Class 9 Science

Quality of Sound


One of the qualities of sound is called timber of sound. Timber is one of the characteristics of sound that enables us to differentiate between two different types of sound.

The sound of flute and harmonium or other musical instruments can be differentiated, because of timbre. You are able to recognize the sound of one person from that of another because of timbre.

Tone and Note of sound

Sound of a single frequency is called tone and sound of mixture of several frequencies is called note. A note is pleasant to listen.

Noise is unpleasant to hear.

Music is pleasant to hear because of good quality, i.e. timbre while noise is unpleasant because of bad quality. A pleasant sound is called music, while an unpleasant sound is called noise.

Sound Wave Needs Medium for Propagation

Since sound wave travels by compression and rarefaction of particles, so a medium is necessary for propagation of sound wave. Sound cannot propagate without a medium.

Sound wave cannot propagate through vacuum. This can be demonstrated using the bell jar apparatus.

Bell Jar Experiment

bell jar experiment

A bell jar is a bottomless jar. During the experiment, an electric bell is put inside the bell jar. The gap through which wiring of electric bell passes is made airtight with suitable means. A vacuum pump is attaches to the bottom so that air can be sucked out of the bell jar.

When air is present in bell jar, the sound of electric bell is heard outside. This shows that sound travels through air.

When air is sucked out of bell jar, the sound of the bell is not heard outside. This happen because there is no medium present, consequently no particles are present for compression and rarefaction in the bell jar through which sound waves could propagate.

Propagation of sound through different media

Sound propagates through solid, liquid and gas. In all these media, sound wave propagates by compression and rarefaction of particles of the medium.

Velocity of sound wave in different media

Velocity of sound waves depends upon the following three factors:

Nature of medium

The velocity of sound is maximum in solid, moderate in liquid and minimum in gas. Following table shows velocity of sound in different media.

MediumVelocity of Sound
Iron5130 m/s
Water1500 m/s
Air344 m/s


The velocity of sound is directly proportion to temperature. This means, velocity of sound increases with increase in temperature. For example: sound wave at 0⁰C in air is 332m/s. The velocity of sound waves at 20⁰C in air is 344m/s. This is the reason, we hear more clearly on a hot day than on a cold day.

Humidity present in air

Since, velocity of sound wave in water is more than that in air, so humidity in air increases the velocity of sound. Therefore, an increase in humidity in air increases the velocity of sound and a decrease in humidity in air decreases the velocity of sound. This is the cause that we can hear more clearly in rainy season than in summer.


See Answer

1: (c) Timbre, 2: (a) Sound of a single frequency, 3: (d) Sound of a mixture of many frequencies, 4: (c) Steel, 5: (b) 344 meter per second, 6: (a) Very hot day, 7: (a) 0.1 second, 8: (b) 17.2 meter, 9: (c) Less than 20 Hz, 10: (a) More than 20,000 Hz