CBSE Class 9 Science Sound Exam Notes

CBSE Class 9 Science Sound Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

CBSE Class 9 Science Sound exam notes covering periodic and oscillatory motion, wave motion and its types, wave velocity, frequency and wavelength with worked examples, the production and propagation of sound, loudness, pitch and quality, reflection, echo and reverberation, audible, ultrasonic and infrasonic waves, sonar, sonic boom and the structure and working of the human ear.

PERIODIC MOTION

(i) When a body repeats its motion continuously on a definite path in a definite interval of time, then its motion is called periodic motion.
(ii) The constant interval of time after which the motion is repeated is called the time period of motion (T).
Example: Time period of the hour hand of a clock is 12 hours.

OSCILLATORY MOTION

(i) If a body in periodic motion moves along the same path to and fro about a definite point (equilibrium position), then the motion of the body is called vibratory motion or oscillatory motion.
Note: The resultant force acting on the particle is zero in the equilibrium condition.
(ii) Every oscillatory motion is periodic, but every periodic motion is not oscillatory.

INTRODUCTION TO WAVE MOTION

Most of us have seen the formation of ripples when a small stone (pebble) is dropped into a pond. The disturbance created by the stone in the water produces ripples which move outwards towards the shore of the pond.

If you examine the motion of a leaf floating near the disturbance for a short while on a steady day, you would see that the leaf moves up and down about its original position, but does not move away or towards the source of disturbance. This indicates that the disturbance (such as a water wave) moves from one place to another, but the water is not carried with it. The water particles simply move up and down about their mean positions.

The formation of ripples on the surface of water is an example of wave motion.

Wave Motion: The movement of a disturbance produced in one part of a medium to another, involving the transfer of energy but not the transfer of matter, is called wave motion.

Examples:
(i) Formation of ripples on the water surface.
(ii) Propagation of sound waves through air or any other material medium.

Characteristics of Wave Motion

(i) In wave motion, the particles of the medium vibrate about their mean positions. The particles of the medium do not move from one place to another.
(ii) A wave motion travels at the same speed in all directions in the given medium. The speed of a wave depends upon the nature of the medium through which it travels.
(iii) During a wave motion, energy is transferred from one point of the medium to another. There is no transfer of matter through the medium.

CLASSIFICATION OF WAVE MOTION

(A) On the Basis of Necessity of Medium Required

Mechanical Waves: The waves which propagate only in a material medium are called elastic or mechanical waves. Example: Sound waves, water waves (ripples), waves on stretched strings, earthquake waves and the shock waves produced by a supersonic aircraft are mechanical (or elastic) waves.

Electromagnetic Waves: Waves which do not require any material medium for their propagation are called electromagnetic waves. Example: Light waves, radio waves, television waves and X-rays are electromagnetic waves. Thus, light waves, radio and television waves, and X-rays can also travel through vacuum.

Difference between Mechanical Waves and Electromagnetic Waves

(B) On the Basis of Mode of Vibration of the Particle

Transverse Waves
A wave in which the particles of the medium oscillate about their mean position in a direction perpendicular to the direction of propagation of the wave is called a transverse wave. Transverse waves can travel through solids and over the surface of liquids, but not through gases.

Examples of transverse waves:
(i) The water waves (ripples) produced on the surface of water are transverse waves. In water waves, the molecules of water move up and down from their mean positions.
(ii) A pulse on a slinky when it is given a jerk is a transverse wave.
(iii) All electromagnetic waves, e.g., light waves, radio waves, etc., are transverse waves.
(iv) The waves produced in a stretched string when plucked are transverse waves. When the string of a sitar or guitar is plucked, transverse waves are produced in the string.

[Figure: Displacement-distance graph for a transverse wave. The y-axis is labelled Displacement, with + above the mean line and - below it, and the x-axis is labelled Distance. Three crests are labelled Crest along the top and two troughs are labelled Trough along the bottom.]

Crest: The highest point on the hump in a transverse wave is called a crest. Thus, the point of maximum positive displacement on a transverse wave is called a crest.
Trough: The lowest point on the depression in a transverse wave is called a trough. Thus, the point of maximum negative displacement on a transverse wave is called a trough.

