Acoustics research centre
Frequently asked questions
The views expressed here are our own, and not necessarily those of DEFRA who funded this work.
General questions about LFN
Why is LFN a problem?
A number of people around the country, and in fact around the world, are disturbed by low frequency sound, which they typically describe as something like a low rumble. Only a small proportion of people are affected, but those who are can be severely distressed, can lose sleep and suffer other symptoms like depression. Sometimes the complaint can be traced to a sound source, typically an industrial site, or an adjoining property. If the source can be found then there is a chance the noise can be controlled. However, in many cases no sound can be traced that could account for the disturbance.
What does Low Frequency mean?
All noise consists of pressure fluctuations in the air. For LFN these fluctuations occur between 20 and 160 times per second. Most everyday sounds fluctuate much faster than this (up to 16 thousand time per second), so the term ‘low frequency’ means that the fluctuations are relatively slow compared with other types of sound. Sounds in this frequency range would typically be heard as a low rumble. Sometimes there is also a sensation of vibration or pressure on the ears. The scientific way of writing the frequency range is 20Hz to 160Hz: -the abbreviation Hz is short for Hertz and means ‘cycles per second’.
How does LFN get into my home?
In the majority of cases where a sound source can be traced, it is ‘airborne’, meaning that it travels through the air as a sound wave and enters the dwelling through windows, roof, etc. In theory sound can also be ‘ground-borne’, meaning that it travels through the ground and is converted into sound in the dwelling, or is sensed as vibration. However, cases of ground-borne LFN seem to be very uncommon.
The nearest industrial site is a long way off. Could this be the source of the sound?
One of the characteristics of low frequency sound is that it can travel relatively long distances without much attenuation (reduction in level). It is not uncommon that a source is traced to a site several kilometres away from the complainant’s property.
Do LFN sufferers have unusually sensitive hearing?
In studies where the hearing thresholds of LFN sufferers was measured it has been found that their hearing thresholds vary much like any other group of individuals with the same age and sex profile. So on average, LFN sufferers cannot be said to have particularly sensitive hearing in the low frequency range.
If no sound can be measured then what am I hearing?
No one really knows why some people develop sensitivity to LFN, and still less why some people react strongly to a sound that cannot be detected by measurement. Recent ideas from the medical world suggest that we are programmed by evolution to get ready to fight or run when we hear a sound that signals a threat to our survival. If we begin to associate a particular sound with a threat then this ‘alerting’ reaction takes place, possibly even for quiet sounds, for example like a barely audible creak on the stairs that could be a burglar. It could be that this survival mechanism is triggered in sufferers (whether or not they are conscious of it), and places them on a constant state of alert so that they can’t sleep. However, these ideas are speculative at the moment, and no one can really explain why some people end up suffering from LFN.
Why can no one else hear the sound?
There are many cases where only one person can hear the sound. This can be distressing, because to the sufferer the sound could appear to be loud, even deafening whereas others cannot hear it. A partial explanation for some cases is as follows: disturbance by LFN is known to occur at levels only slightly higher than hearing threshold, which varies from one individual to the next. Also, loudness varies rapidly with level at low frequencies so a sound only slightly above one person’s threshold could appear loud to them yet inaudible to someone else. Although there is some logic in this argument it does not explain anything like all cases.
I can feel a vibration as well as a sound.
This experience is common among sufferers. Many report that they feel the sound as vibration through various parts of the body, like the elbow. However, in the majority of cases when vibration levels are measured they are found to be below the published thresholds of perception. This means there is probably no direct sensing of vibration, but the sensation is arising perhaps as an association. This is not the same as imagining it because the associations would arise automatically.
Could I be sensing the sound as a vibration in my body rather than through my ears?
Many people are familiar with the pulsing feeling in the body, particularly the chest when listening to amplified music. For this reason some people think that they could be sensing the LFN through the body rather than through the ears. However, this is not the case because the ear is very much more sensitive to pressure fluctuations in the air than the body. In other words, if the sound level is high enough to be sensed in the body then it would be clearly audible to anyone except perhaps to the profoundly deaf or to those with a high hearing loss. Therefore, body vibration does not explain most cases of LFN.
Is LFN harmful to health?
In cases where a noise can be found that is responsible for the disturbance the levels of sound are typically around the threshold of hearing. Sounds at this level do not contain anything like enough energy to cause any physical harm. So low frequency noise in itself is not harmful. If a person starts to become stressed or to lose sleep for example then their health might be affected. If so, the effect is indirect (for example due to the lack of sleep) rather than being a direct effect of the sound itself. In, other words, if there are any health effects they are the result of the reaction to the sound rather than the sound itself. This is a very important distinction because belief and cognition plays a role in the sufferer’s reaction.
Why does LFN have to be treated separately to other types of environmental noise?
