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Sound and Hearing.

posted Feb 2, 2015, 3:51 AM by   [ updated Oct 15, 2016, 1:07 PM by Upali Salpadoru ]

FigH.P. - High Pressure,   L.P. - Low Pressure    R- Rarefaction,  C- Compression..1.

Fig 1. Passing of a sound wave.                            Fig 2 Sound waves entering the ear

Sound waves are pressure waves. High pressure alternating with low pressure rhythmically.
 How come? 
If a drum is struck, as shown in Fig,2 the skin will go back. This will make the particles of air behind condense. In front of it pressure will reduce. 
Next moment, the drum skin  will curve to the left and then spring back to right. These movement will continue for some time.
This kind of a ‘to and fro’ movement is called a vibration. If the number of  vibrations in one second is more than 20 and less than 20.000, you will hear a sound.

 The vibrations of the drum skin  will produce ripples or pressure waves in the air, right round. The dots in the diagram represent air molecules. As the skin moves to the left the molecules on that
side have become compact. This is called a compression. On the other side they are far apart. It is rarefaction.So the sound travels as compressions and rarefactions in the air. 

The pressure changes during the process are indicated by the graph above. During a compression the pressure is high and low during a rarefaction..

 This type of a wave, where the particles vibrate parallel to the direction of the wave is a Longitudinal wave.
In transverse waves, such as the ripples in water, the particles vibrate perpendicular to the wave.

Slinky 2.jpg

Fig.3. A plastic slinky with which longitudinal waves may be demonstrated.

If you push and pull one end of this a pressure wave will travel.

This is shown in Fig.4.


Fig.4. A longitudinal wave ina spring.

When the oscillation of a single loop is converted to a time and displacement graph we get something similar to this.

Velocity W.jpg

Fig.5. Time and distance graph of a wave.

Data: Total time = 0.08 seconds.

         Number of cycles = 2.   Number of wavelengths  =2.

  1. Calculate the frequency. (Number of oscillations in one second)

In 0.08 seconds there are   2 waves. That is 2 oscillations.

Therefore in 1 second  there will be = 2/ 0.08 oscillations.

                                                   = 25 Frequency = 25 Hz.

    2. If the velocity of the wave is 1000ms-1 what is the wavelength?

Velocity - Length of a wave x number of waves in 1 second.

Velocity = Wave length x Frequency

        V =  f x  λ


Fig.6. An oscilloscope converts longitudinal waves to transverse waves.

Hearing of a sound.

The blue lines, in Fig.2,  indicate the pressure waves spreading from the source of the sound. These are collected by the flap of the outer ear (pinna) and funneled into the auditory canal. (ear canal). which is about 3cm long. It narrows down into the middle ear. 

The narrowing of the tube has an amplifying effect. When the pressure waves of a sound note reaches the middle ear , the ear drum is set to vibrate. A compression forces the eardrum inward and a rarefaction forces the eardrum outward, thus vibrating the eardrum at the same frequency of the sound wave. Ear drum (tymphanic membrane) is an extremely sensitive diaphragm consisting of living cells.

There are three tiny inter connected bones, called ossicles which further amplify the sound
wave. The third bone, the stirrup pulls and pushes the oval window. The distance the stirrup
will move is much more than the distance ear drum will move.  Unlike in a normal lever, the
distance ratio can be controlled. This ability gives some protection against loud noises.

In the middle ear sound is not transmitted as pressure waves but as vibrations of ossicles.


 Fig. 7.  Arrows  show the direction of movement at one moment
.    .

 What's the difference between sound and noise?
Unpleasant loud sounds are described as noise. 

In the inner ear the vibrations that are conveyed by the ossicles are converted back to a pressure wave. This time it is hydraulic pressure. A structure, about the size of a pea receives the pressure fluctuations. These are conveyed to the brain via auditory nerves as electrical signals.
      The whole process of hearing could be summarized as follows.

1,  Source of sound : -  Vibrations

2.   Mode of travel

a.  In air

b.  Outer ear

c.   Middle ear.

d. Inner ear.

e.  Ear to brain.

Pressure  waves  (longitudinal waves)

Pressure  waves  (longitudinal waves)

Vibrations of ossicles.

Hydraulic pressure

Electrical in nerves


     Speed of sound
     The Echo method.
      Fire a shot a few meters, say 100m. away from a wall. Some waves will reflect off the barrier and return. A stop watch can measure the time taken for the waves to reach the wall and return. Assume the time taken for the echo to be 0.6 seconds.
      Calculation..............  Velocity = displacement / time.
                                             V   = 2xd / t.
                                             V   = 200 / 0.6..................= 333. ms-1.
Q.1.0 Label the numbered parts of the ear.

Select the answers from these.

Semilunar canal.Ear..jpg

Ear canal.

Ear membrane.





Eustachian tube.



2 x 10 =20 marks


In what form does a sound signal  1. Reaches the ear.

2.travels in the ear canal  3. In the Middle ear  4. In the Inner ear  5  in the nerves.
4x 5 = 20 marks



The fig. Shows a sound wave reaching the ear. Answer the following questions in respect to this.

  1. What do the dots indicate in wave 1 ?.

  2. Which way do they move or vibrate ?

  3. What type of a wave is shown in no.1?  What are correct?

 (1).  Electromagnetic wave. (ii). Mechanical wave       (iii). Longitudinal wave                           (iv). Transverse wave.

             A- all are correct.   B- All are wrong    C. Only(iii) is correct  D. (ii) and (iii) are correct.

  1. How many wavelengths are shown here ?

  2. If the time taken for all oscillations here t is 0.5 S., What is the  the frequency?

  3. If the speed of the wave is 340 ms-1, what is the wavelength?

  4. What is the period?

  5. Can this be considered as a sound wave? Give a reason.

  6. If the listener is wearing a ear plug or a earmuff, what would happen to the wave?

  7. What is the function of the outer ear flap? (Pinna)

3 x 10 = 30 marks


1. The velocity of a sound wave will depend on which of these?

  1. Amplitude.  B, Wave length C Nature of medium    D. Frequency..

2. Thunder and lightening occur at the same time.  But we hear the sound a few seconds later. What could be the reason for this?

  1. Light is an electro magnetic wave while sound is a mechanical wave.

  2. Light travels faster than a sound wave.

  3. Sound needs a medium while light can travel in a vacuum.

  4. Our eyes detect light while our ears detect sound.

3.  Two microphones 3.4 m apart detects a passing sound wave. A mili timer has detected the time difference to e  0.01 s. Which would give the speed of sound?

A- 3.4 x 0.01    B.3.4 ÷ 0.01  C- 0.01÷ 3.4  D. 2x3.4 / 0.01

4x3 = 12 marks


The list below shows different origins of sound waves. Mention what primarily vibrate in producing these.

  1. A tuning fork.     2.  A whistle.   3. A baby crying.   4. Lightning  5. Guitar.

6. Loud speaker

3x 6 = 18 marks.

For the answers click Answer-Sc.