Propagation of Sound

In air, sound is transmitted by pressure variations from its source to the surroundings.

The sound level decreases as it gets further and further away from its source. While absorption by air is one of the factors attributing to the weakening of a sound during transmission, distance plays a more important role in noise reduction during transmission.

The reduction of a sound is called attenuation.

The effect of distance attenuation depends on the type of sound sources. Most sounds or noises we encountered in our daily life are from sources which can be characterized as point or line sources.

If a sound source produces spherical spreading of sound in all directions, it is a point source. Please click on the demo button to see the details.

[Animation of source dispersion from a point source]

For a point source, the noise level decreases by 6 dB per doubling of distance from it.

If the sound source produces cylindrical spreading of sound as shown in this diagram, such as stream of motor vehicles on a busy road at a distance, it may be considered as a line source. Please click on the demo button to see the details.

[Animation of source dispersion from a line source]

For a line source, the noise level decreases by 3 dB per doubling of distance from it.