Unwanted sound in the environment is commonly referred to as environmental noise. The Environmental Health section of your local authority is responsible for ensuring that environmental noise does not cause a nuisance. There are established procedures for assessing environmental noise from industrial sites which are based on measurements of the so called dB(A). The term dB is an abbreviation for decibels which are the units for measuring sound. The (A) means that the sound is filtered to mimic the varying sensitivity of the human ear at different frequencies. Whilst this procedure works well for most industrial noise, the ‘A’ filtering operation strongly attenuates (reduces) the low frequency content of the sounds. In unusual circumstances when a LFN is present, applying the usual dB(A) measurement will not show up any problem. Therefore, several countries have adopted separate guidelines for assessment of low frequency noise.
Aren’t LFN sufferers just making it up to get attention?
No, there is more to it than this. In the past there was a tendency to categorise sufferers in this way. Nowadays, people who interview sufferers are usually convinced of the genuineness of the complaint, even if they can’t find an explanation for it. Many sufferer’s reports are careful, detailed and objective, which would have been difficult to fake. Furthermore, reports from diverse situations around the world describe remarkably similar phenomena even when the individuals concerned have no knowledge of other sufferers. Nowadays the consensus opinion is that the suffering is genuine and should be taken seriously even if can’t be explained.
Aren’t the complaints driven by ulterior motives like trying to shut down local industry?
In many cases the sufferer has no idea where the sound comes from. This being the case it is unlikely that the complaint is motivated by ill feeling because there is no perceived ‘culprit’ to direct the feelings against. Cases where the source of the perceived sound is not known are generally consistent with other cases, so ulterior motives do not explain the phenomena that are observed.
Why does the noise always get quieter whenever anyone comes to measure it?
It is a very common experience that the sound appears to get quieter when someone, usually the Environmental Health department, visits to measure it. In some cases the complainant starts to suspect that someone is ‘tipping off’ the person or site suspected of causing the noise so that they can reduce the level while measurements are being taken. We have conducted surveys on several occasions where we were certain that no one except the complainants and ourselves knew we were coming. Yet, still the complainant described the sound as becoming much quieter as soon as we arrived. This happens so often that we are becoming convinced that there is some phenomenon at work. A speculative explanation is as follows: if the sound is perceived (whether consciously or not) as signalling a threat (see above), then the presence of other people would diminish the threat and diminish the fight or flight response. This explanation has not been scientifically proven so should not be taken as fact, but it does at least provide some possible rational basis for an aspect of LFN that can be quite distressing.
What are the acceptable limits for LFN?
There are no acceptable limits as such. This is because disturbance depends not just on level but on a variety of other factors such as the character of the sound and background noise. Salford University with funding from DEFRA (Department for Environment Food and Rural Affairs) has produced a guidance document to assist Environmental Health Officers to assess complaints about LFN. The guidance takes the form of a procedure to determine whether a measured sound could be the cause of disturbance or not.
My local authority Environmental Health Department say they can’t do anything. What should I do?
In the past it was extremely difficult for Environmental Health officers to deal with complaints about LFN, partly because specialist skills and equipment are needed and partly because no official guidance was available to support them. DEFRA (Department for Environment Food and Rural Affairs) has now addressed the second point by funding Salford University to produce a guidance document specifically for Environmental Health officers (EHos). It should be easier for EHos to make sense of the situation now that they have a detailed procedure to follow. The procedure is designed to help separate cases where there is clearly an environmental noise that correlates with the complaints, from those where no noise exists that is within their power to control.
The Procedure for the assessment of low frequency noise complaints can be downloaded from the Defra website, and supporting reports can be found on the Defra website on LFN.
What happens if the Environmental Health department decides there is nothing they can do?
We expect that there will be cases where, even after a thorough investigation, the Environmental Health officer decides there is no action they can take to control the noise. This could happen if they can’t find a noise that is consistent with the complaints, or if they have no power to control the source (for example they have no power to control aircraft and traffic noise). If the noise itself cannot be controlled then the best option for the complainant is to try to control their reaction to the noise. DEFRA will shortly produce some guidance leaflets to help with this. It is sometimes comforting to sufferers in this situation to know that there are others around the world in similar situations.
Doesn’t the complainant just have tinnitus?
Not usually. Tinnitus is a ringing noise in the ears and in some ways its effects are similar to LFN. However, studies have found that most LFN sufferers do not have tinnitus, or if they do that they can hear the LFN separately to the tinnitus. So tinnitus and LFN are separate phenomena and tinnitus cannot explain the majority of cases of LFN.
Where can I find out more about LFN?
The reports we published for Defra are available free as a download from the Defra website on LFN or from the University of Salford Institutional Repository (USIR).
On the same website you will find an excellent review of published research on Low Frequency Noise and its Effects’, written by Geoff Leventhall, a world authority on the subject.
If you have questions about LFN by all means email me. I’m afraid I can’t get into personal correspondence because I get more enquiries about LFN than I can cope with. However, I will read your email, and if you raise any issues that are not answered already I will try to add the question and answer to this list within a couple of weeks.
Unfortunately I don’t usually have time to respond to phone calls, but again if you leave a brief message with your question I will try to answer it on these pages.
Contact Prof. Andy Moorhouse
+44 (0) 161 295 5490