The System of Sound Production in Man:

• In human being, the sound is produced in the voice box, called the larynx, which is present in the throat.
• It is located at the upper end of the windpipe.
• The larynx has two ligaments called ‘vocal cords’, stretched across it.
• The vocal cords have a narrow slit through which air is blown in and out.
• When a person speaks, the air from the lungs is pushed up through the trachea to the larynx.
• When this air passes through the slit, the vocal cords begin to vibrate and produce a sound.
• By varying the thickness of the vocal cords, the length of the air column in the slit can be changed.
• This produces sounds of different pitches.
• Males generally have thicker and longer vocal cords that produce a deeper, low pitch sound in comparison with females.

Human Ears Work:

• Ear is the important organ for all animals to hear a sound.
• We are able to hear sound through our ears.
• Human ear picks up and interprets high frequency vibrations of air.
• Ears of aquatic animals are designed to pick up high frequency vibrations in water.
• The outer and visible part of the human ear is called pinna (curved in shape).
• It is specially designed to gather sound from the environment, which then reaches the ear drum (tympanic membrane) through the ear canal.
• When the sound wave strikes the drum, the ossicles move inward and outward to create the vibrations.
• These vibrations are then picked up by special types of cells in the inner ear.
• From the inner ear the vibrations are sent to the brain in the form of signals.
• The brain perceives these signals as sounds.

Sound Waves:

• Sound needs a material medium like air, water, steel etc., for its propagation.
• It cannot travel through vacuum.
• This can be demonstrated by the Bell – Jar experiment.
• An electric bell and an airtight glass jar are taken. The electric bell is suspended inside the airtight jar.
• The jar is connected to a vacuum pump.
• If the bell is made to ring, we will be able to hear the sound of the bell.
• Now, when the jar is evacuated with the vacuum pump, the air in the jar is pumped out gradually and the sound becomes feebler and feebler.
• We will not hear any sound, if the air is fully removed (if the jar has vacuum).

Resonances:

• An echo is the sound reproduced due to the reflection of the original sound from various rigid surfaces such as walls, ceilings, surfaces of mountains, etc.
• If you shout or clap near a mountain or near a reflecting surface, like a building you can hear the same sound again.
• The sound, which you hear is called an echo.
• It is due to the reflection of sound.
• One does not experience any echo sound in a small room.
• This does not mean that sound is not reflected in a small room.
• This is because smaller rooms do not satisfy the basic conditions for hearing an echo.

Conditions necessary for hearing echo:

• The persistence of hearing for human ears is 0.1 second.
• This means that you can hear two sound waves clearly, if the time interval between the two sounds is at least 0.1s.
• Thus, the minimum time gap between the original sound and an echo must be 0.1s.
• The above criterion can be satisfied only when the distance between the source of sound and the reflecting surface would satisfy the following equation:

Velocity = distance travelled by sound / time taken

v = 2d / t

d = vt / 2

Since, t = 0.1 second, then d = v x 0.1 / 2 = v / 20

• Thus the minimum distance required to hear an echo is 1/20^th part of the magnitude of the velocity of sound in air.
• If you consider the velocity of sound as 344 ms^–1, the minimum distance required to hear an echo is 17.2 m.

Applications of echo:

• Some animals communicate with each other over long distances and also locate objects by sending the sound signals and receiving the echo as reflected from the targets.
• The principle of echo is used in obstetric ultrasonography, which is used to create real-time visual images of the developing embryo or fetus in the mother’s uterus.
• This is a safe testing tool, as it does not use any harmful radiations.
• Echo is used to determine the velocity of sound waves in any medium.

Applications Reflection of Sound:

Sound board:

• These are basically curved surfaces (concave), which are used in auditoria and halls to improve the quality of sound.
• This board is placed such that the speaker is at the focus of the concave surface.
• The sound of the speaker is reflected towards the audience thus improving the quality of sound heard by the audience.

Ear trumpet:

• Ear trumpet is a hearing aid, which is useful by people who have difficulty in hearing.
• In this device, one end is wide and the other end is narrow.
• The sound from the sources fall into the wide end and are reflected by its walls into the narrow part of the device.
• This helps in concentrating the sound and the sound enters the ear drum with more intensity.
• This enables a person to hear the sound better.