Longitudinal Waves
A wave in which the particles of the medium oscillate (vibrate) to and fro about their mean position in the direction of propagation of the wave is called a longitudinal wave. Longitudinal waves can be produced in any medium, viz., in solids, liquids and gases.

Examples of longitudinal waves:
(i) Sound waves are longitudinal waves.
(ii) The waves produced in a spring (slinky) by compressing a small portion of it and releasing it are longitudinal waves.

[Figure: Graphical representation of a longitudinal wave as a row of vertical dotted lines, with the dense regions labelled C (compression) and the sparse regions labelled R (rarefaction), in the order C, R, C, R, C.]

Compression: The part of a longitudinal wave in which the density of the particles of the medium is higher than the normal density is called a compression.
Rarefaction: The part of a longitudinal wave in which the density of the particles of the medium is lower than the normal density is called a rarefaction.

Difference between Transverse and Longitudinal Waves

TERMS USED FOR DEFINING WAVE MOTION

Wavelength: The distance between two nearest points in a wave which are in the same phase of vibration is called the wavelength. In simple words, it is the length of one complete wave. It is denoted by λ (lambda). The SI unit is metre (m).

[Figure: A wave with the distance between two successive crests marked as the wavelength, and the amplitude marked from the mean line to a crest.]

Amplitude: The amplitude of a wave is the magnitude of the maximum displacement of the vibrating particles on either side of their mean position. It is denoted by the letter A and its SI unit is metre (m).

Time Period: The time required to produce one complete wave (or cycle) is called the time period of the wave. It is denoted by T and its SI unit is second (s).

Frequency: The frequency of an oscillating particle is the number of oscillations completed in one second. The unit of frequency is hertz (Hz). The frequency of the wave is ν = 1/T. It is generally represented by ν (nu).

Wave Velocity: The distance travelled by the wave in one second is called the wave velocity. It is represented by v and its unit is m s^-1.

Phase: All the points on a wave which are in the same state of vibration are said to be in the same phase.

[Figure: A wave with points labelled A to K along its length and the direction of propagation marked to the right. Points A, E and I are on the mean line, B, F and J are on the crests, C, G and K are on the mean line, and D and H are on the troughs.]

(a) Points B, F and J are in the same phase, as they all lie on the crests.
(b) Points D and H are in the same phase, as they both lie on the troughs.
(c) Points A, E and I are in the same phase. All these points are just about to start their vibration in the upward direction from their mean positions.
(d) Points C, G and K are in the same phase. All these points are just about to start their vibration in the downward direction from their mean positions.

RELATION BETWEEN WAVE VELOCITY, FREQUENCY AND WAVELENGTH FOR A PERIODIC WAVE

Wave velocity = distance covered / time taken
Wave velocity = wavelength / time taken
So, v = λ / T ...(1)
Since ν = 1/T, equation (1) can also be written as
v = νλ ...(2)
Wave velocity = Frequency × wavelength

Worked Examples

Example 1: If 50 waves are produced in 2 seconds, what is its frequency?
Solution:
1. Frequency, ν = number of waves produced / time taken
2. ν = 50 / 2 = 25 Hz

Example 2: A source produces 50 crests and 50 troughs in 0.5 second. Find the frequency.
Solution:
1. 1 crest and 1 trough = 1 wave
2. So, 50 crests and 50 troughs = 50 waves
3. Frequency, ν = number of waves / time = 50 / 0.5 = 100 Hz

Example 3: Sound waves travel with a speed of 330 m/s. What is the wavelength of sound waves whose frequency is 550 Hz?
Solution:
1. Given velocity, v = 330 m/s and frequency, ν = 550 Hz
2. Wavelength, λ = v / ν = 330 / 550 = 0.6 m

Example 4: The wavelength of sound emitted by a source is 1.7 × 10^-2 m. Calculate the frequency of the sound, if its velocity is 343.4 m s^-1.
Solution:
1. The relation between velocity, frequency and wavelength is v = νλ.
2. Here, v = 343.4 m s^-1, λ = 1.7 × 10^-2 m and ν = ?
3. 343.4 = ν × 1.7 × 10^-2
4. ν = 343.4 / (1.7 × 10^-2) = 2.02 × 10⁴ Hz
5. Thus, the frequency of the sound is 2.02 × 10⁴ hertz.

