5.1.4. The meaning of commonly used terms

Sound Pressure

Atmospheric pressure has a typical value of 100,000 Pa whereas typical sound pressures are a small fraction of a Pascal (as low as 0.00002 Pa at some frequencies). The range of human hearing is 20 Pa; this tiny pressure variation is known as the threshold of hearing, to 200 Pa, known as the threshold of pain.

Sound Intensity

Sound is a form of energy, and we would expect the amplitude of noise to be associated in some way with the amount of energy arriving at the ear within a given area. The sound intensity or acoustic intensity of sound is a measure of its strength and is defined as the sound energy (watts) flowing per second through one square meter. The range of intensities that can be heard by the human ear is extremely wide, from 10-12 Wm-2 (the threshold of hearing) to 100 Wm-2 (the threshold of pain). If a linear scale is used to represent this accurately it would require 100,000,000,000,000 divisions which would be unworkable. Consequently, a logarithmic scale is used which measures the 10-fold increases in the intensity levels.

Frequency

The frequency of a sound of a single tone is determined by the complete number of compressions and rarefactions passing a point in space each second. This measure of the sound is given in Hertz (Hz) and is perceived by the listener as pitch. The human hearing mechanism has a finite range of perception of pitch and varies from 20 Hz to 20,000 Hz or 20 kHz, but is most sensitive in the range of 4 to 6 kHz.

The Decibel Scale

•         The ear can detect pressures over a very wide range, from 20 µPa to 20 Pa (Pascals)

•         The decibel (dB) is used to measure noise

•         A decibel is a unit of sound pressure (intensity) measured on a logarithmic scale from a base level taken to be the threshold of hearing (0dB)

Typical noise levels include:

Weighting scales – the terms dB(A) and dB(C)

•         Sound level meters use weighting scales or filters

•         Most widely used is the A scale

•         Measurements made on this scale are expressed as dB(A) or referred to as ‘A weighted’

•         The C scale is used to determine peak sound pressure levels

•         Has a broader spectrum than that of the A weighted scale.

•         Measurements made on this scale are expressed as dB(C)

•         The range of frequencies that we encounter is often divided into octave bands.