LIWA

Sound

ICSE Class 8 · Physics · Chapter 7

What this chapter covers

Key definitions

Pitch is the property of a sound that lets the ear tell a high, shrill sound from a low, deep one.

Frequency is the number of complete vibrations that a vibrating object makes in one second.

Hertz is the SI unit of frequency, and one hertz means one complete vibration every second.

A musical instrument is a device that makes pleasant sounds of chosen pitches by setting some part of it into vibration.

Wind instruments are musical instruments in which a column of air inside a tube is made to vibrate to produce sound.

Membrane instruments are musical instruments in which a stretched skin or sheet, called a membrane, vibrates when it is struck to produce sound.

String instruments are musical instruments in which stretched strings vibrate to produce sound.

A monotone is a sound of one single frequency, with no other frequencies mixed in.

Loudness is the property of a sound that tells how strong or weak the sound seems to the ear.

Amplitude is the greatest distance that a vibrating object moves away from its rest position on either side.

The decibel is the unit used to measure the loudness of sound, and its symbol is dB.

Noise is unwanted or unpleasant sound, usually loud and without a regular pattern.

Key points

Questions answered

What is meant by the pitch of a sound? Name one shrill sound and one deep sound.
Pitch is the property of a sound that lets the ear tell a high, shrill sound from a low, deep one. A mosquito's buzz is shrill; a lion's roar is deep.
A string makes 1,500 vibrations in 5 seconds. Find its frequency.
Frequency = (1500)/(5 s) = 300 Hz.
How is the pitch of a sound related to its frequency?
The higher the frequency, the higher the pitch; the lower the frequency, the lower the pitch.
Why does a bansuri give a higher note when the player lifts more fingers from its holes?
Lifting the fingers makes the vibrating air column shorter. A shorter air column vibrates faster, so the frequency and the pitch rise.
Give two ways in which a tabla player can raise the pitch of the drum.
By tightening the skin (tapping the wooden blocks on the side of the dayan, or pressing the heel of the hand on the bayan). A smaller, tighter skin vibrates faster.
State three ways of making a guitar string give a higher note.
Make the vibrating part shorter (press it against the neck), make it tighter (turn the tuning peg), or use a thinner string.
What is a monotone? Give one example.
A monotone is a sound of one single frequency, with no other frequencies mixed in. The sound of a tuning fork is an example.
Two drums are struck, one gently and one hard. Which sound is louder, and why?
The drum struck hard is louder, because its skin vibrates with a larger amplitude, and loudness depends on amplitude.