Example 5: A wave pulse on a string moves a distance of 8 m in 0.05 s. (i) Calculate the velocity of the pulse. (ii) What would be the wavelength of the wave on the same string, if its frequency is 200 Hz?
Solution:
1. (i) Velocity of the wave, v = distance covered / time taken = 8 / 0.05 = 160 m/s
2. (ii) A periodic wave has the same velocity as the wave pulse on the same string, so v = 160 m/s.
3. Wavelength, λ = v / ν = 160 / 200 = 0.8 m
4. Thus, the wavelength of the wave is 0.8 m.

Example 6: A person has a hearing range of 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 340 m/s.
Solution:
1. For ν₁ = 20 Hz, λ₁ = v / ν₁ = 340 / 20 = 17 m
2. For ν₂ = 20 kHz = 20,000 Hz, λ₂ = v / ν₂ = 340 / 20,000 = 1.7 × 10^-2 m = 1.7 cm
3. The typical wavelengths are 17 m and 1.7 cm.

Example 7: A longitudinal wave is produced on a toy string. The wave travels at a speed of 30 cm/s and the frequency of the wave is 20 Hz. What is the minimum separation between the consecutive compressions of the string?
Solution:
1. Given, velocity v = 30 cm/s and frequency ν = 20 Hz
2. The minimum separation between two consecutive compressions is equal to one wavelength.
3. λ = v / ν = 30 / 20 = 1.5 cm

Example 8: A wave of frequency 200 Hz produced in a string is represented in the figure. Find out the following: (i) amplitude (ii) wavelength (iii) wave velocity.

[Figure: Transverse wave on a string with the mean line A-E. Points B (crest), C, D (trough), E, G, and H (trough) are marked. A bracket above the first crest is labelled 40 cm, and a vertical scale is marked 10 cm from the mean line to the crest.]

Solution:
1. (i) Amplitude = maximum displacement = 10 cm
2. (ii) Wavelength λ = distance between two successive crests = 40 cm = 0.4 m
3. (iii) Frequency ν = 200 Hz (as given in the question)
4. Wave velocity, v = νλ = 200 × 0.4 = 80 m/s

Example 9: A stone is dropped into a well 44.1 m deep. The sound of the splash is heard 3.13 seconds after the stone is dropped. Calculate the velocity of sound in air.
Solution:
1. Time taken by the stone to reach the water level, using s = ut + ½gt², where s = 44.1 m, u = 0 and g = 9.8 m/s²:
2. 44.1 = 0 × t + ½ × 9.8 × t²
3. t² = (44.1 × 2) / 9.8 = 9, so t = 3 s
4. Time taken by the sound to reach the top of the well, t₂ = 3.13 - 3 = 0.13 s
5. Speed of sound = distance / time = 44.1 / 0.13 = 339.2 m/s

INTRODUCTION TO SOUND

Sound is a form of energy that produces the sensation of hearing in our ears. The frequency range of audible sound for humans is between 20 Hz and 20 kHz.

Sound Needs a Material Medium to Travel: Vibrations produce sound, and to produce vibrations we need a material body. Therefore, a medium is needed for sound to travel. Sound can travel through air (or gases), liquids and solids, but not through vacuum.

Speed of Sound: The speed of sound is the rate at which sound travels from the sound producing body to our ears. The speed of sound depends on the:
(i) Nature of the material (or medium) through which it travels. The speed of sound in air is 344 m/s.
(ii) Temperature: As the temperature increases, the speed of sound in air increases.
(iii) Humidity of air: Sound travels faster in humid air.

The time gap between 'seeing' and 'hearing' is due to the difference between the time taken by light and sound to travel from the source to the observer. The speed of light is 3 × 10⁸ metres per second (30 crore metres per second), and the speed of sound in air under normal conditions is 344 metres per second. So, light travels almost instantaneously, whereas sound takes some time.

Sound Waves are Longitudinal Waves: Sound travels through air in the form of longitudinal waves.

PRODUCTION AND PROPAGATION OF SOUND

Production of Sound
Sound is produced when an object vibrates (moves back and forth rapidly). In other words, sound is produced by vibrations of objects.

Propagation of Sound
When an object vibrates and makes sound, the air layers around it also start vibrating in exactly the same way and carry the sound waves from the sound producing object to our ears. Transmission of sound requires a material medium, so it cannot travel through vacuum.

