Annoyance due to low frequency hums

BMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6925.355 (Published 05 February 1994) Cite this as: BMJ 1994;308:355
  1. C G Rice

    Hums - low frequency noises - cause much annoyance and may have other non-auditory effects on health. The issue is one of the most enigmatic factors in the assessment and control of environmental noise.

    Over the past 20 years both scientific and lay publications have repeatedly drawn attention to mysterious hums, for which there often seems to be no explanation.1, 2 Universities, government departments, research establishments, and industrial and public companies have all been concerned,3, 4 as have learned societies, charities, and sufferers' associations. 5, 6 What are these hums, and why are there no clear explanations?

    Audible sound is in the frequency range 20-20 000 Hz. “Infrasound” is inaudible sound in the frequency range 1-20 Hz - and if it is sufficiently intense it is sensed rather than heard. “Low frequency sound” is audible sound in the range 20-150 Hz. Hums seem to be perceived in the range 1-150 Hz, in which physical measurement is difficult.

    Hums are associated with noise problems that cannot be routinely solved by acoustic consultants or environmental health officers. Typically, in around a tenth of cases no clear cause can be found - and this results in an element of mystery and much conjecture.7 Hums are often linked with the supply of utilities, waste pipes, ionisation and electromagnetic radiation equipment, industrial plants, and pumping and combustion machinery. Common descriptions include “an incessant hum,” “like an airplane stuck in the sky,” “feels like a tremendous surge of energy,” and “continuous throbbing day and night.”

    Someone investigating an elusive hum should pay close attention to the following considerations. Firstly, many cases can be solved by a competent acoustic engineer using sensitive equipment that measures and analyses sound to a standard above that normally expected in routine investigations. The engineer may also eliminate wrongly suspected causes. All this depends on the hum being perceived when the engineer is on site - often not the case - and account being taken of such features as meteorological and propagation effects, ground and structural vibrations, and masking effects of other background noises.

    Attention should also be paid to physiological causes. Tinnitus commonly (but by no means always) accompanies hearing loss and is not caused by any environmental agent. People with normal hearing may also suffer tinnitus. The sounds are commonly described as hissing, ringing, clattering, and humming in the ears. Tinnitus becomes more apparent in quiet surroundings and is more likely to be noticed by someone resting or in bed, and when the ears are occluded. Tinnitus is not frequency specific and consequently does not always manifest itself by a low frequency sound.

    Tinnitus often has no known cause, and its confirmation requires comprehensive audiological investigation. After its diagnosis a course of rehabilitation is often recommended, for little can be done to alleviate the symptom - fortunately not life threatening - which the patient has to learn to live with.

    Sometimes, then, the cause of an annoying hum can be related to tinnitus, particularly if there is a sole complainant and other close family and neighbours cannot hear the noise - though some persistence is often necessary to get the complainant to acknowledge the explanation.8 An important variant of tinnitus should be suspected when descriptions such as incessant hum, energy surges, and continuous throbbing are used, and when the complainant's behaviour changes. When extreme measures are adopted such as attempting physically to escape from the noise by temporarily moving location or altering sleeping habits, and when the complainant is greatly upset by a noise that no one else has heard, a brain tumour should be suspected and neurological investigations begun.

    Increased sensitivity in hearing at low frequencies may account for some people being more able than others to detect quiet but nevertheless annoying hums.9 Laboratory investigations of the fine structure of hearing at frequencies below 150 Hz have so far proved inconclusive, and the special facilities needed to reproduce frequencies down to 20 Hz have limited research to a few specialist institutes. The suggestions that ionisation and electromagnetic radiation may also induce sensations of hearing in some people need further evaluation.

    What conclusions can be drawn about these elusive hums? They are perceived by enough people to be an important source of annoyance, and when the problem is unresolved substantial stress may be caused.10 In theory, if the correct but complex measurements are undertaken by an acoustic engineer then specific sources ought to be either identified or excluded. If this procedure fails then consultant neuroaudiological investigations ought to be carried out to exclude a physiological cause. If these time consuming and expensive procedures fail it is usually very difficult to come up with an explanation despite there being little doubt about the distress being caused. No doubt further research will be carried out, but probably each case will prove to be as different and as individual as the complainants themselves.


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