Meter(m) is the SI unit of amplitude.

and Amplitude is denoted by A. 


part (b) : diagram




  




Fill in the blanks:
Sound can travel in a _____, _______ or _______. 

Sound can travel in a medium solid, liquid, or gas.
Sound travels as a longitudinal wave through a material medium. 


Describe an experiment to show that sound can travel in a solid .

Take two empty ice-cream cups. make a small hole at the bottom of each cup and pass a long thread (about $$ 20 \,m $$ long) through them. Tie a knot or match-stick at each end of the thread so that the thread does not slip out through the holes. This makes a toy -telephone . Now use the toy telephone as shown in figure and talk to your friend . You will be able to hear the sound of your friend . This shows that sound travels through the thread and reaches your ear. Thus , sound can travel through a solid. 


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Define the following term :
Frequency.

$$\text{Frequency (f):}$$ The number of oscillations made by a wave in one second is known as its frequency. 
Frequency is the inverse of the time period.

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Differentiate between a high pitch sound and a low pitch sound.

 High pitched soundLow pitched sound 
1) High pitched sounds have higher frequency. 1) Low pitched sounds have higher frequency. 
2) High pitched sounds have shorter wavelength. 2) Low pitched sounds have higher wavelength. 
3) Sound produced is shrill. 3) Sound produced is flat. 

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Describe an experiment to show the production of sound having low and high pitch.

Take few rubber bands some thicker and longer, few thinner and of shorter length. Cut and stretch these rubber bands by holding one end of the string in your mouth under the teeth and the other end in your hand. Now pluck these rubber bands one by one. The rubber bands thicker and longer will produce sound with a lower pitch. The rubber bands thinner and shorter will produce sound with a higher pitch. 


Which characteristics of sound makes it possible to recognize a person by his voice without seeing him ?

Timbre and pitch are the characteristics of sound that make it possible to recognize a person by his voice without seeing him.


Can sound travel through solids and liquids ? In which of these two does it travel faster ?

Yes, sound can travel through solids and liquids. Speed of sound is more in higher density medium. Hence, Sound travels with the highest speed in solids and sound travels with the lowest speed in gases. 


What causes sound?

Sound is produced by vibrating objects. The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid, or gas. Sound moves through a medium from the point of generation to the listener. When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear. The disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium.


What determines the pitch of a sound?

 Frequency determines the Pitch of a sound. 

  • Pitch is the sensation (brain interpretation) of the frequency of an emitted sound.

  • Faster the vibration of the source, higher is the frequency and higher is the pitch. Similarly, low pitch sound corresponds to low frequency. 


 




How can the pitch of sound produced in a piano be changed ?

In a piano , the string is struck to make the string vibrate and produce sound. The pitch of sound produced can be changed by stretching or loosening the strings of piano.
Pitch here is nothing but the vibration frequency of string.


Complete the following sentences:
(a) The pitch of sound increases if its frequency …………..
(b) If the amplitude of a sound is halved, its loudness becomes ..............

(a) Pitch describes how high or low a tone is and depends upon the rapidity with which a sounding body vibrates, i.e., upon the frequency of vibration. 
Hence the pitch of sound increases if its frequency $$increases$$ .
(b) Loudness of sound depends on square of amplitude .
So if the amplitude of a sound is halved, its loudness becomes $$one-fourth$$ .


A musical instrument takes $$2$$ seconds to complete $$200$$ oscillations then calculate frequency of it.

Frequency $$ = \dfrac{{\text{Total time taken in vibration}}}{{\text{Time taken in vibration}}} $$ 

Frequency $$ = \dfrac{200/ {\text{Vibration}}}{2 \, sec} $$ 
 
       $$ = 100 $$ vibration/sec. or hertz.


What is the importance of air in transmission of sound?

Air acts as a medium for transmission of sound. When a sound wave passes through air, the particles of air vibrate back and forth, parallel to the direction of the sound wave. Thus, when a sound wave travels in the horizontal direction, then the particles of the medium also vibrate back and forth in the horizontal direction. Sound cannot travel without a medium and thus we cannot listen to sounds in the vacuum because of absence of air. 


There is no atmosphere on moon. Can you hear each other on the moon's surface?

No, we cannot hear each other on the surface of the moon because there is not medium available on the moon to propagate the sound.


What is noise pollution ? How does it affect us ? How can it controlled ? Explain it in detail. 

