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Study of Sound

Production of sound

If somebody calls you from behind, you will quickly turn around. What makes you do so?

We turn back in response to a call because of the sound heard by us. We are able to talk to each other because of the sound produced by us. We are able to predict the distance of a train only by listening to the sound it produces. Similarly, we can distinguish between different musical instruments because of the sounds they produce.

How do you realize that an alarm bell is ringing? So, what is sound? Sound is a form energy that produces the sensation of hearing in our ears and vibrating bodies produce sound. 

Do you know how a sound is produced? To find out, let us perform the following activities.

Take a frying pan and suspend it in air with the help of support. Hit the pan with a metal spoon. Now, touch the pan. Can you feel the vibrations? When you beat an object, you can feel its vibrations with the help of your sense of touch. Touch the pan when it is not producing any sound. Can you feel the vibrations now?

 

 

 

Take a rubber band and stretch it between two poles (as shown in the given figure). Now, pluck the rubber band in the middle. Can you hear any sound? Does the rubber band vibrate when it produces a sound? On plucking a stretched rubber band or a stretched string, it vibrates rapidly and produces a sound.

 

 

Take a cooking utensil and pour some water in it. Now, beat the utensil with a rod. You will hear a sound. Carefully, observe the surface of water in the utensil. Do you see concentric circles moving on the water surface? These are vibrations in water, produced by vibrations of the utensil body, on beating.

Therefore, it can be concluded that a vibrating body produces sound.

The back and forth movement of an object produces sound. An object moving back and forth is said to be in vibration. Hence, sound is produced by vibrating objects.  

If somebody calls you from behind, you will quickly turn around. What makes you do so?

We turn back in response to a call because of the sound heard by us. We are able to talk to each other because of the sound produced by us. We are able to predict the distance of a train only by listening to the sound it produces. Similarly, we can distinguish between different musical instruments because of the sounds they produce.

How do you realize that an alarm bell is ringing? So, what is sound? Sound is a form energy that produces the sensation of hearing in our ears and vibrating bodies produce sound. 

Do you know how a sound is produced? To find out, let us perform the following activities.

Take a frying pan and suspend it in air with the help of support. Hit the pan with a metal spoon. Now, touch the pan. Can you feel the vibrations? When you beat an object, you can feel its vibrations with the help of your sense of touch. Touch the pan when it is not producing any sound. Can you feel the vibrations now?

 

 

 

Take a rubber band and stretch it between two poles (as shown in the given figure). Now, pluck the rubber band in the middle. Can you hear any sound? Does the rubber band vibrate when it produces a sound? On plucking a stretched rubber band or a stretched string, it vibrates rapidly and produces a sound.

 

 

Take a cooking utensil and pour some water in it. Now, beat the utensil with a rod. You will hear a sound. Carefully, observe the surface of water in the utensil. Do you see concentric circles moving on the water surface? These are vibrations in water, produced by vibrations of the utensil body, on beating.

Therefore, it can be concluded that a vibrating body produces sound.

The back and forth movement of an object produces sound. An object moving back and forth is said to be in vibration. Hence, sound is produced by vibrating objects.  

Reflection of Sound

When you sing in the bathroom or shout in an open field, your sound gets reflected off various obstacles. This reflection of sound results in echo and reverberation. There is an old wives’ tale that a duck’s quack has no echo. The tale would be true if the duck quacks in your living room. However, in suitable conditions, a duck’s quack will surely echo. 

When sound falls on a hard surface (solid or liquid), it bounces and changes its direction—just like light or a rubber ball. This bouncing back of sound on striking a surface is called reflection of sound. Hard surfaces such as a metal box and concrete wall are good reflectors of sound waves. Soft surfaces such as a cushion are bad reflectors of sound because they absorb sound. 

Laws of reflection of sound:

(i)The incident sound wave, the reflected sound wave and the normal to the surface at the point of incidence, all lie in the same plane, i.e., reflection is a two-dimensional phenomenon.