Mega phone:

• A megaphone is a horn-shaped device used to address a small gathering of people. Its one end is wide and the other end is narrow.
• When a person speaks at the narrow end, the sound of his speech is concentrated by the multiple reflections from the walls of the tube.
• Thus, his voice can be heard loudly over a long distance.

Reflection of Sound:

• Sound bounces off a surface of solid or a liquid medium like a rubber ball that bounces off from a wall.
• An obstacle of large size which may be polished or rough is needed for the reflection of sound waves.
• The laws of reflection are:
• The angle in which the sound is incident is equal to the angle in which it is reflected.
• Direction of incident sound, the reflected sound and the normal are in the same plane.

Uses of multiple reflections of sound:

Musical instruments:

• Megaphones, loud speakers, horns, musical instruments such as nathaswaram, shehnai and trumpets are all designed to send sound in a particular direction without spreading it in all directions.
• In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward direction towards the audience.

Stethoscope:

• Stethoscope is a medical instrument used for listening to sounds produced in the body.
• In stethoscopes, these sounds reach doctor’s ears by multiple reflections that happen in the connecting tube.

Applications of reflection of sound waves:

• Stethoscope:
• It works on the principle of multiple reflections.
• It consists of three main parts:
• Chest piece: It consists of a small disc-shaped resonator (diaphragm) which is very sensitive to sound and amplifies the sound it detects.
• Ear piece: It is made up of metal tubes which are used to hear sounds detected by the chest piece.
• Rubber tube:
• This tube connects both chest piece and ear piece.
• It is used to transmit the sound signal detected by the diaphragm, to the ear piece.
• The sound of heart beats (or lungs) or any sound produced by internal organs can be detected, and it reaches the ear piece through this tube by multiple reflections.

• Echo:
• An echo is a repetition of sound produced by the reflection of sound waves from a wall, mountain or other obstructing surfaces.
• The speed of sound in air at 20°C is 344 ms^–1.
• If we shout at a wall which is at 344 m away, then the sound will take 1 second to reach the wall.
• After reflection, the sound will take one more second to reach us.
• Therefore, we hear the echo aft er two seconds.
• SONAR:
• Sound Navigation and Ranging.
• Sonar systems make use of reflections of sound waves in water to locate the position or motion of an object.
• Similarly, dolphins and bats use the sonar principle to find their way in the darkness.
• Reverberation:
• In a closed room the sound is repeatedly reflected from the walls and it is even heard long after the sound source ceases to function.
• The residual sound remaining in an enclosure and the phenomenon of multiple reflections of sound is called reverberation.
• The duration for which the sound persists is called reverberation time.
• It should be noted that the reverberation time greatly affects the quality of sound heard in a hall.
• Therefore, halls are constructed with some optimum reverberation time.

Sonic Boom:

• When the speed of any object exceeds the speed of sound in air (330 ms–1) it is said to be travelling at supersonic speed.
• Bullets, jet, aircrafts etc., can travel at supersonic speeds.
• When an object travels at a speed higher than that of sound in air, it produces shock waves.
• These shock waves carry a large amount of energy.
• The air pressure variations associated with this type of shock waves produce a very sharp and loud sound called the ‘sonic boom’.
• The shock waves produced by an aircraft have energy to shatter glass and even damage buildings.

Doppler Effect:

• The whistle of a fast moving train appears to increase in pitch as it approaches a stationary listener and it appears to decrease as the train moves away from the listener.
• This apparent change in frequency was first observed and explained by Christian Doppler (1803-1853), an Austrian Mathematician and Physicist.
• He observed that the frequency of the sound as received by a listener is different from the original frequency produced by the source whenever there is a relative motion between the source and the listener.
• This is known as Doppler Effect this relative motion could be due to various possibilities as follows:

• The listener moves towards or away from a stationary source
• The source moves towards or away from a stationary listener
• Both source and listener move towards or away from one other
• The medium moves when both source and listener are at rest
• For simplicity of calculation, it is assumed that the medium is at rest. That is the velocity of the medium is zero.
• Let S and L be the source and the listener moving with velocities v_S and v_L
• Consider the case of source and listener moving towards each other.
• As the distance between them decreases, the apparent frequency will be more than the actual source frequency.
• Let n and n’ be the frequency of the sound produced by the source and the sound observed by the listener respectively.
• Then, the expression for the apparent frequency n’ is
• Here, v is the velocity of sound waves in the given medium.

