LIWA

Sound

ICSE Class 7 · Physics · Chapter 6

What this chapter covers

Key definitions

Sound is a form of energy that is produced by vibrating objects and that we can hear with our ears.

Vibration is a rapid to-and-fro or back-and-forth movement of an object about a fixed middle position.

Amplitude is the greatest distance that a vibrating particle or object moves away from its middle (rest) position.

Loudness is the property of a sound that tells us how strong or weak it seems to our ears.

Frequency is the number of complete vibrations made by a vibrating object in one second.

Pitch is the property of a sound that tells us how shrill (high) or flat (deep) it sounds.

Reflection of sound is the bouncing back of sound from a surface that it strikes.

Absorption of sound is the taking in of sound energy by a material, so that less of the sound is reflected or passed on.

Key points

Questions answered

What is common to every source of sound?
Every source of sound produces sound by vibrating, that is, by moving rapidly to and fro.
Name the part of the human body that produces the voice, and say which part of it vibrates.
The larynx, or voice box, produces the voice. The vocal cords inside it vibrate when air from the lungs passes between them.
Name the vibrating part in (a) a sitar, (b) a flute, (c) a dholak.
(a) A stretched string, (b) the column of air inside the tube, (c) the stretched skin or membrane.
Why is sound called a longitudinal wave?
In a sound wave the particles of the medium move back and forth along the same direction in which the sound travels, which is the mark of a longitudinal wave.
What are a compression and a rarefaction?
A compression is a region where the particles of the medium are crowded together and the pressure is higher. A rarefaction is a region where they are spread apart and the pressure is lower.
A student beats a drum first gently and then hard. What changes in the vibration of the drum skin, and what change does the student hear?
The amplitude of the vibration becomes larger when the drum is beaten hard, so the sound becomes louder.
A string makes 900 vibrations in 3 seconds. Find its frequency.
f = (900)/(3 s) = 300 Hz.
Why can two astronauts on the Moon not hear each other directly?
There is no air on the Moon, and sound needs a medium to travel. They talk using radio sets, since radio waves can travel through empty space.