In the laboratory, a vibrating tuning fork is used for producing sound. During vibration, the prong of the tuning fork moves from one extreme position to another about its mean position (the position when it is at rest).

[Figure: A vibrating object creating a series of compressions (C) and rarefactions (R) in the medium, shown in the order C, R, C, R.]

Sound as a Longitudinal Wave
When a sound wave travels through air, the molecules in the air oscillate to and fro about their mean positions in the direction of propagation of the sound wave. Therefore, sound waves are called longitudinal waves. Sound waves propagate in any material medium as a series of compressions or rarefactions.

Sound Wave in Terms of Pressure and Density
Sound waves can be studied in terms of pressure and density with distance and time. Pressure and density are high for the particles whose amplitude is less, i.e. at the compression state.

CHARACTERISTICS OF SOUND

(A) Loudness
Loudness of a sound depends on the amplitude of the vibration producing that sound. Greater the amplitude of vibration, louder is the sound produced. The loudness of a sound also depends on the quantity of air that is made to vibrate. Loudness is measured in decibel (dB).

(B) Pitch
The shrillness of a sound is called its pitch. The pitch of a sound depends upon its frequency. Higher the frequency of a sound, higher is its pitch. The voice of a child or a woman has a higher frequency than the voice of a man.

The faster the vibration of the source object, the higher is the frequency and therefore the higher is the pitch. A higher pitch of any sound corresponds to a larger number of compressions and rarefactions passing a point per unit time.

[Figure: Two sine waves drawn on the same axis with the Wave Distance on the horizontal axis. The low pitch wave has longer wavelength and fewer waves, and the high pitch wave has shorter wavelength and more waves.]

The stretched membrane of a tabla or mridangam produces sound of a higher frequency (or of higher pitch).

(C) Quality
Quality of a sound is also called timbre. It is the characteristic which enables us to distinguish between the sounds produced by different sources. A more pleasant sound is said to be of rich quality.

A sound of a single frequency (called a pure sound) is called a tone. A tuning fork produces the sound of a single frequency. A sound which is a mixture of several frequencies is called an impure sound (or note), and it is pleasant to listen to.

Different instruments, depending on their shape and size, produce different numbers of harmonics of different relative loudness. As a result, the sound produced by an instrument can be distinguished from that produced by other instruments.

REFLECTION OF SOUND

Sound waves, like light waves, also get reflected from plane and spherical surfaces. During reflection, sound waves obey the laws of reflection. The reflection of sound from a hard surface can be observed by performing a simple experiment with a hard board, a watch and an ear placed at points A and B.

[Figure: Reflection of sound from a hard surface. A hard board is placed between a watch at point A and an ear at point B, with the sound reflected from the board.]

Sound waves have much longer wavelengths than light waves. Therefore, unlike light waves, sound waves do not need smooth surfaces for reflection. That is why a brick wall, a wooden board, a row of trees and a hill can serve as reflectors of sound waves.

To have an appreciable reflection of sound waves from any surface, the surface should have dimensions equal to or larger than the wavelength of the sound waves falling on it. A smaller object will not reflect sound waves of larger wavelength.

ECHO

The sound returning back towards the source after suffering reflection from a distant obstacle (a wall, a row of buildings, etc.) is called an echo.

When sound is reflected repeatedly from a number of obstacles, more than one echo, called multiple echoes, is heard. Multiple echoes may be heard one after the other when sound gets repeatedly reflected from distant high rise buildings or hills. The rolling of thunder is an example of multiple echo formation.

The two sounds, one direct and the other an echo, can be heard distinctly provided the distance between the observer and the reflecting surface is large enough to allow the reflected sound to reach him without interfering with the direct sound. The sensation of sound persists for 1/10 second after it is produced, so the echo can be heard distinctly only if it reaches at least 1/10 second after the original sound is produced.

Minimum Distance between the Observer and the Obstacle for an Echo to be Heard
Let the distance between the observer and the obstacle be d, the speed of sound in the medium be v, and the time after which the echo is heard be t.
1. The sound travels a distance 2d in time t, so v = 2d / t.
2. Therefore, t = 2d / v, or d = vt / 2.
3. Speed of sound in air at 25 °C = 343 m s^-1.
4. For an echo to be heard distinctly, t ≥ 0.1 s.
5. Then d ≥ (343 × 0.1) / 2, or d ≥ 17.2 m.
6. Thus, the minimum distance (in air at 25 °C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.

