The human hearing is generally not sensitive at low-frequencies and relatively high sound pressure levels are needed before the sound is audible. However, if the sound is audible then slight changes in the level gives relative large changes in the perception of the sound which is reflected in the compression of the equal-loudness-level contours at low-frequencies. Combined with individual differences in the hearing function with possible extraordinary low-frequency hearing this could explain cases where only one person in the household complain about a low-frequency sound which is not audible to the rest of the household. However, there are cases where no apparent sound source is found and noise measurements are not able to show any sound that could be causing annoying. This raises the fundamental question if it is a physical sound that causes the annoyance. To answer this question a selection of twenty-one cases of low-frequency noise complaints were investigated with sound recordings in the home and laboratory experiments in a new low-frequency test facility which allows for great control of the sound exposure with low background noise and low distortion. The facility uses digital signal processing in order to create a homogeneous sound field for the entire low-frequency region. The low-frequency hearing threshold and an equal-loudness contour were measured for each complainant. The sound recordings were played back to the complainant in blind tests in order to reveal if the physical sound in the home is audible to the complainant. In cases of audible sound, recognition tests provided information whether the sound was similar to the annoying sound. The audible sounds were filtered into four different frequency ranges which were presented to the complainant in another series of blind tests and recognition tests in order to find the audible and annoying frequency components. Finally a matching experiment was used to approximate the frequency and level of the annoying sound.
No cases of extraordinary hearing were found among the complainants. The results shows that in seven cases the complainant is annoyed by a physical sound in the home, while in six cases low-frequency tinnitus is responsible. In the remaining eight cases the complainants could hear the recorded sound but it is not clear whether it is physical sound or low-frequency tinnitus that is responsible for the annoyance. In none of the cases is infrasound responsible for the annoyance. It is not audible even at 10 dB above the recorded level. Comparisons between results obtained by the Danish, Swedish and a three-dimensional corner measurement procedure show that especially the Danish method gives much variation and both the Swedish and the Danish method gives lower values than the three-dimensional corner method. For the seven cases clear cases with annoyance from a physical sound only three of the cases had levels exceeding the Danish limits for low-frequency noise even if the three-dimensional corner method is used. This shows that further research is needed on finding acceptable limits for low-frequency noise.
Ph. D. thesis by Christian Sejer Pedersen
Acoustics, Department of Electronic Systems Aalborg University, Denmark
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