## 12. Study of Sound

Sound is a form of energy which creates the sensation of hearing in our ears. This energy is in the form of waves. A medium is necessary for the propagation of sound waves. Sound waves give rise to a chain of compression (place of higher density) and rarefaction (place of lower density) in the medium. The particles of the medium oscillate about their central or mean positions, in a direction parallel to the propagation of the wave. Such waves are called longitudinal waves. On the other hand, in the waves created by dropping a stone in still water, the particles of water oscillate up and down. These oscillations are perpendicular to the direction of propagation of the wave, such waves are called transverse waves.

A sound wave can be shown in the form a graph. At any moment during the propagation of a sound wave we would find alternate bands of compression and rarefaction of the medium i.e. bands of greater and lesser density.

Figure 12.1 A shows the changes in density while figure B shows the changes in pressure. The changes in density or pressure are shown in the form of a graph in figure C.

The wavelength of sound waves is indicated by the Greek letter lambda (l), while its frequency is indicated by nu (u). The amplitude, which is the maximum value of pressure or density, is indicated by A. The time taken for one oscillation of pressure or density at a point in the medium is called the time period and is indicated by T. The value of frequency determines the pitch (high or low) of the sound while the value of the amplitude determines its strength or loudness

Velocity of sound

1. Take your friend to a place where there are iron pipes e.g. the school verandah, a staircase or a fence. 2. Stand near one end of the pipe and ask the friend to stand 20 to 25 feet away from you near the pipe.
• Ask the friend to gently tap the pipe with a stone. Press your ear to the pipe and listen to the sound coming through the pipe. 4. The same sound coming through the air can also be heard. But which sound reaches you first? From the above activity you can see that you hear the sound through the iron pipe much before you hear it coming through air. This shows that sound travels faster in iron than in air

The distance covered by a point on the wave (for example the point of highest den[1]sity or lowest density) in unit time is the velocity of the sound wave.

Thus, the velocity of sound = wavelength x frequency.

In any medium at fixed physical conditions the velocity of sound of different frequencies is very nearly the same. The velocity is highest in solids and least in gases. It increases with an increase in the temperature of the medium.

Audible, infra- and ultra-sound

The limits of hearing of the human ear are 20 Hz to 20,000 Hz. That is, the human ear can hear sounds of frequencies in this range. These sounds are called audible sounds. Our ears cannot hear sounds of frequencies lower than 20 Hz and higher than 20,000 Hz (20 kHz). Sound with frequency smaller than 20 Hz is called infra sound. The sound produced by a pendulum and the sound generated by the vibrations of the earth’s crust just before an earthquake are examples of such sounds. Sound waves with frequency greater than 20 kHz are called ultrasound.

The dog, mouse, bat, dolphin etc have a special ability to hear infra sounds. Thus, they can sense some noise which are inaudible to us. Children under 5 years of age and some creatures and insects can hear waves with frequency up to 25 kHz. Bats, mice, dolphins, etc, can also produce ultrasound.

Reflection of sound

1. Make two sufficiently long, identical tubes out of a cardboard. 2. Arrange them on a table, in front of a wall as shown in figure 12.2. 3. Place a clock near the end of one of the tubes and try to hear the ticking of the clock at the end of the other tube by placing your ear there. 4. Adjust the angles of the two tubes till you can hear the sound very clearly. 5. Measure the angle of incidence q1 and the angle of reflection q2 . Try to see if they are related in any way.

Like light waves, sound waves, too, get reflected from a solid or a liquid surface. These waves also follow the laws of reflection. A smooth or a rough surface is needed for the reflection of sound. The direction of the incident sound wave and reflected sound wave make equal angles with the perpendicular to the surface and all these three lie in the same plane.

Good and bad reflectors of sound

How much of the incident sound gets reflected decides whether a reflector is a good or a bad reflector. A hard and flat surface is a good reflector while clothes, paper, curtains, carpet, furniture, etc. absorb sound instead of reflecting it and, therefore are called bad reflectors.

