Decibels and dB (A)

To comprehend the rudiments of noise control, it is necessary to have a basic understanding of the nature and size of sound.

Sound is comprised of pressure waves: its amplitude is measured in decibels (dB) and its content and nature is expressed as its frequency. Most adults enjoy an audio frequency range of around 40 Hz to 15 kHz. Our hearing, however, is not equally sensitive to all frequencies.

At modest sound levels the ears response can be plotted as a curve. This is applied as a weighting curve to some sound level measurements, which is known as an A-weighted sound level dB (A). Consequently dB (A) is an approximation to our hearing.

In terms of amplitude our hearing has a range from around 0dB (known as the threshold of hearing) to 140dB (the threshold of pain). A noisy factory is typically in the range 90 – 100 dB (A), while a pneumatic drill registers 120dB (A).

The decibel scale is a logarithmic scale and as such is not subject to the normal rules of addition and subtraction.

For example, two noise sources of 50dB (A) thankfully do not combine to give 100 dB (A). In fact the result is 53 dB (A): a difference of 3 dB is only just perceptible. In terms of a subjective impression to compare loudness, an increase of 10 dB is roughly equivalent to twice as loud.
The decibel scale and loudness should be borne in mind when considering noise control.