## Sound

Chapter:- 6

Sound

Production of Sound

Sound is produced by vibrating objects. Vibration means a kind of rapid to and fro motion of an object. The sound of human voice is produced due to vibrations in the vocal cords. We can produce sound by striking the tuning fork, by plucking, scratching, rubbing, blowing or shaking different objects. They all produce sound due to vibrations.

Propagation of Sound

When an object vibrates, it sets the particles of the medium around it in vibration. The particles of the medium do not move forward but the disturbance is carried forward. Sound waves require a medium to travel so, they are called mechanical waves. Compression is the part of a longitudinal wave in which they particles of medium are closer to one another than they normally are and it is the region of high pressure. It is denoted by C. Rarefaction is the part of a longitudinal wave in which the particles of the medium are farther apart than they normally are and it is the region of low pressure. It is denoted by R.  As the object moves back and forth rapidly, a series of compressions and rarefactions is created in air. These make the sound wave that propagates through the medium. Propagation of sound can be visualized as propagation of density variations or pressure variations in the medium in a given  volume. More density of the particles in the medium gives more pressure and vice-versa.

Sound Needs a Medium to Travel

The substance through which sound travels is called a medium. It can be solid, liquid or a gas. Sound wave is a mechanical wave and requires a material medium like air, water, steel, etc., for its propagation. Sound wave cannot travel in vacuum.

Types of waves

Mainly there are two types of waves

(i) Longitudinal waves

(ii) Transverse waves

Logitudinal Waves

In longitudinal 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 positions of rest. This is exactly how a sound wave propagates, hence sound waves are longitudinal waves. Longitudinal waves can be produced in al the three media such as solid, liquids and gases.

The waves which travel along a spring when it is pushed and pulled at one end, are the longitudinal waves.

When coils are closer together than normal, compressions (C) and observed in spring. When coils are farther apart than normal, rarefactions (R) are observed. A long flexible spring which can be compressed or extended easily is called slinky.

Transverse Waves

In transverse waves, the individual particles of the medium move about its mean position in a direction perpendicular to the direction of wave propagation.

For Example:- The water waves (or ripples) formed on the surface of water in a pond (when a stone is dropped in the pond of water) are transverse waves.

Terms to Describe Sound Waves

Sound waves can be described by its

(i):- Wavelength              (ii):- Frequency                 (iii):- Time period

(iv):- Amplitude               (v):- Speed

## Wavelength

Wavelength

The distance between the two consecutive compressions (C) or two consecutive rarefactions (R), is called the wavelength. Wavelength is the minimum distance in which a sound wave repeats it self.

In other words, it is the combined length of a compression and an adjacent rarefaction. It is represented by a lambda λ. Its SI unit is metre (m).

Frequency

The number of complete waves produced in one second is called frequency of the wave. It is the number of vibrations that occur per second. If we can count the number of the compressions or rarefactions that cross us per unit time, we will get the frequency or sound wave. The frequency of a wave is fixed and does not change even when it passes through different substances. It is denoted by v (Greek letter, nu).

• Its SI unit is hertz (symbol, Hz)
• 1 hertz is equal to 1 vibration per second
• 1 kHz = 1000 Hz

Time Period

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, the time required to produce one complete wave (or oscillations) is called time period of the wave. It is denoted by symbol T. Its SI unit is second (s). The time period of a wave is the reciprocal of its frequency, i.e.,

T = 1/v

or Time period = 1/Frequency or Frequency = 1/ Time period

Amplitude

The maximum displacement of the particles of the medium from their original mean positions on passing a wave through the medium, is called amplitude of the wave. It is usually denoted by the letter A. Its SI unit is metre (m).

The amplitude of wave is the same as the amplitude of the vibrating body producing the wave.

Speed

The distance travelled by a wave in one second is called speed of the wave or velocity of the wave. Under the same physical conditions, the speed of sound remains same for all frequencies. It is represented by letter v. Its SI unit is metre per second (m/s or ms–1). Relationship between speed, frequency and wavelength of a wave:

Suppose distance travelled by a wave is λ (wavelength), in time T, then the speed is given by v = λ/ T

We know that frequency, v = 1/T

Therefore,   v = λ × v    or     v =

or    Speed (velocity) = Frequency × Wavelength

Loudness

It is the measure of the sound energy reaching the ear per second. Greater the sound energy reaching our ear per second, louder the sound will appear to be. If the sound waves have a small amplitude, then sound will be faint or soft but if waves have a large amplitude, then the sound will be loud.

