Sound is the form of energy which gives the sensation of hearing. It travels in the form of waves. The wave which can travel without medium is called non mechanical or electromagnetic wave. The wave which requires medium for its propagation is called mechanical wave.

Mechanical waves are of two types:

1. Longitudinal waves
2. Transverse waves

The wave in which vibrations are along the direction of wave propagation are longitudinal waves. They travel in the form of rarefactions and compressions. These types of waves are sound waves.

Vibrations produce sound and the sound of frequency 20 Hz to 20000 Hz is audible to humans. The sound of frequency greater than 20000 Hz is called ultrasonic and less than 20 Hz is called infrasonic.

The transverse wave travels in the form of crest and trough. The minimum distance between two successive crest or troughs is called wavelength. The minimum distance between two rarefactions or compressions is also the wavelength of the wave.

The time of one oscillation or one vibration is called time period. The number of oscillations or vibrations in one second is called frequency. The velocity of the wave (v) is product of wavelength (λ) and frequency (ν).

v = νλ

Sound is a longitudinal mechanical wave which travels in air with the speed of 333 ms–1 at NTP. The velocity of sound in solids is more than in liquids. The speed of sound depend upon medium and temperature. It increases with increase in temperature and decreases with the increase in the density of the medium. It also increases with increase in humidity in the air.

Sound level is measured in unit of decibel (dB). It is a unit which compares the levels of power of two sources. For average human ears, the whisper is about 30 decibel. The normal conservation is about 65 decibels while jet plane taking off makes a noise of about 150 decibels. Beyond 85 decibels, sound is damaging and can lead to temporary loss of hearing.

We can not have sound communication on moon, because moon has negligible atmosphere. In such cases the effective communication is done by electromagnetic waves. They do not require medium for their propagation. Radio, micro, IR and X-rays are examples of electromagnetic waves. They travel in free space with speed of light - 3 × 108 ms–1.