Sound

pitch

Reason — Pitch is the characteristic of sound which distinguishes a shrill sound from a flat sound.

500 Hz

Reason — We can hear sounds of frequency in the range of 20 Hz to 20,000 Hz.

Hence, we can hear sound of frequency of 500 Hz.

vacuum

Reason — Sound needs a medium for its propagation so it cannot travel in vacuum.

17 m

Reason — To hear an echo clearly in air the reflecting surface should be at a minimum distance of 16.5 m from the source of sound.

So, 17 m is the correct answer.

metre

Reason — Wavelength is measured in metre.

1500 m s^-1

Reason — The speed of sound in water is nearly 1500 m s^-1.

solids

Reason — Sound travels much faster in solids than liquids and gases.

0.1 sec

Reason — Sound persists in the human ear for about 0.1 second due to a phenomenon called persistence of hearing.

This means that even after the sound source stops vibrating, the sensation of sound continues in the ear for a short duration.

This property helps in the continuity of sound, such as in speech and music, preventing abrupt breaks between successive sounds.

larynx

Reason — The larynx is called the voice box of humans because it contains the vocal cords, which vibrate to produce sound when air passes through them.

Both A and R are true and R is the correct explanation of A

Explanation

Assertion (A) is true because the speed of sound in air is about 330 m s^-1, and for an echo to be heard distinctly, a minimum time gap of 0.1 s is required.

In this time, sound travels a total distance of:

330 × 0.1 = 33 m

Since this distance includes the to-and-fro journey of sound, the reflecting surface must be at least 16.5 m away.

Thus, the total distance travelled by sound is about 33 m.

Reason (R) is true because a distinct echo requires a minimum time delay of 0.1 s, which is the persistence of hearing of the human ear.

The Reason justifies the Assertion because a distinct echo needs a time gap of 0.1 s. In this time, sound travels about 33 m (to and fro) which leads to a minimum distance of about 16.5 m to the reflecting surface, explaining the assertion.

Therefore, both A and R are true and R is the correct explanation of A.

A is false but R is true

Explanation

Assertion (A) is false because sound does not travel faster in air than in liquids but it travels faster in liquids because particles are closer together and transmit vibrations more efficiently.

Reason (R) is true because particles in air are more loosely packed compared to liquids due to less intermolecular forces.

Therefore, A is false but R is true.

(a) Sound is produced when a body vibrates.

(b) The number of times a body vibrates in one second is called its frequency.

(c) The pitch of a sound depends on its frequency.

(d) Sound can travel in a medium solid, liquid or gas.

(e) We can hear sounds of frequency in the range of 20 Hz to 20,000 Hz.

(f) Sound requires a medium for propagation.

(g) Sound travels faster in solids than in liquids.

(h) The sound heard after reflection is echo.

(i) Sound produces sensation in ears.

(a) False
Correct Statement — Sound cannot travel in vacuum. Sound needs a medium for its propagation.

(b) True

(c) True

(d) False
Correct Statement — Larger is the amplitude, louder is the sound produced.

(e) True

(f) False
Correct Statement — Loudness depends on amplitude.

(g) False
Correct Statement — Waveforms of two different stringed instruments are different.

(h) True

(i) False
Correct Statement — A ticking clock sound is heard faster when heard through a metal.

(j) False
Correct Statement — Sound waves with frequencies lower than 20 Hz are called infrasonic sound and sound waves with frequencies above 20,000 Hz are called ultrasonic sound.

The oscillatory (to and fro) motion in which the body assumes a new shape during its motion is called vibratory motion. In Vibratory motion, a part of the body oscillates keeping its other part fixed.

Sound is a form of energy that produces the sensation of hearing in our ears.

Sound is produced when a body vibrates.

The two sources of sound are:

1. Tuning fork
2. Musical instruments

We produce sound when our vocal cords vibrate on blowing air through them by our lungs.

Bees make sound by moving their wings up and down very fast.

The wave in which the particles of the medium vibrate about their mean positions, in the direction of propagation of sound is called longitudinal wave.

1. Amplitude (a) — The maximum displacement of a wave on either side of its mean position, is called the amplitude of wave.

2. Time period (T) — The time taken by a wave to complete its one vibration is called the time period of the wave.

3. Frequency (f or n) — The number of vibrations produced by a source of sound wave in one second is called the frequency of the wave.