Echo

You must have visited an echo point at a hill station. If you shout loudly at such a point, you hear the same sound just after a little while. This sound is called an echo.

An echo is the repetition of the original sound because of reflection by some surface. In order to be able to hear the original sound and its reflection distinctly, at 22 0 C, what must the minimum distance be between the source and the reflecting surface?

At 22 0 C, the velocity of sound in air is 344 m/s. Our brain retains a sound for 0.1 s. Thus, for us to be able to hear a distinct echo, the sound should take more than 0.1 s after starting from the source to get reflected and come back to us. Thus we can determine the minimum distance between the source and the reflecting surface as follows:

Distance = speed ´ time = 344 m/s ´ 0.1 s = 34.4 m Thus, to be able to hear a distinct echo, the reflecting surface should be at a minimum distance of half of the above i.e. 17.2 m. As the velocity of sound depends on the temperature of air, this distance depends on the temperature.

Reverberation

Sound waves get reflected from the walls and roof of a room multiple times. This causes a single sound to be heard not once but continously. This is called reverberation. The time between successive reflections of a particular soundwave reaching us becomes smaller and the reflected sounds get mixed up and produce a continuous sound of increased loudness which cannot be deciphered clearly. This is the reason why some auditoriums or some particular seats in an auditorium have inferior sound reception. This is shown in figure 12.3.

SONAR

SONAR is the short form for Sound Navigation and Ranging. It is used to determine the direction, distance and speed of an underwater object with the help of ultrasonic sound waves. SONAR has a transmitter and a receiver, which are fitted on ships or boats.

The transmitter produces and transmits ultrasonic sound waves. These waves travel through water, strike underwater objects and get reflected by them. The reflected waves are received by the receiver on the ship.

The receiver converts the ultrasonic sound into electrical signals and these signals are properly interpreted. The time difference between transmission and reception is noted. This time and the velocity of sound in water give the distance from the ship, of the object which reflects the waves. SONAR is used to determine the depth of the sea. SONAR is also used to search underwater hills, valleys, submarines, icebergs, sunken ships etc.

Sonography

Sonography technology uses ultrasonic sound waves to generate images of internal organs of the human body. This is useful in finding out the cause of swelling, infection, pain, etc. The condition of the heart, the state of the heart after a heart attack as well as the growth of foetus inside the womb of a pregnant woman are studied using this technique.

This technique makes use of a probe and a gel. The gel is used to make proper contact between the skin and the probe so that the full capacity of the ultrasound can be utilized.

The gel is applied to the skin outside the internal organ to be studied. High frequency ultrasound is transmitted inside the body with the help of the probe. The sound reflected from the internal organ is again collected by the probe and fed to a computer which generates the images of the internal organ. As this method is painless, it is increasingly used in medical practice for correct diagnosis.

The ear is an important organ of the human body. We hear sounds because of our ear. When sound waves fall on the eardrum, it vibrates. These vibrations are converted into electrical signals which travel to the brain through nerves. The ear can be divided into three parts: 1. Outer ear 2. Middle ear 3. Inner ear.

Outer ear or Pinna The outer ear collects the sound waves and passes them through a tube to a cavity in the middle ear. Its peculiar funnel like shape helps to collect and pass sounds into the middle ear.

Middle ear There is a thin membrane in the cavity of the middle ear called the eardrum. When a compression in a sound wave reaches the eardrum, the pressure outside it increases and it gets pushed inwards. The opposite happens when a rarefaction reaches there. The pressure outside decreases and the membrane gets pulled outwards. Thus, sound waves cause vibrations of the membrane.

Inner ear The auditory nerve connects the inner ear to the brain. The inner ear has a structure resembling the shell of a snail. It is called the cochlea. The cochlea receives the vibrations coming from the membrane and converts them into electrical signals which are sent to the brain through the nerve. The brain analyses these signals