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What is sound?
Sound is a special kind of to and fro movement that propagates through matter as a wave. We perceive sound by the sense of hearing.

What is the speed of sound?
The speed of sound is the speed at which sound travels and it varies depending on the medium through which the sound waves pass. Sound waves travel through air at about 340 metres per second. Objects moving faster than this are said to be supersonic.

Why do sounds have different loudness?
How loud a sound is depends on the amplitude (or the 'height') of the sound waves that are making it. This is a measure of the energy that the sound waves contain. A loud sound has high waves while a quiet sound has low waves.

Why are some sounds very high while others are very low?
This feature of sound is called pitch. In sound, frequency is the number of waves per second and the pitch of a sound depends on the frequency of the vibrations that are making it. A low pitch sound has a low frequency while a high pitch sound has a high frequency.

What is infrasound?
Sound with a frequency of less than 20 sound waves per second (or 20 Hz) will have a pitch that is too low for our ears to detect. This type of sound is known as infrasound.

What is ultrasound?
Sound with a frequency of more than 20,000 sound waves per second (or 20,000 Hz) will have a pitch that is too high for our ears to detect. This type of sound is known as ultrasound.

Why do we hear returning sound when we shout at a cliff?
The returning sound is called echo. When sound waves hit a hard surface, they reflect off it and so we hear the sound returning after a time gap.

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