Sound

ICSE Class 10 · Physics · Chapter 7

What this chapter covers

Reflection of sound waves
Echoes and the conditions for hearing them
Uses of echoes
Natural vibrations
Damped vibrations
Forced vibrations
Resonance
Characteristics of sound
Loudness and intensity
Sound level in decibels and noise pollution
Pitch and frequency
Quality and waveforms

Key definitions

Reflection of sound is the bouncing back of sound waves into the same medium when they strike the surface of an obstacle such as a wall, a hill or a cliff.

An echo is the sound heard after reflection from a distant obstacle, arriving after the original sound has ended so that the two are heard separately.

Persistence of hearing is the property of the human ear by which the sensation of a sound lasts for about 0.1 s after the sound has stopped.

Echolocation is the method of finding the position of an object by sending out sound and timing the echo that returns from it.

SONAR is a system that uses ultrasonic waves and their echoes to detect and locate objects under water and to measure the depth of the sea; the name stands for SOund Navigation And Ranging.

Natural vibrations are the vibrations of a body when it is displaced from its position of rest and then left free to vibrate on its own, with no outside force acting on it except the one that restores it.

Natural frequency is the frequency with which a body vibrates when it is set into natural vibrations, and it depends only on the body itself.

Damped vibrations are the vibrations of a body whose amplitude decreases steadily with time because of energy lost to friction or to the resistance of the surrounding medium.

Forced vibrations are the vibrations of a body under the action of an external periodic force, in which the body vibrates with the frequency of the applied force and not with its own natural frequency.

Resonance is a special case of forced vibrations in which the frequency of the applied periodic force is equal to the natural frequency of the driven body, so that the body vibrates with a very large amplitude.

Loudness is the characteristic of sound by which a loud sound can be distinguished from a faint one of the same frequency; it is a measure of the response of the ear to the sound.

Sound level is a measure of the loudness of a sound compared with the faintest sound a normal human ear can hear, and it is expressed in decibels.

Key points

Sound is reflected by large surfaces and obeys the same laws of reflection as light.
An echo is heard only if the reflected sound arrives at least 0.1 s after the original, because of persistence of hearing.
In air at 340 m s^-1, the reflector must be at least 17 m away for a distinct echo.
The distance of a reflector is found from d = Vt/2, where t is the time for the echo to return.
Bats, dolphins, fishermen's echo sounders, medical ultrasonography and SONAR all use echoes of ultrasound.
Natural vibrations occur at the body's natural frequency, which depends only on the body.
Damped vibrations lose amplitude with time; their frequency stays practically the same.
Forced vibrations occur at the frequency of the driver and usually have a small amplitude.
Resonance is the special case of forced vibrations in which the driver's frequency equals the natural frequency, giving a large amplitude.
Applications of resonance include tuning a radio, musical instruments and sympathetic strings; soldiers break step on bridges to avoid it.
Loudness depends on amplitude (loudness is proportional to amplitude squared), area of the vibrating body, distance, density of the medium and resonant bodies.
Loudness, pitch and quality are subjective; intensity, frequency and waveform are objective.
Sound level is expressed in decibels (dB); prolonged exposure above about 85 dB damages hearing.
Pitch depends on frequency: the higher the frequency, the shriller the sound.
Quality depends on the waveform, which is set by the overtones accompanying the fundamental note.

Questions answered

Define an echo. Why can you not hear an echo in a small room?

An echo is the sound heard after reflection from a distant obstacle, arriving after the original sound has ended so that the two are heard separately. In a small room the walls are much closer than 17 m, so the reflected sound returns within 0.1 s and merges with the original sound because of persistence of hearing.

State the conditions necessary for an echo to be heard.

The reflected sound must reach the listener at least 0.1 s after the original, so the reflector must be at least about 17 m away in air; the reflecting surface must be large compared with the wavelength; the sound must be loud enough; and it should be short and sharp.

Why do bats use ultrasound and not audible sound to locate insects?

Ultrasound has a very short wavelength, a few millimetres, so it is reflected even by a small insect. Audible sound has a much longer wavelength and would bend around the insect without giving an echo.

A person hears the echo of a shout from a building 0.8 s after shouting. If sound travels at 340 m s^-1, how far away is the building?

d = Vt/2 = (340 × 0.8)/2 = 136 m.

A SONAR receives an echo from a submarine 1.2 s after sending a pulse. If the speed of sound in sea water is 1500 m s^-1, how far away is the submarine?

d = Vt/2 = (1500 × 1.2)/2 = 900 m.

What are damped vibrations? Why does a pendulum swinging in water stop sooner than one swinging in air?

