Low Frequency Noise (LFN)
Noise has been defined as 'unwanted sound'. The effect of low frequency noise (LFN), however, is often not recognised as a nuisance, even though it may have a profound effect on the psychological and physiological well being of some people.
These complaints are very difficult for Environmental Health to investigate as the levels of noise are often barely audible and the source direction is unclear. Our noise equipment has difficulty detecting such low frequencies.
Nuisance
It's hard for us to know much of a serious nuisance low frequency noise is, but it is often a source of irritation. It's also difficult for us to assess the level of nuisance caused by low frequency noise. Ordinary sound level meters may not be able to detect LFN on the decibel scale as its decibel level is often lower than background noise. A narrow band frequency analyser is required to measure it.
Sources
The following may all be sources of LFN:
- amplified music
- pumps
- fans
- boilers
- ventilation plants
Sound
The frequency of a sound is the number of sound waves which pass a particular point in 1 second, and is measured in Hertz (Hz). Sound audible to the adult human ear is in the range 5 - 18000 Hz. Low frequency sound may be loosely defined as having a frequency below 150 Hz. Sound in the lower frequency range is around us all the time, but we are not always aware of it as people's sensitivity varies considerably. Problems arise when the levels of low frequency noise are such that they interfere with our everyday lives.
Low frequencies travel further than high frequencies
LFN can be more noticeable indoors, which is why it is often associated with disturbed sleep. In the open air other noises such as traffic may mask the annoying low frequencies. Indoors, middle and high frequency noise from outside is reduced because the insulating effect of the building increases with sound frequency. Noises from the lower frequency bands, may remain the same or even increase, hence rattling windows etc. Another problem is that LFN travels further than higher frequencies, so the source is often difficult to trace. Sleep may be disturbed by the hum of a distant boiler, or the rattle of a window caused by passing traffic.
Hums
Throughout the country there are cases of people who can hear 'hums'. These sounds have no obvious source. In some areas small groups of people apparently complain about noise from the same, unidentified source. These have been blamed on industrial sources and on gas piplines. In some areas they have been investigated extensively, but their actual sources remain unconfirmed.
Control
Apart from the difficulties of tracking down the source of LFN, and assessing its magnitude, practical methods of control are technically difficult and often too expensive. Sound proofing in buildings is usually impracticable as the design, particularly of modern buildings, can enhance the effect. Enclosing the noise source is a better option and will provide a more comprehensive solution. This is often difficult and expensive as it involves enclosing the source in a combination of massive structures to reduce sound transmission. LFN from machinery can sometimes be reduced by the use of vibration absorbing mountings.
The answer to eliminating low frequency noise lies in the design of the sources themselves. There is also a need to develop agreed standards for measuring and controlling LFN.
To make a complaint
If you wish to make a complaint about low frequency noise contact Environmental health, who will be able to discuss your situation further with you.
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