Oud, M. Low Frequency Noise: a Biophysical Phenomenon

Mireille Oud, The Netherlands

Presented at Congres Geluid, Trillingen, Luchtkwaliteit en Gebied & Gebouw 2012 [Noise, Vibration, Air Quality, and Field & Building in The Netherlands], 6–7 November, 2012

Abstract:

Complaints on low-frequency noise were till recently fairly unexplained, but audiological research shed light on the mechanisms that enable perception of frequencies below the threshold of average normal hearing. It was shown that exposure to low-frequency sound may alter the inner ear. This results in an increase of sensitivity to low-frequency sounds, and as a result, previously imperceptible sounds becomes audible to the exposed person. Interactions between inner-ear responses to low and higher frequencies furthermore account for perception of low-frequency sound, as well as the property of the hearing system to perceive so-called difference tones.

… In Figure 2 the sensitivity curves of the inner and the outer hair cells are shown, along with the noise spectrum of a Dutch wind turbine. It is seen that noise above 50 Hz can be heard by the average normal hearing person. Noise below 5 Hz is not audible for anyone. The region in between is not audible, unless the sensitivity of a persons outer hairs cells are altered. …

Frequencies lower than about 20 Hz cannot be heard by the average person, but they can be sensed as vibrations, as most people will have experienced when standing near e.g. a subwoofer. A minority of people, however, are able to hear these frequencies as well. …

Legislatory control of noise necessarily rests on noise-level standards for the average person, as these standards cover the majority of people. The ear of the average person is generally assumed to have a frequency-sensitivity characteristic according to the dBA-standard. When this standard is applied in the assessment of noise, as a weighting, the amount of low-frequency noise produced by public infrastructure seems small. The unweighted low-frequency level, however, can be considerable. For wind-turbine noise, this is shown in Figure 4. A growing number of people suffer from LFN-induced enhanced hearing sensitivity for low frequencies, with enhancements of 20 dB or more. …

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