Sound is a form of energy made when objects vibrate (move to and fro rapidly). As an object vibrates it sets the air around it vibrating, too: molecules of the gases in the air press close together and then pull apart. These regions of higher and lower air pressure move away from the source and are called sound waves. Sounds also travel through liquids and solids. Sound waves usually move away from the sound source in all directions, like ripples in a pond.

Sound waves

Sound waves travel as a series of alternating high and low pressure regions, called compressions and rarefactions. As they move, the distance between compressions and rarefactions remains the same. The distance between one compression and the next is the wavelength of the sound. The difference between the low pressure of a rarefaction and the high pressure of a compression is the amplitude of the sound wave. The greater the amplitude of the wave, the greater its energy.

Frequency

The number of compressions that passes a point in one second is called the frequency of the wave: this is always the same for a particular wave. The unit of frequency, a measure used to compare frequencies of all waves (including sound, radio and light waves), is the hertz (Hz), named after the German physicist Heinrich Hertz (1857–94).

Echoes

As sound waves spread out, their energy has to spread over a greater and greater area. So the sound gradually gets weaker and fades away. However if there is a hard, smooth surface in the way, such as a wall, then some sound waves bounce off it and come back again. The bouncing is known as reflection and, if the surface is many metres away, we hear the returning sound as an echo.

Acoustics

The way sounds are reflected around an internal space such as a room or concert hall is called acoustics. Large buildings that have many bare surfaces, such as cathedrals, may be full of reflected sounds. If the walls are close together, or if the reflected sounds overlap, we do not hear separate echoes. Instead, we hear a jumbled sound that may take many seconds to die away. This effect is called reverberation. If there is too much reverberation, the original sound may become lost and difficult to hear; if there is too little, the sound may be too thin.

The speed of sound

Sound travels through the air, of course, but it also travels through liquids, such as water, and solids, including metals. The speed of sound varies according to the substance it moves through. It goes at a slower speed slower through more compressible substances—those that are easier to squeeze smaller.

Doppler effect

When a vehicle sounding a siren approaches you, the pitch of the sound gets higher; it then becomes lower as the vehicle moves away. This is because each sound wave takes slightly less time to reach you than the previous one as the vehicle comes nearer and nearer. The sound waves "bunch together", giving them a higher frequency. They spread apart as the vehicle gets further and further away, resulting in a lower frequency. This is called the Doppler effect.