The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air for an echo to be heard clearly at temperatures higher than 25 °C is more than 17.2 m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.

REVERBERATION

The repeated reflection that results in the persistence of sound in a large hall is called reverberation.

Excessive reverberation in any auditorium or hall is not desirable, because the sound becomes blurred and distorted. Reverberation can be minimised or reduced by covering the ceiling and walls with sound absorbing materials, such as fibre board, rough plaster, draperies and perforated cardboard sheets.

PRACTICAL APPLICATIONS OF MULTIPLE REFLECTION OF SOUND

Some simple devices based on multiple reflection of sound are:
(i) Stethoscope
(ii) Megaphone, loudhailer and horns
(iii) Trumpet and shehnai
(iv) Curved ceiling of concert hall, conference hall or cinema hall
(v) Sound boards

Stethoscope: A stethoscope is a medical diagnostic instrument based on multiple reflection of sound waves. It is used by doctors for listening to the sounds produced inside the body, particularly in the heart or lungs.

Megaphone: A megaphone is a horn-shaped tube. Megaphones are used for addressing a small group of people. A speaking tube is a hollow tube, with one end at the speaker's end and the other at the listener's end.

In these devices, the sound waves suffer reflection repeatedly and the energy of the waves remains confined to the tube. The sound waves are prevented from spreading out. It is for this very reason that loudspeakers also have horn-shaped openings.

Ear Trumpet (or Hearing Aid): An ear trumpet or hearing aid is used by persons who are hard of hearing. The sound waves received by the wide end of the trumpet are reflected into a much narrower area, leading to the ear. This increases the amplitude of the vibrating air inside the ear and helps in improving hearing.

Sound Boards and Curved Ceiling and Walls in Large Halls: The arched ceiling and walls of large halls or auditoriums often reflect sound waves. These reflected sound waves interfere with the words of the speaker. This problem is solved by hanging curtains, putting up screens or by using sound boards. A sound board is often a concave rigid surface. The speaker is located at the focus of the sound board, placed behind the speaker. The sound board makes the reflected sound waves parallel. This enables the sound to reach large distances. The sound board prevents the spreading out of the sound waves in different directions.

[Figure: Sounding board placed behind a source of sound S, with the reflected sound waves shown travelling parallel to each other.]

Example 10: A girl hears the echo of her own voice from a distant hill after 3 seconds. The speed of sound in air is 340 m/s. What is the distance of the hill from the girl?
Solution:
1. Let d be the distance of the hill from the girl. The total distance travelled by the sound in going and coming back is 2d.
2. Speed v = total distance travelled / time taken, so 340 = 2d / 3
3. 2d = 1020, so d = 510 m

AUDIBLE, ULTRASONIC AND INFRASONIC WAVES

(a) Audible Waves: The human ear is sensitive to sound waves of frequency between 20 Hz and 20 kHz. This range is known as the audible range, and these waves are known as audible waves. Example: Waves produced by a vibrating sitar, guitar, organ pipe, flute, shehnai, etc.

(b) Ultrasonic Waves: A longitudinal wave whose frequency is above the upper limit of the audible range, that is, above 20 kHz, is called an ultrasonic wave. It is generated by very small sources. Example: Quartz crystal.

(c) Infrasonic Waves: A longitudinal elastic wave whose frequency is below the audible range, that is, below 20 Hz, is called an infrasonic wave. It is generally generated by a large source. Example: Earthquake.

ULTRASOUND

Sound waves having frequency higher than 20,000 Hz are called ultrasonic waves or ultrasound. Human beings cannot hear ultrasound. Dogs, bats and dolphins can hear ultrasound. For example, bats and dolphins can hear sound waves having frequencies of about 1,50,000 Hz. Bats and dolphins detect the presence of any obstacle by hearing the echo of the sound produced by them.

Characteristics of Ultrasound
Ultrasound is found very useful for the following reasons:
(i) Ultrasound is a high frequency sound with a short wavelength. These short wavelength waves can be reflected back from smaller objects. Thus, ultrasound can detect smaller objects (less than 1 cm in size). Audible sound waves cannot detect or 'see' objects smaller than a few tens of centimetres to a few metres.
(ii) An ultrasound beam is highly directional and can be aimed towards any target just like a torch. These waves remain undeviated over long distances.

