[Introduction] Low-frequency noise (LFN) is sound focused in the frequency range below ~100 Hz. For example, in Germany sound is defined as low frequency if the C- and A-weighted sound pressure level (SPL) differs by more than 20 dB. For a growing number of people LFN is an urgent problem, and many questions remain unanswered: Why does LFN seem to be a greater problem today than in the past? Are people more sensitive? Is LFN increasing?
It can be assumed that LFN was less common in the past, which doesn’t mean that it was quieter. It is difficult to prove, but not improbable, that A-weighting is in part responsible for increasing LFN, because A-weighting attenuates LFN strongly. Therefore, in the endeavor to meet the limits in regulations, which are mostly defined in dB(A), it is easier (meaning in most cases less expensive) to shift resonances of machines to the low-frequency range than to attenuate the vibration or the sound by technical means. In addition, A-weighting is to blame for an underestimation of the annoyance of LFN.
Another reason for increasing LFN may be found in the growing application of all kinds of noise protection, e.g., noise barriers, special windows, etc. All of these measures have a common property: low-frequency waves can pass through, over, or across them more easily than waves in the middle- or even high-frequency range. Therefore, it isn’t impossible that noise protection bears some responsibility for the problem of LFN. As reported by Persson Waye et al. (2003), after measures had been installed against noise coming from outside, people were suddenly hearing LFN from inside the house and were so annoyed that some of them preferred to sleep with open windows despite the resulting high noise level. Di et al. (2005) reported a similar problem.
In searching for an answer to the question of why some people prefer to endure a louder noise with a broader spectrum than LFN at a lower level, few clues are found in the literature. One clue can be found in the detailed LFN report by Leventhall (2003). According to Bryan (1976) referenced there, the annoyance of LFN is determined by edge steepness limiting the spectrum of LFN to higher frequencies in the way, that a steeper edge causes an unacceptable annoyance while a moderately steep edge is acceptable. The core question is: What is the basic cause for all of these reactions?
In Hansen (2007), many contributions dealing with the effects of LFN on people came to the assumption that the special effects of LFN are caused less by the peripheral processing in the outer, middle, and inner ear but more by the subsequent processing in the nervous system. This might explain the direct influence of LFN on mental health [Persson Waye et al. (2001)], which can be found also in physiologic investigations [Persson Waye et al. (2002)].
Presented at Acoustics ’08, Paris, June 29–July 4, 2008
Univ. of Wuppertal, Wuppertal, Germany
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