Production and Propagation of Sound

Sound

Everyday we hear sounds from various sources like humans, birds, bells, machines, vehicles, televisions, radios etc. Sound is a form of energy which produces a sensation of hearing in our ears. There are also other forms of energy like mechanical energy, heat energy, light energy etc. We have talked about mechanical energy in the previous chapters. You have been taught about conservation of energy, which states that we can neither create nor destroy energy. We can just change it from one form to another. When you clap, a sound is produced. Can you produce sound without utilising your energy? Which form of energy did you use to produce sound? In this chapter we are going to learn how sound is produced and how it is transmitted through a medium and received by our ear.

 

Production of Sound

Activity-1:

• Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear.
• Do you hear any sound?
• Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends.
• Now, suspend a table tennis ball or a small plastic ball by a thread from a support [Take a big needle and a thread, put a knot at one   end of the thread, and then with the help of the needle pass the thread through the ball]. Touch the ball gently with the prong of a vibrating tuning fork (Fig. 1).
• Observe what happens and discuss with your friends.

 

Fig.1.

In the above activities we have produced sound by striking the tuning fork. We set the objects vibrating and produce sound. Vibration means a kind of rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords. When a bird flaps its wings, do you hear any sound? Think how the buzzing sound accompanying a bee is produced. A stretched rubber band when plucked vibrates and produces sound. If you have never done this, then do it and observe the vibration of the stretched rubber band.

 

Propagation of 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. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle.

As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound reaches your ear. The disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium.

A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. They in turn produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during propagation of sound in a medium, hence sound can be visualised as a wave.

Sound waves are characterised by the motion of particles in the medium and are called mechanical waves. Air is the most common medium through which sound travels. When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C), as shown in Fig. 2. This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R), as shown in Fig. 2. As the object moves back and forth rapidly, a series of compressions and rarefactions is created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure. Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium.

 

Fig.2.

 

Sound Needs a Medium to Travel

Sound is a mechanical wave and needs a material medium like air, water, steel etc. for its propagation. It cannot travel through
vacuum, which can be demonstrated by the following experiment.

Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump, as shown in Fig. 3. If you press the switch you will be able to hear the bell. Now start the vacuum pump. When the air in the jar is pumped out gradually, the sound becomes fainter, although the same current is passing through the bell. After some time when less air is left inside the bell jar you will hear a very feeble sound. What will happen if the air is removed completely? Will you still be able to hear the sound of the bell?

 

Fig.3.

 

Sound Waves are Longitudinal Waves

Activity-2:

• Take a slinky. Ask your friend to hold one end. You hold the other end. Now stretch the slinky as shown in Fig. 4. Then give it a sharp  push towards your friend.
• What do you notice? If you move your hand pushing and pulling the slinky alternatively, what will you observe?
• If you mark a dot on the slinky, you will observe that the dot on the slinky will move back and forth parallel to the direction of the propagation of the disturbance.

 

Fig.4.

 

The regions where the coils become closer are called compressions (C) and the regions where the coils are further apart are called rarefactions (R). As we already know, sound propagates in the medium as a series of compressions and rarefactions. Now, we can compare the propagation of disturbance in a slinky with the sound propagation in the medium. These waves are called longitudinal waves.

In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest. This is exactly how a sound wave propagates, hence sound waves are longitudinal waves.

There is also another type of wave, called a transverse wave. In a transverse wave particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels. Thus a transverse wave is the one in which the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation. Light is a transverse wave but for light, the oscillations are not of the medium particles or their pressure or density – it is not a mechanical wave. You will come to know more about transverse waves in higher classes.

 

Characteristics of a Sound Wave

We can describe a sound wave by its

  • frequency
  • amplitude and
  • speed.

A sound wave in graphic form is shown in Fig. 5(c), which represents how density and pressure change when the sound wave moves in the medium. The density as well as the pressure of the medium at a given time varies with distance, above and below the average value of density and pressure. Fig. 5(a) and Fig. 5(b) represent the density and pressure variations, respectively, as a sound wave propagates in the medium.

Compressions are the regions where particles are crowded together and represented by the upper portion of the curve in Fig. 5(c). The peak represents the region of maximum compression. Thus, compressions are regions where density as well as pressure is high. Rarefactions are the regions of low pressure where particles are spread apart and are represented by the valley, that is, the lower portion of the curve in Fig. 5(c). A peak is called the crest and a valley is called the trough of a wave.