The audible range of frequency for the normal human ear is 20 Hz to 20,000 Hz.

Sounds of frequencies higher than 20,000 Hz are called ultrasonic sounds. We cannot hear ultrasonic sound.

Sounds of frequency lower than 20 Hz are called infrasonic sounds. We cannot hear the infrasonic sounds.

Bat produces ultrasonic sound while flying. Ultrasonic waves reflect back after striking any obstacles in their way and bats hear it. Thus, they detect the obstacles in their path and change their path without colliding with the obstacle.

Loudness and pitch.

The loudness of a sound depends on:

1. The amplitude of vibration of the vibrating body producing the sound.
2. The area of the vibrating body.

The loudness of sound produced is directly proportional to the vibrating area of the body. Greater the area of the vibrating body, louder is the sound produced.

The pitch of a sound depends on its frequency.

Quality is the characteristic which differentiates two sounds of the same pitch and same loudness.

Quality of sound makes it possible for the musician to recognize the musical instrument by hearing the sound produced by it, even without seeing the instrument.

A musician playing on a flute changes the pitch of sound produced by it by changing the length of vibrating air column on closing the different holes present in it.

Musical instruments are provided with more than one string to produce sound of different pitch. Musical instruments are provided with number of strings of different thickness and under different tensions so that each string produces sound of a different pitch.

The pitch of sound produced in a piano can be changed by the following ways:

1. By changing the place of plucking on the string.
2. By changing the tension and thickness of the string.

(i) 330 m s^-1.

(ii) 1500 m s^-1.

(iii) 5960 m s^-1.

Light travels much faster than sound so it takes negligible time for light to reach us as compared to sound. Thus, during a thunderstorm, the sound of a thunder is heard after the lightning is seen.

Yes, sound can travel through solids and liquids. It travels faster in solids as compared to liquids.

The return of sound after striking a surface in the same medium is called reflection of sound.

The reflection of sound is used in making the trumpet, sound board, megaphone.

Echo is the distinct sound heard after reflection from a distant rigid surface such as a cliff, a hillside, the wall of a building, etc.

The minimum distance required between the source of sound and the reflecting surface to hear an echo is 16.5 m.

Speed of sound in air is 330 m s^-1 and the time interval needed to identify two different sounds is 0.1 s, so sound will travel a distance of 330 x 0.1 = 33 m. Since sound has to travel an equal distance in going up to the reflecting surface and in coming back from the reflecting surface, so it must travel nearly $\frac{\text{33}}{\text{2}}$ = 16.5 m either way.

Clothes, paper, thermocol, curtains are good absorbers of sound.

The following experiment shows that each source of sound is a vibrating body:

1. Take a ruler.
2. Press its one end on the table with the left hand as shown in the figure below.
3. Pull down the other end of the ruler with the right hand and then leave it.

We will notice that the ruler vibrates i.e., the ruler moves to and fro and a humming sound is heard. After sometime the ruler stops vibrating. No sound is then heard.

This shows that the humming sound is produced only because of the vibrations of the ruler. Hence, we can say that each source of sound is a vibrating body.

No, sound cannot travel through a vacuum.

It is shown by the following experiment:

1. Take an electric bell and an air-tight glass jar connected to a vacuum pump. Suspend the electric bell inside the jar.
2. Connect the bell to a battery through a switch.
3. On pressing the switch bell starts ringing and a sound is heard.
4. Now start the vacuum pump. It withdraws the air from the jar.

We will notice that as the air from the jar is evacuated the sound gradually becomes more and more feeble. After sometime when no air is left within the jar, no sound is heard. But we can notice the hammer of the electric bell is striking the gong.

This shows that sound cannot travel in vacuum because when there is air in the jar, we can hear the sound but when air is evacuated from the jar through vacuum pump, we cannot hear the sound although the bell is vibrating.

The following experiment shows that sound can travel in water:

1. Fill a balloon with water and hold it near your ear.
2. Now keep a watch gently to the other side of the balloon.

We will hear the ticking sound of the watch.

This shows that sound can travel in water.

The following experiment show that sound can travel in a solid:

1. Take two empty ice-cream cups. Make a small hole at the bottom of each cup and pass a long thread through them.
2. 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.
3. Now use the telephone as shown in the below figure and talk to your friend.