The disturbance (irregular sound waves) produced in the environment by the undesirable , loud and harsh sound from the various sources is called noise pollution. 
Noise pollution is a recent phenomenon of the twentieth century. Increasing dependence of the man of various kinds of machines at home, at work place in factories , etc. has contributed a lot to the noise pollution.
The noise pollution at a particular place is determined by following :
(a) Loudness of the sound
(b) Duration of noise at a particular place. 
Harmful effects :
  1. Noise in the surrounding interferes with speech and talk with another person.
  2. A long exposure to noise pollution may result in the loss of hearing of deafness.
  3. Noise pollution reduces concentration and results in the loss of work efficiency.
  4. Noise causes anger , tension and interferes with the sleep pattern of the individuals.
  5. Noise produces headaches , irritability and nervous tension.
  6. Noise can causes loss of night vision as well as cause colour blindness.
Preventive and control of noise :
In the modern society we can not eliminate noise, but can lower its level to bearable limits by taking following measures: 
  1. The machines should be designed in such a way that they produce minimum noise.
  2. All automobiles , electric generators etc. should be provided with improved silencers 
  3. The heavy vehicles should not allowed in residential areas. 
  4. The use of loudspeakers for various social or religious functions should be banned.
  5. The factories should be relocated far away from the residential areas.
  6. At homes, the T.V, the radio , the power music system etc. should be played at low volume.


Define frequency and periodic time. Represent the relation of them by a formula. 

Frequency: The number of vibrations or oscillations per second is called frequency.
Time period: The time required to complete one oscillation or vibration is called a time period. 
Following is the relationship between frequency and time period.
Time period $$ = \dfrac{1}{{\text{Frequency}}} $$


Complete the following sentence:
Sound is produced by a ______ body.

Sound is produced by vibrating objects.


What are differences in audible , infrasonic , ultrasonic.

If the frequency of the vibrations is less than $$20\,Hz $$ , then the human ear cannot perceive them. Such vibrations are called infrasonic vibrations . 
If the frequency of vibrations is between $$ 20\,Hz$$ to $$ 20,000\,Hz $$ , then they can be perceived by the human ear . Such vibrations are called sonic vibrations or sound vibrations . It has been further found that higher the frequency , the sharper or shriller is the effect of sound on the ear. 
However, if the frequency of vibration is above $$ 20000\,Hz$$ , then the human ear cannot perceive . Such vibrations are called ultrasonic vibrations or ultrasonic. 


Fill in the blanks :
The international unit of frequency is .................

The international unit of frequency is Hertz


Which object is vibrating when the following sounds are produced ? 
The sound of a sitar or veena.

Strings of sitar or veena vibrate to produce sound.


What is frequency ? How is it represented ? Write its S.I unit.

The number of vibration/oscillation per second is called the frequency of the wave. It is represented by the symbol $$ \nu $$ (nu). Its S.I unit is Hertz (Hz).


Which object is vibrating when the following sounds are produced? 
The sound of the tabla.

In Tabla (or any percussion instrument, whereon leather is fitted), sound gets generated, when the leather surface thumped, wither by fingers or palm.
The skin of the tabla vibrate to produce sound.


Which object is vibrating when the following sound is produced? 
The buzzing of a bee or a mosquito.

The wings of a mosquito or a bee vibrate to produce sound.


Which object is vibrating when the following sounds are produced ? 
The radio.

In the radio speakers vibrate to produce sound.


What is the most common medium through which sound travels ? 

Sound is a mechanical wave which need a medium to travel.
Air is the most common medium through which sound travel.


What is time period ? Write its S.I unit. 

The time taken by one complete oscillations in the density of the medium is called time period. It is denoted by the capital letter T. Its S.I unit is second.


Write the definition of the following :
Frequency 

Frequency: The change in density from the maximum value to the minimum value, again to the maximum value, makes one complete oscillation. The number of such oscillations per unit time is called the frequency of the sound wave, represented by f. It's S.I unit is Hertz (symbol, Hz).


In which of the three media, air, water, or iron, does sound travel the fastest at a particular temperature?

Sound travels fastest through iron.
As the density increases the speed of the sound also increases. Iron is solid so it has high density.


Write the definition of the following :
Time period

Time period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave. It is represented by the symbol T. Its S.I. the unit is second (s).


How do we produce sound ?

The sound is produced when an object is set to vibrate or we can say that vibrating objects produce sound. The sound of the human voice is produced due to vibration in the vocal cord. We can produce sound by striking the tuning fork, by plucking, scratching, rubbing, blowing or shaking two objects. All objects produce sound due to vibration. However we can perceive these vibrations as sound only when they lie in our audible range (i.e. $$20 \ Hz$$ to $$20,000 \ Hz$$)


Which wave property determines the loudness of sound?