Damped vibrations are vibrations whose amplitude decreases steadily with time because energy is lost to friction or to the resistance of the medium. Water resists the motion far more than air, so the pendulum loses its energy faster.

Distinguish between forced vibrations and resonance.

In forced vibrations the driver's frequency differs from the natural frequency of the driven body and the amplitude is small. In resonance the two frequencies are equal and the amplitude is very large.

Why are soldiers asked to break step while crossing a suspension bridge?

If the frequency of their marching steps equalled a natural frequency of the bridge, resonance would make the bridge vibrate with a very large amplitude, which could damage it.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Two sound waves, X and Y have the same amplitude and the same wave pattern, but their frequencies are 60 Hz and 120 Hz respectively, then:

X will be shriller, and Y will be grave.
X will be grave, and Y will be shriller.
X will differ in quality than Y.
X will be louder than Y.

Show answer

X will be grave, and Y will be shriller.

ICSE 2027 sample paper · 3 marks

A boy is located asymmetrically between two parallel walls, closer to the left wall than the right. He is X m from the left wall and 68 m from the right wall. If he claps, the time interval between successive echoes is 0.25 s, then find the value of X in metres. [speed of sound in air = 340 m/s.]

Show answer

Echo from the right wall: t = 2d/v = (2 × 68)/340 = 0.4 s The echo from the nearer (left) wall arrives first, 0.25 s earlier: t = 0.4 − 0.25 = 0.15 s 0.15 = 2X/340 → 2X = 51 X = 25.5 m

ICSE 2027 sample paper · 4 marks

Two wires A and B of the same material and length are fixed to a wooden board and stretched under the same tension. The cross-sectional area of wire A is greater than that of wire B. When set into vibration using identical tuning forks of frequency 300 Hz, only wire B produces a loud sound. (a) Name and explain the phenomenon responsible for the loud sound produced by wire B. (b) A loud sound is heard from wire A when the experiment is repeated using a tuning fork of frequency f. Is 'f' greater than, less than or equal to 300 Hz? Justify your answer.

Show answer

(a) Resonance. The frequency of the tuning fork (300 Hz) equals the natural frequency of wire B, so B vibrates with a large amplitude and produces a loud sound. (b) f < 300 Hz. Wire A is thicker (greater cross-sectional area, hence greater mass per unit length) than wire B under the same tension and length, so its natural frequency is lower than that of B; resonance needs a fork of lower frequency.

ICSE 2026 sample paper · 1 mark

Two sound waves X and Y have the same amplitude and the same wave pattern but their frequencies are 60 Hz and 120 Hz respectively, then:

X will be shriller and Y will be grave.
X will be grave and Y will be shriller.
X will differ in quality than Y.
X is louder than Y.

Show answer

X will be grave and Y will be shriller.

ICSE 2026 board paper · 1 mark

Equal volumes of water are added to three cylindrical jars A, B and C of same height and radii rA, rB and rC respectively with rB < rA < rC. If you blow air into the mouth of these jars, which tube will produce the shrillest note?

A
B
C
All will produce the notes of same shrillness

Show answer

ICSE 2026 sample paper · 2 marks

(a) Name the wave used for echo depth sounding. (b) Give one reason why the waves mentioned in (a) is used for the above purpose.

Show answer

(a) Ultrasonic waves (ultrasound). (b) Ultrasonic waves have very high frequency (short wavelength), so they can be sent as a narrow beam in one direction and travel long distances through water without spreading much or being absorbed easily.

ICSE 2026 board paper · 2 marks

(a) Define natural vibrations. (b) How is this vibration different from damped vibrations in terms of their amplitudes?

Show answer

(a) Natural (free) vibrations are the periodic vibrations of a body, once set vibrating, at its own natural frequency in the absence of any external force (or resistance). (b) In natural vibrations the amplitude remains constant with time, whereas in damped vibrations the amplitude decreases with time (and finally the vibrations stop).

ICSE 2026 sample paper · 3 marks

A person standing in front of a cliff fires a gun and hears its echo after 3s. Speed of sound in air is 336 ms⁻¹: (a) Calculate the distance of the person from the cliff. (b) After moving a certain distance from the cliff, he fires the gun again and this time the echo is heard 1.5 s later than the first. Calculate the distance that the person has moved.

Show answer

(a) 2d = v × t, so d = (336 × 3)/2 = 504 m (b) Taking the second echo to be heard 1.5 s after firing (as in the official scheme): d′ = (336 × 1.5)/2 = 252 m, so the person has moved 504 − 252 = 252 m (towards the cliff). (If the second echo takes 1.5 s longer, i.e. 4.5 s: d′ = 336 × 4.5/2 = 756 m, and the person has moved 756 − 504 = 252 m away from the cliff — the same distance, 252 m.)