You will be able to hear the sound of your friend.

This shows that sound can travel through solid as sound travels through the thread and reaches the ear.

Quality of sound makes it possible to recognize a person by hearing his voice on a telephone without seeing him. The vibrations produced by the vocal cord of each person have a characteristic wave form which is different for different persons.

The following experiment shows the production of sound having low and high pitch.

1. Take four test tubes. In first test tube take a little amount of water in it.
2. Blow air in the tube by placing your lip on the mouth of the test tube.

You will hear a flat sound i.e. sound of low pitch.

3. Now add more and more water in the remaining three test tubes as shown in below figure.
4. Again blow air in each of these three test tubes.

You will notice sound becomes more and more shrill i.e. high pitch.

In first tube water is very less, so flat sound (low pitch) is produced. As we add more and more water in the tubes, the length of vibrating air column above water level decreases, so sound becomes more shriller (high pitch).

Sound travels much faster in solids as compared to liquids and gases. The speed of sound in steel is 5960 m s^-1. So, sound produced by the moving wheels of train travels much faster through the track than air. So you can predict the arrival of a train by placing your ear on the rails without seeing it.

Estimation of speed of sound in air:

1. Suppose we choose two hills A and B about a kilometre apart.
2. A person at hill A fires a gun.
3. Another person at hill B starts a stop watch as he sees the flash of fire and stops it on hearing the sound.

He measures the time interval between the sight of flash and hearing the sound. Let it be t second.

Measure the distance between the hills A and B. Let it be d metre.

So speed of sound (V) = $\frac{\text{distance (d)}}{\text{time (t)}}$ = $\frac{\text{d}}{\text{t}}$ m s^-1.

Experimentally it is found that speed of sound in air is 330 m s^-1.

Following measures should be taken when designing a sound-proof room:

1. The roof of the enclosure must be covered by plaster of paris after putting the sheets of thermocol.
2. The wall of the enclosure should be covered by wooden strips.
3. The floor must be laid down by thick carpets.
4. Thick curtains should be used to cover the doors and keep them closed.
5. Thick stripping must be used to cover the openings of doors and windows.
6. The machine parts of all the electrical equipments like fans, air conditioners etc. must be placed outside the enclosure.

Given,

• Distance between two boys = 1020 m
• Time taken to hear the echo = 3 s

Speed of sound is given by,

$$\begin{aligned}\text {Speed of sound} = \frac{\text{Distance between two boys}}{\text{Time taken to hear the echo}} \\[1em] = \frac{\text{1020}}{\text{3}} \\[1em] = 340 \text { m s}^{-1}\end{aligned}$$

So, the speed of sound is 340 m s^-1.

Given,

• Speed of sound = 330 m s^-1
• Time to hear the echo = 4 s

Now, $$\begin{aligned}\text {Speed of sound} = \frac{\text {Distance between the two hills}}{\text{Time to hear the echo}} \\[1em] \Rightarrow \text {Distance between the two hills} = \text {Speed of sound} \times \text{Time to hear the echo} \\[1em] = 330 \times 4 \\[1em] = 1320\ \text m\end{aligned}$$

So, the distance between two hills is 1320 m.

The solved crossword puzzle is given below:

No, two persons cannot hear each other on moon's surface because sound needs a medium for its propagation but on moon's surface there is vacuum, so sound cannot propagates through it.

The outer case of the bell in a temple is made big to increase the surface area of the vibrating body. It helps in producing a louder sound as greater the area of the vibrating body, louder is the sound produced, which can be heard to a longer distance.

The voice of a woman is shriller than that of a man because the voice of a woman has a higher pitch.

(i) Initially, the auditorium had hard, bare surfaces like walls and floor, which are good reflectors of sound. So, the sound waves were reflected back, producing an echo.

(ii) Carpets, curtains, and cushioned chairs are soft and absorbent materials. They absorb sound waves instead of reflecting them, so the echo disappeared and the sound became clearer.

(iii) An echo is heard when the reflected sound reaches the ear after a sufficient time gap i.e., at least 0.1 s, so it is heard separately from the original sound.

(iv) The reflecting surface should be at least 16.5 m away from the source of sound. This allows the sound to travel about 33 m to and fro, producing a distinct echo.