APPLICATIONS OF ULTRASOUND

In ultrasonic spectacles for blind people: Such a spectacle is fitted with a transmitter and a receiver. The receiver produces a high or low sound in the person's ear, depending upon whether the object causing the echo is near or far.

For medical use: Ultrasound is used to detect any infirmity or deformity in an unborn baby (X-rays cannot be used for this purpose because X-rays may harm the unborn baby). In this method, an ultrasonic transmitter and receiver are moved across the mother's stomach. Different tissues (skin, muscles, bones) reflect the sound waves differently to produce many echoes. The machine uses these echoes to construct a picture on the screen. Any deformity or infirmity in the baby can be detected, and proper treatment can be prescribed.

This technology has been misused to find out the sex of the unborn baby. Many people force the would-be mother to have an abortion if the unborn baby is identified as a female. This practice is unethical and a social crime. The Government has banned such practices. All of us should work together to eradicate this social menace.

In echocardiography: In this medical diagnostic technique, ultrasonic waves are used to construct the image of the heart.

For determining the depth of the sea: Ships use ultrasound to determine the depth of the sea by the echo-sounding method. A transmitter on the ship sends ultrasound towards the sea-bed and the receiver receives the echo. From the time gap between the two signals, the depth of the sea can be estimated.

This is illustrated in the sonar figure given below under SONAR.

For cleaning hard to reach places: Ultrasonic waves are used for cleaning hard to reach places, such as spiral tubes, odd shaped machine parts, components and electronic components. The object to be cleaned is kept in a cleaning solution, and the solution is subjected to ultrasonic waves. The high frequency waves stir up the dust and dirt particles. These particles get detached, and the object is thoroughly cleaned.

Echolocation in bats: Bats fly in the darkness of night without colliding with other objects by the method of echolocation. Bats emit high frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from the objects in their path. From the time taken by the echo to be heard, bats can judge the distance of the objects in their path and avoid them by changing direction. Bats also search for their prey at night by echolocation.

[Figure: A bat emitting an ultrasonic squeak towards a flying insect (prey), with an echo returning to the bat.]

SONAR

The word SONAR stands for Sound Navigation and Ranging.

(A) Principle of Sonar: Sonar is an apparatus used to find the depth of the sea or to locate underwater things like shoals of fish and enemy submarines. Sonar works by sending short bursts of ultrasonic sound from a ship down into sea water, and then receiving the echo produced by the reflection of the ultrasonic sound from underwater objects like the bottom of the sea, a shoal of fish or a submarine.

(B) Working of Sonar: A sonar device has (i) a transmitter, for emitting ultrasonic waves, and (ii) a receiver, for detecting ultrasonic waves. The transmitter is made to emit a pulse of ultrasonic sound with a very high frequency of about 50,000 hertz. This pulse travels down in the sea water towards the bottom of the sea. When the pulse strikes the bottom of the sea, it is reflected back to the ship in the form of an echo. This echo produces an electrical signal in the receiver part of the sonar device. The sonar device measures the time taken by the ultrasonic pulse to travel from the ship to the bottom of the sea and back to the ship. Half of this time gives the time taken by the ultrasound to travel from the ship to the bottom of the sea.

[Figure: Sonar set-up on a ship. The transmitter sends an ultrasound pulse down through the water to the sea bottom, and the receiver picks up the reflected ultrasound pulse (echo) returning to the ship.]

Depth of sea d = (v × t) / 2, where d is the depth of the sea, v is the velocity of sound in sea water, and t is the time recorded by the recorder.

Example 11: The ultrasonic waves take 4 seconds to travel from the ship to the bottom of the sea and back to the ship. What is the depth of the sea? (Speed of sound in water = 1500 m/s.)
Solution:
1. Time taken by the ultrasound to travel from the ship to the sea-bed = 4 / 2 = 2 s
2. Speed = distance / time, so 1500 = distance / 2
3. Distance = 1500 × 2 = 3000 m
4. Thus, the depth of the sea is 3000 m.

