Ever been driven crazy by the low hum of a generator, a pump or an air-conditioning system, or by deep bass music? Then you will know what it is like to be disturbed by low frequency noise.

What is low frequency noise?

‘Low frequency noise (LFN)’ should not be confused with ‘low level noise’. The latter is low in volume, whereas the former is low in pitch (e.g. the lowest two octaves on a piano) and can be as loud as you like in volume. Environmental LFN can be a component of such sounds as the rumble of heavy traffic or the deep hum of industrial machinery or a neighbour’s music system. In recent years it has become worse with the increase in road and air traffic and the proliferation of machinery such as generators and air-conditioning systems in, or near, domestic homes.

The term ‘infrasound’ is used for sound waves whose frequency is less than 20 cycles per second. (By comparison, the lowest note on a normal piano is about 27 cycles and middle C is about 256.) So this is extremely low in pitch, like a growling or rumbling. The term ‘infrasound’ is misleading and frequently misinterpreted as meaning ‘inaudible’. In fact frequencies as low as 1 cycle per second can be heard by a normal human ear if loud enough.

Does low frequency noise affect everybody?

No, it does not, because the audibility threshold varies from person to person. But for those who do hear it, it can be very distressing. It is not the same as tinnitus, which is a ‘ringing in the ears’ which would continue even when one is away from the noisy environment. In any case tinnitus is a high-pitched sound, quite different from the low pitch of LFN.

With very low frequencies (below about 60 or 70 cycles per second, including infrasound) the audibility and pain thresholds are close together. So if one can hear the sound at all then it has only to increase by a few decibels to become painful. Also experience suggests that prolonged exposure may lower one’s audibility threshold still further.

How is it being tackled?

Not at all well. When Environmental Health Officers (EHO’s) visit complainants at home, frequently either they cannot hear the noise or they find it faint because their audibility threshold differs from that of the complainant. Either way there is a tendency to dismiss the sound as not being loud enough to constitute a Statutory Nuisance. Recent recommendations following research by Salford University for the Department of the Environment, are designed to rectify these problems, but their effect remains to be seen.

What are A and C Weighting?

When measuring sound levels an adjustment has to be made in order to assess how loud a sound will appear to the human ear. An amount is deducted from the decibel reading for this purpose. The A-weighted scale was devised to achieve this adjustment with speech and music. Sound Level Meters with this A-weighted adjustment built into them, are issued to EHO’s. But A-weighting was never intended for, and does not work with, LFN because it subtracts far too much, in effect eliminating any LF component from the reading. Leading scientists advise C-weighting or some other alternative.

What do the courts make of low frequency noise?

Magistrates argue that the law concerning Statutory Nuisance from noise, applies only to people of ‘normal’ hearing. They equate ‘normal’ with ‘average’, and tend to argue that anyone bothered by LFN does not have normal hearing and therefore is outside the protection of the law.

What needs to be done?

▪ A-weighting should be scrapped for low frequencies and replaced by C-weighting.

▪ Enact legislation governing the installation of industrial and domestic equipment to achieve insulation against the transmission of vibrations and low frequency noise.

▪ Have a pool of qualified acousticians on whom EHO’s can call when faced with a LFN complaint for which they lack the expertise or the necessary meter.

▪ The ability to hear lower frequencies than the average person should not disqualify someone from protection of the law concerning noise.

Further research on the physiological effects of very low frequencies (1-70).

UK Noise Association, PO BOX 551

Chatham, Kent ME4 9AJ

Tel: 01634 863 852 www.ukna.org.uk info@ukna.org.uk

Information Sheets: Low Frequency & Infrasound