Class 9 Science

Quality of Sound


The quality 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.

Because of timbre; the sound of flute and harmonium or other musical instruments can be differentiated. For example; the frequency of a particular note 'Do' or 'Sa' produced by all the musical instruments is equal. In spite of that, because of different timbre one can differentiate the sound of same frequency of different musical instruments.

Different persons produce sound of different timbre. Because of different timbre in sound, the voice of different persons can be recognized.

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.

Sound Wave Needs Medium for Propagation:

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

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

bell jar experiment

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

Propagation (travelling) of sound through different media:

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

Velocity of sound wave in different media:

The 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. For example:

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.

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