 

Fig.5

 

The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength, as shown in Fig. 5(c), The wavelength is usually represented by λ (Greek letter lambda). Its SI unit is metre (m).

Frequency tells us how frequently an event occurs. Suppose you are beating a drum. How many times you are beating the drum per unit time is called the frequency of your beating the drum. We know that when sound is propagated through a medium, the density of the medium oscillates between a maximum value and a minimum value. 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 the frequency of the sound wave. If we can count the number of the compressions or rarefactions that cross us per unit time, we will get the frequency of the sound wave. It is usually represented by  ν (Greek letter, nu). Its SI unit is hertz (symbol, Hz).

The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the wave. In other words, we can say that the time taken for one complete oscillation in the density of the medium is called the time period of the sound wave. It is represented by the symbol T. Its SI unit is second (s). Frequency and time period are related as : ν =1/T .                                      

A violin and a flute may both be played at the same time in an orchestra. Both sounds travel through the same medium, that is, air and arrive at our ear at the same time. Both sounds travel at the same speed irrespective of the source. But the sounds we receive are different. This is due to the different characteristics associated with the sound. Pitch is one of the characteristics.

How the brain interprets the frequency of an emitted sound is called its pitch. The faster the vibration of the source, the higher is
the frequency and the higher is the pitch, as shown in Fig. 6. Thus, a high pitch sound corresponds to more number of compressions and rarefactions passing a fixed point per unit time.

Objects of different sizes and conditions vibrate at different frequencies to produce sounds of different pitch.

 

Fig.6.

 

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, as shown in Fig. 6. For sound its unit will be that of density or pressure.

The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon
the force with which an object is made to vibrate. If we strike a table lightly, we hear a soft sound because we produce a sound wave of less energy (amplitude). If we hit the table hard, we hear a loud sound. Can you tell why?

Loud sound can travel a larger distance as it is associated with higher energy. A sound wave spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases. Fig. 7. shows the wave shapes of a loud and a soft sound of the same frequency.

Fig.7.

The quality or timber of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness. The sound which is more pleasant is said to be of a rich quality. A sound of single frequency is called a tone. 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.

 

Speed of Sound in Different Media

Sound propagates through a medium at a finite speed. The sound of a thunder is heard a little later than the flash of light is seen.So, we can make out that sound travels with a speed which is much less than the speed of light. The speed of sound depends on the properties of the medium through which it travels. You will learn about this dependence in higher classes. The speed of sound in a medium depends on temperature of the medium. The speed of sound decreases when we go from solid to gaseous state. In any medium as we increase the temperature the speed of sound increases. For example, the speed of sound in air is 331 m s–1 at 0ºC and 344 m s–1 at 22ºC. The speeds of sound at a particular temperature in various media are listed in Table.1. You need not memorise the values.

 

 

 

Speed of sound in different media at 25 ºC
State Substance Speed in m/s
Solids Aluminium 6420 6420
Nickel 6040
steel 5960
Iron 5950
Brass 4700
Glass (Flint) 3980
Liquids Water (Sea) 1530
Water (distilled) 1498
Ethanol 1207
Methanol 1103
Gases Hydrogen 1284
Helium 965
Air 346
Oxygen 316
Sulphur dioxide 213

 

Sonic Boom

When the speed of any object exceeds the speed of sound it is said to be travelling at supersonic speed. Bullets, jet aircrafts etc. often travel at supersonic speeds. When a sound, producing source moves with a speed higher than that of sound, it produces shock waves in air. These shock waves carry a large amount of energy. The air pressure variation associated with this type of shock waves produces a very sharp and loud sound called the “sonic boom”. The shock waves produced by a supersonic aircraft have enough energy to shatter glass and even damage buildings.

 

What you have learnt?

  • Sound is produced due to vibration of different objects.

  • Sound travels as a longitudinal wave through a material medium.

  • Sound travels as successive compressions and rarefactions in the medium.

  • In sound propagation, it is the energy of the sound that travels and not the particles of the medium.

  • Sound cannot travel in vacuum.

  • The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.

  • The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ.

  • The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.

  • The number of complete oscillations per unit time is called the frequency (ν),1/T

  • The speed v, frequency ν, and wavelength λ, of sound are related by the equation, v = λν.

  • The speed of sound depends primarily on the nature and the temperature of the transmitting medium.

 

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