(ii)The angle of reflection of sound is always equal to the angle of incidence.

Quick Questions

Question 1: Is the law of reflection of sound similar to the law of reflection of light?

Solution: Yes, the two laws are similar.

Question 2: Does the frequency of sound change after reflecting off a surface?

Solution: No, it does not. The frequency of sound depends only on the source of sound.

Activity to Show the Reflection of Sound

Echo 

The repetition of sound caused by its reflection off a hard surface is known as echo. If you shout once in an auditorium, then you will hear the original sound at first and then the reflected sound. This reflected sound is the echo of the original sound.

Echo 

The sensation of a sound exists in the human brain for about 0.1 s. This means that if two sounds reach our ears within one-tenth of a second, then we will not hear them as separate sounds. So, if a reflected sound is to be heard separately from the original sound, there needs to be a time interval of at least one-tenth of a second (i.e., 0.1 s) between them

Now, we know that:

The speed of sound in air at 20°C is about 344 m/s. 

The minimum time difference needed between a sound and its reflection for the echo to be heard is 0.1 s.

Therefore, the total distance travelled by the sound and its reflection to produce the echo is given as:

Total distance = Speed × Time 

= 344 × 0.1 = 34.4 m

So, the sound travels 34.4 m during the time between which it is transmitted and the echo is heard. This distance is twice the actual distance between the source of the sound and the reflector of the sound. Therefore, the actual distance between the source of the sound and the reflector of the sound is 17.2 m.

Project Ideas

Visit your school auditorium with a friend. One of you should stand at a corner and the other should stand at the adjacent corner that is farther from it. One of you should clap. The other should measure the time interval between the clap and its echo using a stopwatch. Then, taking the speed of sound to be 330 m/s, calculate the distance between the two of you. Find out the actual length of the auditorium and compare it with the distance calculated.

Solved Examples

Medium

Example 1: 

A person is standing between two vertical cliffs. He is 540 m away from the nearest cliff. He shouts and hears the first echo after 3 s. Calculate the speed of sound in air.

Solution: 

Total distance covered by the sound and its reflection = 2 × 540 m

Time taken for the echo to be heard = 3 s

Let the speed of sound in air be v.

We know that: Speed=DistanceTime⇒v=2×5403∴v=360 m/s

Example 2:

Rajeev claps his hands near a mountain and hears the echo of the sound after 6 s. If the speed of sound in air is 346 m/s, then calculate the distance between Rajeev and the mountain.

Solution: 

Time taken for the echo to be heard = 6 s

The time taken by the sound to reach the mountain is half of the time taken for the echo to be heard, i.e., 3 s.

Speed of sound in air = 346 m/s

Let the distance between Rajeev and the mountain be s.

We know that: Distance = Speed × Time ⇒s=346×3∴ s=1038 m

Reverberation

A sound produced in an auditorium exists for some time because it undergoes multiple reflections off the walls, ceiling and floor. This is called reverberation. The duration of an echo in this case is so short that several echoes overlap with the original sound. If the reverberation is too long, then the sound becomes distorted, noisy and confusing.

Solved Examples

Easy

Example 1:

A fishing boat using sonar detects a school of fish 150 m below it by transmitting an ultrasound signal. How much time elapses between the transmission of the signal and its return to the boat? (Speed of sound in sea water = 1500 m/s) 

Solution: 

It is given that:

Speed of sound in sea water = 1500 m/s

Distance between the boat and the fish = 150 m

Distance covered by the ultrasound signal = (2 × 150) m = 300 m

Let the time taken by the signal to return to the boat be t.

We know that: Time=DistanceSpeed⇒t=3001500∴t=0.2 s

Medium

Example 2: 

A man standing at a point between two parallel walls fires a pistol. He hears the first echo after 0.5 s and the second one after 0.7 s. Find the distance b

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