Auditory thresholds and temporal integration of infrasound and very low frequency signals
Résumé
We know now that sounds below 20 Hz can be heard, provided that they are loud enough. Hearing thresholds in the so-called “audible” frequency range (20 - 2000 Hz) have been the subject of a lot of studies, leading to the establishment of international standards. However, hearing perception at very low frequencies (between 20 and about 200 Hz) and infrasound (f< 20 Hz) has been little studied in comparison with higher frequencies. To date, there are no standards of auditory thresholds in infrasound frequencies, and standards for very low frequencies have been established on the basis of few studies. In practice, many studies refer to ISO 226 for thresholds above 20 Hz, and to Møller and Pedersen's (Noise&Health 2004) proposal for those below 20 Hz. In order to study infrasound perception, a special cabin has been built that makes it possible to reproduce these frequencies without audible distortions. To broadcast very low and infrasonic frequencies the room is equipped with 20 bass speakers and 32 subwoofer speakers on opposite wall that allows to reach the needed levels to make these frequencies audible. Hearing thresholds were measured between 4 and 125 Hz. We found lower thresholds than those reported in the literature. By calculating the excitation patterns of our signals, we verified that the system did not generate audible distortions. At 4 frequencies from 20 to 125 Hz we tested several threshold measurement methods (two different 2I-2AFC procedures with a 2 down-1up rule, method of limit), with different restitution systems (in the cabin, in a room dedicated to sound listening with a subwoofer, under headphones), on a panel of 30 listeners, confirming the thresholds we previously found at these frequencies. Inter-individual differences were also highlighted in these experiments. We also measured temporal integration for infrasonic frequencies. We confirmed that integration time increases with decreasing frequency, and showed that at 4 Hz, integration is still not complete for a duration of 5 s.
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