Since the amplitude of a sound wave is equal to the amplitude of vibrations of the source producing the sound waves. 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. The loudness of sound is measured in decibel (dB).

Intensity

The amount of sound energy passing each second through unit area is known as the intensity of sound. Loudness and intensity are not same terms. 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 becomes our ear detects it in better way.

• The SI unit of intensity is watt per square metre (W/m2) .

Pitch or Shrillness

It is that characteristic of sound by which we can distinguish between different sounds of the same loudness.

• Pitch of a sound depends on the frequency of vibration.

The faster the vibration of the source, the higher is the frequency and hence, higher is the pitch, as shown in figure.

Quality or Timbre

The quality or timbre of sound is that characteristic of sound which enables us to distinguish one sound from another having the same pitch and loudness. The pleasant sound is said to be of a rich quality. Noise is unpleasant to ear, music is pleasant to ear and is of rich quality. The sound produced by different musical instruments like flute, violin, sitar, tanpura.

## Speed of Sound in Different Media

Sonic Boom

When the speed of any object exceeds the speed of sound, it is said to have supersonic speed.  Many objects such as some aircrafts, bullets and rockets, etc travel at supersonic speeds. When a sound producing source moves with a speed higher than that of sound, it produces shock waves in air, which carry a large amount of energy. The tremendous air pressure variations caused by the hock waves produce a loud burst of sound, known as sonic Boom.

Echo

When a person shouts in a big empty hall, we first hear his original sound, after that we hear the reflected sound of that shout. This reflected sound is echo. the repetition of sound caused by reflection of sound waves is called an echo.

The sensation of sound persists in our brain for about 0.1 s. To hear a distinct echo, the time interval between the original sound and the reflected one must be atleast 0.1 s.

The speed of sound in air is 344 m/s. The distance travelled by the sound in

0.1 s = speed × time = 344 × 0.1= 34.4 m

So, echo will be heard if the minimum distance between the source of sound and the obstacle is

To hear an echo, our distance from the reflecting surface should be atleast 17.2 m.

Reverberation

The persistence of a sound in a big hall due to repeated reflections from the walls, ceiling and floor of the wall is known as reverberation.

A short reverberation is desirable in a concert hall, where music is being played, as it boots the sound level. Excessive reverberation is highly undesirable, because sound becomes blurred, distorted and confusing due to overlapping of different sounds.

Uses of Multiple Reflection of Sound

The reflection of sound is used in the working of devices such as megaphone, horns, stethoscope and sound board.

Range of Hearing

The average frequency range over which the human ear is sensitive is called audible range.

The audible range of sound for human beings is from 20 Hz to 20000 Hz. Children under the age of 5 and some animals such as dogs can hear upto 25000Hz. As people grow older, their ears becomes less sensitive to higher frequencies.

Infrasonic Sound

The sound of frequencies lower than 20 Hz are known as infrasonic sounds or infrasound, whales, elephants and rhinoceroses produce infrasonic sound of frequency 5 Hz.

Ultrasonic Sound

The sounds of frequencies higher than 20000 Hz are called ultrasonic sounds Dogs can hear ultrasonic sounds of frequency upto 50000 Hz. Monkeys, bats, cats, dolphins, leopard and porpoises can also hear ultrasonic sounds.

Hearing Aid

This is a device used by people who are hard of hearing. It receives sound through a microphone which converts the sound waves to electrical signals.

These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. Which converts the amplified electrical signals to sound.

Ultrasound and its Applications

Ultrasounds are high frequency waves.  They travel in straight line without bending around the corners. They can penetrate into matter to a large extent. Due to these properties, ultrasound is used in industry and in hospitals for medical purposes.

Some of the important applications of ultrasound are given below

(i):-Ultrasound is used to clean parts located in hard-to-reach-places, such as spiral tubes, odd-shaped machines and electronic components, etc.

(ii):-Ultrasound is used in industry f

(iii):- Ultrasound is used to investigate the internal organs of human body such as liver, gall bladder, pancreas, kidneys, uterus and heart, etc.

