ABSTRACT
Humans can detect fairly low noise levels and are also subject to much louder noises. In order to measure this range of noise intensity on a reasonable scale, the logarithm of noise is measured. The ‘unit’ used for noise measurement is the decibel (dB), although it should be noted that it is not truly a ‘unit’, but a ratio. The scale is defined such that 0 dB corresponds to a noise level at the bottom end of the human hearing range (just audible, but only just). The sound pressure level of various noises is shown below:
Noise Approximate sound pressure level (dB(A))
Jet engine 25 m away 140
Submarine engine room 120
Pneumatic drill 7 m away 90
Vacuum cleaner 80
Inside car 70
Quiet bedroom 35
The scale used for measurement of the noise levels is the dB(A) scale. This is simply the decibel scale, with adjustments made to accommodate the characteristics of human hearing. Humans hear sound in the frequency range 1,000-5,000 Hz well, and so no adjustment is needed in that region. However, humans hear less well as the frequency rises above 5,000 Hz or below 1,000 Hz. Noises at the limit of our frequency range bother us less, because we hear them less well. The A weighting takes this into account by subtracting an amount from the measured level. This is done to convert meter readings (a meter will detect the pressure of the sound wave, no matter what the frequency of the sound) into readings which approximate to the response of the human ear, which does vary with frequencies.
