Sound Class 8 Physics Selina Concise

Selina Concise ICSE Solutions for Class 8 Physics Chapter 7 Sound

Part A: Objective Type Questions

Part B: Short/Long Answer Type Questions

A. Objective Questions

1. Write true or false for each statement

(a) When sound propagates in air, it does not carry energy with it.

Answer. False.

(b) In a longitudinal wave, compression and rarefaction are formed.

Answer. True.

(c)The distance from one compression to nearest rarefaction is called wavelength.

Answer. False.

(d) The frequency is measured in second.

Answer. False.

(e) The quality of a sound depends on the amplitude of wave.

Answer. False.

(f) The pitch of sound depends on frequency.

Answer. True.

(g) Decibel is the unit of pitch of a sound.

Answer. False.

2. Fill in the blanks(Answers in bold)

(a) The time period of a wave is 2 s. Its frequency is 0.5 /s

(b) The pitch of a stringed instrument is increased by increasing tension in string.

(c) The pitch of a flute is decreased by increasing length of air column.

(d) Smaller the membrane, higher is the pitch.

(e) If a drum is beaten hard, its loudness increases.

(f) A tuning fork produces sound of single frequency.

3. Match the following

4. Select the correct alternative(Answers in bold)

(a) Sound can not travel in

  1. solid

  2. liquid

  3. gas

  4. vacuum

(b) When sound travels in form of a wave

  1. the particles of medium move from the source to the listener

  2. the particles of medium remains stationary

  3. the particles of medium start vibrating up and down

  4. the particles of medium transfer energy without leaving their mean positions.

(c) The safe limit of loudness of audible sound is

  1. 0 to 80 dB

  2. above 80 dB

  3. 120 dB

  4. above 120 dB

(d)The unit of loudness is

  1. cm

  2. second

  3. hertz

  4. decibel

(e) In a piano, pitch is decreased by

  1. using thicker string

  2. increasing tension

  3. reducing length of string

  4. striking it hard 

Part B: Points to remember:-

.    • SOUND: “Is energy which produces in us the sensation of hearing.” It is produced by vibration of body.

    • Sound needs a medium for its propagation. Sound cannot travel in vacuum.

    • Speed of sound is maximum in solids. 5000 ms-1 in steel, in water 1500 ms-1 and in air it is least 330 ms-1 nearly.

    • When a body vibrates, the particles of medium also start vibrating and K.E. of particles changes into potential energy and P.E. into

K.E. This is why sound in energy.

    • Sound travels in a medium in the form of wave.

    • Longitudinal wave : When the particles of medium move in the direction of motion of wave by forming compression and rarefaction.

    • AMPLITUDE : “The maximum displacement of the particle of medium on either side of mean position.”

    • TIME PERIOD: “The time taken by a particle of medium to complete its one vibration” “t”

    • FREQUENCY: “The number of vibrations made by a particle of

the medium in one second. ƒ measured in Hertz (Hz)

    • FREQUENCY ƒ = 1/ t or t = 1 / ƒ

    • WAVE LENGTH: “The distance travelled by the wave in one one time period of vibration of particle of medium.”

Or

“The distance between two consecutive compressions or between two consecutive rarefactions.” It is denoted by ‘ λ ’ and S.I. unit of wave length is metre (m).

    • CHARACTERISTICS OF SOUND :

    (i) Loudness.

    (ii) Pitch (or shrillness).

    (iii) Quality (or timbre or wave form).

LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be distinguished from a faint sound, both having same frequency and same wave form.

    • It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) decibel.

    • PITCH: It depends on number of vibrations per second or frequency : more frequency is high pitch shrilled sound and low frequency is flat sound.

    • QUALITY: is the characteristic which distinguishes two sounds’of the same pitch and same loudness. It depends on wave form.