Audibility of very low frequency and infrasound signal

Overview & Synopsis

While the human ear is less sensitive to very low frequency (20 < f < 200Hz) and infrasound (f < 20Hz) they are still audible even though absolute thresholds quickly raise with decreasing frequencies. Today no standard provides absolute pure-tone thresholds in infrasound, but many data have been published. Our own experiments show lower thresholds at these frequencies than the rest of the literature, suggesting greater sensitivity than previously reported. Concerning complex low frequency noises, a key problem is estimating their audibility, which is often done by comparing third-octave noise spectra with threshold proposals from the literature and standards. However, some studies show that with decreasing frequencies, auditory filters are larger with respect to their central frequency. In particular, lowest auditory filter is centered around 50 Hz and seems to process every frequency below. As such, comparing third-octave spectra to absolute thresholds might be an erroneous approach as it might ignored spectral integration when assessing the audibility of very low frequency and infrasound signal. The results of our thresholds measurement as well of an ongoing experiment on spectral integration show that human hearing might be more sensitive to very low and infrasonic frequencies than expected from standards.