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Noise has been defined as unwanted sound. We are all aware of the noises that surround us in our everyday lives, many of which are a source of nuisance. 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. SoundThe frequency of a sound is the number of sound waves which pass a particular point in one 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 Frequency NoiseThere are many sources of low frequency noise which can cause disturbance and/or annoyance both indoors and outdoors. Sound enters buildings through their structure, through open windows, or can be generated inside the building. Sound with a very long wavelength may be heard as noise (primary noise), cause rattling windows, doors or furniture (secondary noise), and may be difficult to distinguish from a structural vibration. Both of these forms of noise can cause disturbance, particularly at night. 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, however, 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. Sources The following may all be sources of LFN:amplified music pumpsfansboilersventilation plantfoundriesblasting/quarryingroad rail and air trafficelectrical installations Effects of Low Frequency NoiseA low frequency noise is very often characterised by a hum or rumble. It may be confused with tinnitus (ringing/hissing in the ears with no obvious cause). Health The hearing sensitivity range of people varies; therefore people who are disturbed by LFN may feel isolated if no one else is annoyed by the noise. Symptoms of LFN annoyance are those associated with stress. These include feelings of irritation and unease, fatigue, headache, nausea and disturbed sleep. It is not clear at what level Low Frequency Noise may be physically damaging; however, the unpleasant symptoms it can induce are sufficient to cause disruption and significant social and economic penalties to sufferers. A review of research into low frequency noise is available in PDF format from the Defra website. NuisanceHow far low frequency noise presents a serious nuisance problem is uncertain, but it is often a source of irritation. Assessing the level of nuisance caused by low frequency noise is difficult. 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. Noise nuisance is determined by the magnitude of the noise on the decibel scale. In the case of LFN this scale is not appropriate so it is often difficult to prove a nuisance. HumsThroughout 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 pipelines. There have been extensive investigations into some of these, but their actual sources remain unconfirmed - it is very difficult to determine the origin of some of these sounds. Controlling Low Frequency Noise Apart from the difficulties of tracking down the source of low frequency noise, and assessing its magnitude, practical methods of control are technically difficult and often prohibitive in cost. 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. If you are persistently troubled by low frequency noise or hum, contact the environmental health department of your local authority and ask them to investigate it for you.
