ACOUSTIC CLIMATE

The levels of noise present in control rooms are not generally high enough to cause hearing damage. Nonetheless, considerable disturbance can be caused by noise. Noise is defined as an undesirable sound. It is produced as a succession of pressure waves in the air from sources such as vibrating surfaces (e. g., loudspeaker cones, equipment panels, or a person’s vocal cords) or air turbulence caused by, for exam­ple, fans or compressed air outlets. The noise level or ‘sound pressure level’ is usu­ally measured in decibels (dB).

The full frequency range of human hearing lies between 20 Hertz (Hz) and 20,000 Hz. As the human ear is less sensitive to sounds at low frequencies and very high frequencies, a special weighting is applied to sound measured at different fre­quencies so that their effect can be compared directly. Known as the A-weighting, this mimics both the sensitivity of the ear to sounds at different frequencies and also the likelihood of damage occurring to the ear at different frequencies. The A – weighted decibel is written dBA, although the older notation dB(A) is still seen.

Following a European Directive published in 2002, all countries in the EU now have noise regulations that limit the daily average noise levels to which people may be exposed at work. The Lower Exposure Action Value (LEAV) is an average over the day of 80 dBA, at which point a noise risk assessment should be carried out. The Upper Exposure Action Value (UEAV) is set at 85 dBA, and this is taken as the point at which the risk of hearing damage starts to rise (with consequent implications for health surveillance). The regulations set a daily limit value (ELV) of 87 dBA. Each of these criterion values has an associated peak limit level, but sudden peaks of noise should not normally be a problem in control rooms.