Auditory perception of infrasonic signals

Infrasound may be below the conventional frequency range of audible sound (20 Hz to 20 kHz), but several studies have demonstrated that sound may be perceived down to 2 Hz. Such psychoacoustic measurements are usually performed with single tones. No direct conclusions can be drawn from such measurements as to the actual perception of complex signals with more than one infrasound frequency component – as is the case with real environmental noises. Together with the Department of Experimental Audiology at the Otto von Guericke University Magdeburg, PTB, in the project titled “Infrasound and its relevance for audible sound”, has dealt with issues such as the question as to what effect the number of infrasound components has on the detection threshold. The project is funded by the Deutsche Forschungsgemeinschaft (DFG – German Research Foundation).

Since the perception of infrasound only begins at very high sound pressure levels, such investigations require a correspondingly efficient playback system. Here, the challenge consists in generating the required sound pressure levels without audible distortion. Such a system was developed within the scope of the above-mentioned project and based on commercially available electrodynamic earphone transducers. This system allows several infrasound tones to be presented at the same time in the ear canal by means of an insert-earphone system.

With this system, PTB investigated whether one of the mechanisms known from the conventional frequency range of audible sound can explain the detection thresholds of complex tones consisting of two and three different infrasound components, respectively. The results have shown that the detection threshold is not determined by a dominant component within the signal. The threshold drops a little further with every additional component, namely to the same extent as would be expected if the intensity of the components were integrated. The more components an infrasound complex tone has, the better it can therefore be heard.

Further investigations with a larger database and much more complex signals are planned in order to better understand this mechanism as well as its limitations.