• Let us consider different possibilities of motions of the source and the listener.
• In all such cases, the expression for the apparent frequency.

Case No.	Position of source and listener	Note	Expression for apparent frequency
1.	·        Both source and listener move ·        They move towards each other	·        Distance between source and listener decreases. ·        Apparent frequency is more than actual frequency.
2.	·        Both source and listener move ·        They move away from each other	·        Distance between source and listener increases. ·        Apparent frequency is less than actual frequency. ·        vS and vL become opposite to that in case-1.
3.	·        Both source and listener move ·        They move one behind the other ·        Source follows the listener	·        Apparent frequency depends on the velocities of the source and the listener. ·        vS becomes opposite to that in case-2.
4.	·        Both source and listener move ·        They move one behind the other ·        Listener follows the source	·        Apparent frequency depends on the velocities of the source and the listener. ·        vS and vL become opposite to that in case-3.
5.	·        Source at rest ·        Listener moves towards the source	·        Distance between source and listener decreases. ·        Apparent frequency is more than actual frequency. ·        vS = 0 in case-1.
6.	·        Source at rest ·        Listener moves away from the source	·        Distance between source and listener increases. ·        Apparent frequency is less than actual frequency. ·        vS = 0 in case-2.
7.	·        Listener at rest ·        Source moves towards the listener	·        Distance between source and listener decreases. ·        Apparent frequency is more than actual frequency. ·        vL = 0 in case-1.
8.	·        Listener at rest ·        Source moves away from the listener	·        Distance between source and listener increases. ·        Apparent frequency is less than actual frequency. ·        vL = 0 in case-2.

• Suppose the medium (say wind) is moving with a velocity W in the direction of the propagation of sound.
• For this case, the velocity of sound, ‘v’ should be replaced with (v + W).
• If the medium moves in a direction opposite to the propagation of sound, then ‘v’ should be replaced with (v – W).

Conditions for no Doppler effect:

• Under the following circumstances, there will be no Doppler effect and the apparent frequency as heard by the listener will be the same as the source frequency.
• When source (S) and listener (L) both are at rest.
• When S and L move in such a way that distance between them remains constant.
• When source S and L are moving in mutually perpendicular directions.
• If the source is situated at the center of the circle along which the listener is moving.

Applications of Doppler Effect:

To measure the speed of an automobile:

• An electromagnetic wave is emitted by a source attached to a police car.
• The wave is reflected by a moving vehicle, which acts as a moving source.
• There is a shift in the frequency of the reflected wave.
• From the frequency shift, the speed of the car can be determined.
• This helps to track the over speeding vehicles.

Tracking a satellite:

• The frequency of radio waves emitted by a satellite decreases as the satellite passes away from the Earth.
• By measuring the change in the frequency of the radio waves, the location of the satellites is studied.

RADAR (RAdio Detection And Ranging):

• In RADAR, radio waves are sent, and the reflected waves are detected by the receiver of the RADAR station.
• From the frequency change, the speed and location of the aero planes and aircrafts are tracked.

SONAR:

• In SONAR, by measuring the change in the frequency between the sent signal and received signal, the speed of marine animals and submarines can be determined.

Resonance:

• It is a special case of forced vibrations where the frequency of external periodic force (or driving force) matches with the natural frequency of the vibrating body (driven).
• As a result the oscillating body begins to vibrate such that its amplitude increases at each step and ultimately it has a large amplitude.
• Such a phenomenon is known as resonance and the corresponding vibrations are known as resonance vibrations.
• Example: The breaking of glass due to sound.
• Soldiers are not allowed to march on a bridge.
• This is to avoid resonant vibration of the bridge.
• While crossing a bridge, if the period of stepping on the ground by marching soldiers equals the natural frequency of the bridge, it may result in resonance vibrations.
• This may be so large that the bridge may collapse..