Sound

Distance (d) = 1020 m

time (t) = 3 s

Speed of sound = $\dfrac{\text{distance}}{\text{time}}$ = $\dfrac{\text{1020}}{\text{3}}$ = 340 m s^-1.

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

Speed of sound = 330 m s^-1

time (t) = 4 s

Speed of sound = $\dfrac{\text{distance}}{\text{time}}$

distance = speed x time = 330 x 4 = 1320 m

So, distance between two hills = 1320 m .

(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.

(a) vibrates

(b) frequency

(c) frequency

(d) a medium solid, liquid or gas

(e) 20 Hz to 20,000 Hz

(f) medium

(g) solids

(h) echo

(i) 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.

vacuum

Reason — Sound needs a medium for its propagation. 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.

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.

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 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.

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.

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.

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 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 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 pitch of a sound depends on its frequency.

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

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

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.

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.

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).

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.

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.

(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. So during a thunderstorm, the sound of a thunder is heard after the lightning is seen.

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 = $\dfrac{\text{distance (d)}}{\text{time (t)}}$ = $\dfrac{\text{d}}{\text{t}}$ m s^-1.

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

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 $\dfrac{\text{33}}{\text{2}}$ = 16.5 m either way.

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

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.