Example 12: A submarine emits a sonar pulse which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m s^-1, how far away is the cliff?
Solution:
1. Given: v = 1531 m s^-1 and time for the return journey t = 1.02 s
2. Let the distance of the underwater cliff be S. The pulse travels a total distance of 2S in the return journey.
3. 2S = vt, so S = vt / 2
4. S = (1531 × 1.02) / 2 = 780.8 m

Reasons for Using Ultrasonic Waves in Sonar
(i) Ultrasonic waves have a very high frequency, due to which they can penetrate deep into sea water without being absorbed.
(ii) Ultrasonic waves cannot be confused with noise, such as the sound of the engines of a ship. This is because ultrasonic waves are not perceived by the human ear.

SONIC BOOM

When a body moves with a speed greater than the speed of sound in air, it is said to be travelling at supersonic speed. Jet fighters, bullets, etc., often travel at supersonic speed, and when they do so, they produce a sharp, loud sound called a sonic boom.

When the source moves at a speed greater than that of sound, the sound waves travelling at the speed of sound are left behind. The high-pressure layers due to sound waves originating at different points bunch together. These layers lie on the surface of an imaginary cone, of which OA and OB are a part. The total pressure on the surface of this cone is very high. The source is at the apex of this cone. As the source moves ahead, it drags the cone along with it. When the surface of the cone reaches a person, the ears experience a sudden increase in pressure. After the surface crosses him, the pressure is suddenly reduced. This causes the person to hear a sharp, loud sound, the sonic boom.

[Figure: Imaginary cone with its apex at O, the moving source at the apex, and the cone surface edges labelled OA and OB. A man stands on the ground at the cone's path.]

A region consisting of a very high-pressure layer followed by a lower-pressure layer travels through space together with the cone. This is called a shock wave. This shock wave gives rise to the sonic boom when it reaches a person.

The shock waves produced by supersonic aircraft have enough energy to shatter glass and even damage weak structures.

THE HUMAN EAR

(a) Structure of the Human Ear
The ear has three compartments: the outer ear, the middle ear and the inner ear.

Outer ear: The part of the ear which we see outside the head is called the outer ear. It consists of a broad part called the pinna and a passage called the ear canal, about 2 to 3 centimetres long. At the end of the ear canal there is a thin, elastic, circular membrane called the ear-drum (also called the tympanum). The outer ear contains air.

Middle ear: The middle ear contains three small and delicate bones called the hammer, anvil and stirrup. These ear bones are linked to one another. One end of the hammer touches the ear-drum, and its other end is connected to the anvil. The other end of the anvil is connected to the stirrup, and the free end of the stirrup is held against the membrane over the oval window of the inner ear. The middle ear also contains air. The lower part of the middle ear has a narrow tube called the Eustachian tube, which goes to the throat. The Eustachian tube connects the middle ear to the throat and ensures that the air pressure inside the middle ear is the same as that on the outside.

Inner ear: The inner ear has a coiled tube called the cochlea. One side of the cochlea is connected to the middle ear through the elastic membrane over the oval window. The cochlea is filled with a liquid. The liquid contains nerve cells which are sensitive to sound. The other side of the cochlea is connected to the auditory nerve, which goes into the brain.

[Figure: Structure of the human ear. Labelled parts from outside to inside: Pinna, Ear canal, Ear drum, Hammer, Anvil, Stirrup, Oval window, Cochlea, Auditory nerve (goes to brain), and Eustachian tube (goes to throat). Outer ear, middle ear and inner ear are marked as three sections.]

(b) Working of the Human Ear
1. Sound waves from outside are collected by the outer ear (pinna) and reach the ear-drum through the ear canal.
2. When the sound waves strike the ear-drum (tympanic membrane), it starts vibrating.
3. These vibrations are passed on to the oval window by three bones (the hammer, anvil and stirrup), which act as a lever with the pivot at point P. They magnify the force of the vibrations.
4. The oval window has a smaller area than the ear-drum. So, this increases the pressure on the oval window and on the liquid in the cochlea.
5. The vibrations of the liquid in the cochlea affect thousands of auditory nerves, which send messages to the brain.

Care of the Ears
Our ears are very delicate and fragile organs. Proper care must be taken to keep them healthy.
(i) Never insert any pointed object into the ear. It can damage the ear-drum and make a person deaf.
(ii) Never shout loudly into someone's ear.
(iii) Never hit anyone hard on his or her ear.

Ø CHARACTERISTICS OF WAVE MOTION

The main characteristics of wave motion are described below :