The amplitude of vibrations determines the loudness of sound.


Which wave property determines the Pitch of sound? 

A frequency of the sound determines its pitch. Higher the frequency, the more is the pitch of the sound.


How do human beings contribute to noise pollution?

The major reasons for noise pollution are:
  1. Air horn
  2. Loudspeaker
  3. The sounds of vehicles etc


Explain an activity to show that sound can do work.

Take a thin strip of paper. Hold it against sound from a loud speaker the paper starts moving. This shows that sound can do work.


Table the following as those having high pitch and low pitch separately.
Cuckoo's cry, lion's roar, female voice, duck's sound, male voice, air horn.

high pitchlow pitch
cuckoo's cryLion's roar
Female voiceDuck's sound
Air hornMale voice


The frequency of a source of sound is $$ 250 \,Hz $$ . Calculate the number of times , the sound vibrates in one minute. Also , calculate the time period ?

Given frequency $$ v = 250\,Hz $$ 
thus , the number of vibrations of the source of sound in one second $$ = 250 $$ 
Therefore no. of vibrations in one minute $$ = 250 \times 60 = 15000 $$ 
The time period , $$ T = \dfrac{1}{v} = \dfrac{1}{250} $$ 
$$ T = 0.004 $$ second.


What is meant by amplitude of a sound wave?

The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter $$A$$.


Mention the harmful effects of noise pollution.

Unwanted sound leads to noise pollution
Effects of noise pollution
1. Lound sound may cause irritation, headache and loss of concentration
2. Long exposure to such sounds may lead to loss of sleep, tension and stress.


Name the two species of animals which can detect and produce infrasonic waves?

Whales, pigeons, octopus, Elephants, Cod and Giraffe can detect infrasonic waves.


State what change happens to the loudness of sound in the following situation:
Amplitude of vibration decreases.


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What are the demerits of sound pollution?

Causes mental stress
Causes emotional strain
Causes deafness
Increase the blood pressure


What is the available range of sound frequencies?

Humans can detect sounds in a frequency range from about $$20\ Hz$$ to $$20\ kHz$$. (Human infants can actually hear frequencies slightly higher than $$20\ kHz$$, but lose some high-frequency sensitivity as they mature; the upper limit in average adults is often closer to $$15–17\ kHz$$.)


Complete the table.
No. of vibrationstimeFrequency
10__a__2
153___b___
____c___54

Frequency of vibration is no of vibration per second given by:
$$frequency=f=\dfrac{No\quad of\quad vibrations}{Time}$$
So
Case 1
$$2=\dfrac{10}{a}$$
$$a=5s$$
Case2
$$b=\dfrac{15}{3}$$=$$5s^{-1}$$
Case 3
$$4=\dfrac{c}{5}$$
$$c=20vibrations$$


Write the ways to reduce sound pollution.

$$\underline{\text{Ways to reduce sound pollution:}}$$
  • Ban the air horn
  • Control loudspeakers
  • Use silencers in vehicles
  • Control the sound having more than 50dB near the hospitals


Can we hear the sound from Galton whistle having a frequency of $$30000Hz$$? Why?

No. We cannot hear the sound having a frequency greater than $$20000Hz$$. Sounds having frequency more than 20,000 Hz are called Ultrasonic sound. As this is above the normal hearing range for humans, we cannot hear ultrasonic sound.


How is sound produced and how is it transmitted and heard by us?

Sound is a form of energy produced and transmitted by vibrating matter. Sound travels in waves. When sound moves through matter (solid, liquid, or a gas), the molecules of the matter move backward and forward in the direction in which the wave is traveling. This movement is called a vibration.
As sound waves travel, molecules are pressed together in some parts compression) and in some parts are spread out (rarefaction). When a loud sound is heard, it is because many molecules have been vibrated with much force. A soft sound is made with fewer molecules being vibrated with less force. 
Sound travels more quickly through solids than through liquids and gases because the molecules of a solid are closer together. Sound travels the lowest through gases because the molecules of a gas are farthest apart.
In a vacuum sound cannot travel because there is no matter for it to move through.


Describe the bell jar experiment to prove that sound requires a medium to propagate.

