Sound in Air

This page covers the way in which sound moves through air, and the measurable features of a sound wave.

Sound Propagation in Air
The movement (propagation) of sound through the air involves the movement of a 'wave'. This wave is made up of compressions and rarefactions of the air itself. If we imagine a tuning fork (as seen in Fig. 1) with its prongs vibrating back and forth in the air, it pushes the air around it as it moves. When the fork pushes against the air in a given direction, it creates a compression as air particles bump up against one another and get pushed along in a wave. It is the same process as we see when dropping a pebble into a lake. Ripples in water are a good analogue for sound in air, as it also reminds us that sound does not travel in a straight line. Fig. 1 is only illustrating the propagation of sound in one particular direction, but actually the same thing is happening in all directions around the fork.

The regularity with which these compressions are created is referred to as the frequency of the wave. Frequency is what defines the pitch or note we perceive of the sound, and is measured in Hertz (Hz). One Hertz = one compression per second. 1250 Hertz = 1.25 kiloHertz (kHz) = 1,250 compressions/rarefactions a second. As the frequency increases, we perceive a sound higher in pitch.

One of the other features of a wave, as seen in Fig. 1 is its Wavelength. Wavelength is measured in smaller designations of metres, such as micrometres (µm) and nanometres (nm). It is represented in formulae by the greek letter lambda: λ. The wavelength of a wave is the distance over which the shape of the wave is repeated. It is the length of one single cycle of the wave.

Finally, the intensity and strength of the wave is known as its amplitude. Amplitude mainly applies to an electrical signal, in which case it is measured in Volts (covered in more detail in What is Digital Audio?) but its analogue in sound in air is measured in decibels (dB), which is a measurement of volume. These factors relate to the volume level, or loudness, we perceive of the sound.

The simplest way in which a sound wave can be represented is as a sinusoidal wave, as seen in Fig. 2. The peaks in a sine wave correspond to the compressions in a wave in air. Likewise, the troughs correspond to rarefactions. A sine wave could also be understood from a physics perspective as a graph representing the changing air pressure at a given point, from higher to lower and back.

All the features of a wave in air can also be applied to a sine wave. The sine wave is an exact analogue of the sound wave it represents.

Summary

 * A sound wave in air is made up of compressions and rarefactions.


 * This pattern of movement can be represented by a sinusoidal wave (Fig. 2).


 * The features of a sound wave are Frequency, Wavelength and Amplitude.


 * Frequency relates to our perception of pitch.


 * Amplitude relates to our perception of volume.

The way in which these waves in air can be recorded, transmitted electrically and played back is covered in What is Digital Audio?

The way in which we perceive sound waves is covered on the Psychoacoustics wiki page.