HOW SOUND IS PROPAGATED

HOW SOUND IS PROPAGATED?

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 the propagation of sound in 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.

Explanation of propagation of sound

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. 12.5. 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. 12.5.

Figure 12.5 ( explanation of propagation of sound ) : A vibrating object creating a series of compressions(C) and rarefactions(R) in the medium

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.

Can sound make a light spot dance? ( sound-light-dance )

Take a tin-can. Remove both ends to make it a hollow cylinder. Take a balloon and stretch it over the can, then wrap a rubber band around the balloon. Take a small piece of mirror. Use a drop of glue to stick the piece of mirror to the balloon. Allow the light through a slit to fall on the mirror. After reflection the light spot is seen on the wall, as shown in Fig. 12.4. Talk or shout directly into the open end of the can and observe the dancing light spot on the wall. Discuss with your friends what makes the light spot dance.

Figure 12.4 ( sound-light-dance ) : A beam of light from a light source is made to fall on a mirror. The reflected light is falling on the wall

Questions

1.   How does the sound produced by a vibrating object in a medium reach your ear?

Source: This topic is taken from NCERT TEXTBOOK

SOUND NEEDS A MEDIUM TO TRAVEL

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.

Activity 12.3: ( sound cannot travel without medium )

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. 12.6. 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?

Figure 12.6 ( sound cannot travel without medium ) : Bell jar experiment showing sound cannot travel in vacuum

Questions

1.   Explain how sound is produced by your school bell.

2.   Why are sound waves called mechanical waves?

3.   Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Source: This topic is taken from NCERT TEXTBOOK

SOUND WAVES ARE LONGITUDINAL WAVES

SOUNDWAVES ARE LONGITUDINAL WAVES

Activity 12.4:

*  Take a slinky. Ask your friend to hold one end. You hold the other end.

*  Now stretch the slinky as shown in Fig. 12.7 (a). 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.

Figure 12.7: Longitudinal wave in a slinky

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 direction 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 propagation of the wave. When we drop a pebble in a pond, the waves you see on the water surface is an example of transverse wave. 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.

Source: This topic is taken from NCERT TEXTBOOK

CHARACTERISTICS OF A SOUND WAVE

CHARACTERISTICS OF A SOUND WAVE

We can describe a sound wave by its

*   Density/pressure

*   Wavelength and

*   speed

Characteristics of sound

*   Frequency (Pitch)

*   Amplitude (loudness)

*   Quality (timber)

A sound wave in graphic form is shown in Fig. 12.8(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.12.8(a) and Fig.12.8(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.12.8(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.12.8(c). A peak is called the crest and a valley is called the trough of a wave.

Figure 12.8: Sound propagates as density or pressure variations as shown in (a) and (b), (c) represents graphically the density and pressure variations

The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength, as shown in Fig. 12.8(c), The wavelength is usually represented by \(\lambda\) (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 in 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, then 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 \(\nu\) (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 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 follows:

\(\nu=\frac{1}{T}\)

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. 12.9. 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.

Figure 12.9: Low pitch sound has low frequency and high pitch of sound has high frequency

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. 12.8(c). For sound its unit will be that of density or pressure.

The loudness or softness of 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 louder sound. Can you tell why? A sound wave spreads out from its source. As it moves away from the source its amplitude as well as its loudness decreases. Louder sound can travel a larger distance as it is associated with higher energy. Fig. 12.10 shows the wave shapes of a loud and a soft sound of the same frequency.

Figure 12.10: Soft sound has small amplitude and louder sound has large amplitude

The timber or quality 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.

The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time.

We know,

speed, v = distance / time

v=  \(\frac{\lambda}{T}\)                  

Here \(\lambda\) is the wavelength of the sound wave. It is the distance travelled by the sound wave in one time period (T) of the wave. Thus,

v = \(\lambda\nu\)

That is, speed = wavelength \(\times\) frequency.

The speed of sound remains almost the same for all frequencies in a given medium under the same physical conditions.

The amount of sound energy passing each second through unit area is called the intensity of sound. We sometimes use the terms “ loudness and intensity ” interchangeably, but they are not the same. Loudness is a measure of the response of the ear to the sound. Even when two sounds are of equal intensity, we may hear one as louder than the other simply because our ear detects it better.

More to know

Heinrich Rudolph Hertz was born on 22 February 1857 in Hamburg, Germany and educated at the University of Berlin. He confirmed J.C. Maxwell’s electromagnetic theory by his experiments. He laid the foundation for future development of radio, telephone, telegraph and even television. He also discovered the photoelectric effect which was later explained by Albert Einstein. The SI unit of frequency was named as hertz in his honour.

Illustration 12.1:

A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5km?

Sol:

Given,

Frequency, \(\nu\)= 2 kHz = 2000 Hz,

Wavelength, \(\lambda\) = 35 cm = 0.35 m

We know that

speed, v of the wave = wavelength \(\times\) frequency

v = \(\lambda\nu\)

= 0.35 m \(\times\)2000 Hz = 700 m/s

The time taken by the wave to travel a distance, d of 1.5 km is

\(t=\frac{d}{v}=\frac{1.5\times1000m}{700ms^{-1}}=\frac{15}{7}s=2.1s\)

Thus sound will take 2.1 s to travel a distance of 1.5 km

Questions

1.  Which wave property determines

(a) loudness, (b) pitch?

2.  Guess which sound has a higher pitch: guitar or car horn?

3.  What are wavelength, frequency, time period and amplitude of a sound wave?

4.  How are the wavelength and frequency of a sound wave related to its speed?

5.  Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

6.  A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

7.  Distinguish between loudness and intensity of sound.

Source: This topic is taken from NCERT TEXTBOOK

SPEED OF SOUND IN DIFFERENT MEDIA

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 ms^–1 at 0ºC and 344 ms^–1 at 22ºC. The speeds of sound at a particular temperature in various media are listed in Table 12.1. You need not memorise the values.

Table 12.1: Speed of sound in different media at 25 ºC

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 window glass and even damage buildings.

Questions

1.  In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Source: This topic is taken from NCERT TEXTBOOK