Ultrasound helps us to see inside the human body and to give pictures of the inner organs by converting into electrical signals. This technique is called ultrasonography.

Ultrasound is also used for diagnosing heart diseases by scanning the heart from inside. This technique is echocardiography.

Ultrasound may be employed to break small stones formed in the kidneys into fine grains which later get flushed out with urine) This way, the patient gets relief from pain.

## Sonar and Human Ear

Sonar

The word SONAR stands for Sound Navigation and Ranging. Sonar is an apparatus used to find the depth of a sea or to locate the underwater things like shoals of fish, shipwrecks and enemy submarines. It use ultrasonic waves to measure the distance and speed of underwater objects.

SONAR consists of two parts:-

(i):- A transmitter (for emitting ultrasonic waves)

(ii):- A receiver (for detecting ultrasonic waves),

The transmitter produces and transmits ultrasonic waves.

These waves travel down the sea-water towards the bottom of the sea. When the ultrasonic sound pulse strikes the bottom of the sea, it is reflected hack in the form of echo and are sensed by the detector.

This will give us the depth of the sea. Let the time interval between transmission and reception of ultrasound signal be t and the speed of sound through sea-water be v. The total distance, 2d travelled by the ultrasound is, then 2d = v × t. This method is called echo-ranging.

Ultrasonic sound waves are used in SONAR, because

• These waves have a very high frequency and very short wavelength, due to which they can penetrate into sea-water to a large extent to locate the underwater objects or to determine the depth of the sea.
• These waves cannot be confused with engine noises or other sounds made by the ship as they cannot be heard by human beings.

Use of Ultrasonic Waves by Bats

Bats search out prey and fly in dark night by emitting and detecting reflections of ultrasonic waves. The method used by some animals like bats, tortoises and dolphins to locate the objects by hearing the echoes of their ultrasonic squeaks is known as echolocation.

Bats emit high frequency or high pitched ultrasonic squeaks while flying and listen to the echoes produced by reflection of their squeaks from the obstacles or prey in their path. From the time taken by the echo to be heard, bats can determine the distance of the obstacle or prey and can avoid the obstacle by changing the direction or catch the prey.

Human Ear

The ears are the sense organs which helps us in hearing sound. It allows us to convert pressure variations in air with audible frequencies into electric signals which travel to the brain via auditory nerve.

(i):- Outer Ear It consists of a broad part pinna and about 3 cm long passage ear canal. At the end of ear canal, a thin, elastic and circular membrane, eardrum is present, which is also called tympanum or tympanic membrane.

(ii):- Middle Ear contains three small bones – hammer, anvil and stirr up, which are connected with each other. One end of hammer is touching the eardrum and the free end of stir up is touched to oval-window of inner ear. The lower part of middle ear has a narrow tube, Eustachian tube going to the throat. It ensures that the air pressure inside the middle ear is the same as that on the outside.

(iii):- Inner Ear has a coiled tube, cochlea.  One side of cochlea is connected to middle ear through elastic membrane over the oval window. A liquid is filled in cochlea, which contains nerve cells that are sensitive to sound. The other side of cohlea is connecred to auditory nerve going into the brain.

Hearing Aid

This is a device used by people who are hard of hearing. It receives sound through a microphone which converts the sound waves to electrical signals.

These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. Which converts the amplified electrical signals to sound.

Working of Human Ear

Pinna collects the sound waves from the surroundings. These collected sound waves pass through the car canal (auditory canal) and fall on the eardrum (tympanum) or tympanic membrane. Since, sound waves are longitudinal waves, these waves consists of compressions (high pressure regions) and rarefactions (low pressure regions). When a compression of the medium reaches the eardrum the pressure on the outside of the membrane (eardrum) increases and forces increases and forces the eardrum inward. Similarly, when the rarefaction of sound wave falls on the eardrum, the pressure on the outside of the membrane (eardrum) decreases and it moves outward. In this way, when sound waves fall on the eardrum, it starts vibrating back and forth rapidly.

These vibrations are amplifies several times by the three bones (hammer, anvil and strip) in the middle ear and then passes to the liquid in the cochlea. Due to this, the liquid in the cochlea begins to vibrate and the cochlea. These electrical signals are carried by auditory nerve to the brain. The brain interprets them as sound and we get the sensation of hearing.