Bell Jar Experiment : Let us take a bell jar and suspend an electric bell in it and connect it to a battery outside the jar When the circuit is closed (switch is on) we can hear the bell ringing as the jar contains air and sound travels through this air
We then remove the air from the jar with the help of a vacuum pump connected to the bell jar Now as we close the circuit we can hardly hear any sound even though the hammer of the bell is seen striking the gong So we can conclude that sound cannot propagate in the absence of the material medium
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What is vacuum? Explain why sound cannot travel through vacuum?

Sound is a mechanical wave, so to propagate it material medium is required. Vacuum is defined as the empty space in which there are no matter particles present. Sound cannot travel through vacuum as there is no particles present for vibrations to take place.


An alarm bell is kept inside a vessel as shown in Figure. A person standing close to it can distinctly hear the sound of the alarm. Now if the air inside the vessel is removed completely. How will the loudness of alarm get affected for the same person?
1790883_407ba06a7db84199beabf7e88303af83.png

Until the air was present in the vessel the necessary medium was there to help the sound to propagate. But when the air is removed from the vessel vacuum takes place. Sound cannot propagate in a vacuum; therefore the man standing next to the vessel will not hear the sound of the bell.


A person is listening to a tone of $$500 Hz$$ sitting at a distance of $$450 m$$ from the source of the sound. What is the time interval between successive compression from the source?


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Vijay, Shikha, Rajesh and Anupam were arguing about "would someone in space hear the explosion of an asteroid ?"
Vijay said, "they would not hear the explosion."
Shikha said the reason for the same that  "the explosion produce sound of frequency above 20,000 Hz hence they cannot hear it."
Which among these two is correct?

We know that sound is a mechanical wave and it needs a medium to propagate.In space there is no medium .Space is vaccum hence sound can not travel in space that's why explosion can not be heard in space so Vijay is correct.
We also know that sound of frequency more than 20,000Hz can not be heard by humans.
So what Shikha said is also correct statement but it is not the correct reason for what Vijay said.


a) An electric bell is suspended by thin wires in a glass vessel and set ringing. Describe and explain what happens if the air is gradually pumped out of the glass vessel.

b) Why cannot a sound be heard on the moon? How do astronauts talk to one another on the surface of moon?

a) When the air is gradually pumped out of the glass vessel, the ringing of the electric bell cannot be heard as there is a creation of vacuum inside the glass vessel. As there are no air molecules inside the glass jar, sound waves cannot travel.

b) Sound cannot be heard on the moon because there is no air on the moon which acts as a carrier of sound waves. Astronauts talk to one another with the help of radio waves as these waves can travel through the vacuum.



How does sound propagate in a medium?

Sound is produced by vibrating objects. Sound is a mechanical wave and needs a material medium like air, water, steel etc. for its propagation. A vibrating object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave to propagate through the medium. 


How is loudness related to the amplitude of wave?

  • The loudness of a sound depends on the amplitude of vibration of the vibrating body producing sound. Greater the amplitude of vibrations, louder is the sound produced.

  • The loudness of sound is directly proportional to the square of amplitude of wave. 

Loudness ∝ $(Amplitude)^2$

 




How do we heard sound ? 

When the vibrating air hits your eardrum, it causes your eardrum to vibrate. These vibrations are transferred through the tiny bones in your ear to the inner ear. These vibrations are detected by nerves, which send impulses that your brain "hears" as sound.

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Draw a diagram to show the wave pattern of high pitch note and a low pitch note, but of the same loudness.

  • Pitch is the sensation (brain interpretation) of the frequency of an emitted sound.

  •  Faster the vibration of the source, higher is the frequency and higher is the pitch. Similarly, low pitch sound corresponds to low frequency. 


Diagram of high pitch and low pitch wave patterns







The time period of a wave is  2 s. Its frequency is _____.

 

The time period of a wave = 2 s.

  frequency = 0.5 Hertz.

Explanation:

Given,

Time t = 2 sec

We know that, Frequency ƒ = \frac{1}{Time period (T)}

ƒ =\frac{1}{2s}=0.5Hz

Hence,  0.5 Hertz. is the frequency 





If a drum is beaten hard, its loudness _____(increases/decreases).

If a drum is beaten hard, its loudness increases.


Describe an activity to show that the sound requires a material medium for its propagation.

Suspend a mobile phone inside an airtight glass jar as shown in the diagram connect the bell to vacuum pump, when a call is made to that phone, the ringing tone is heard. Start removing the air from the jar by using a vacuum pump.
The ringing tone will get feeble and feeble and finally cannot be heard by anyone. This experiment shows that sound cannot travel in vacuum, it needs a material medium for its propagation.Because sound became inaudible when all air (medium) was removed.
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Class 8 Physics Extra Questions