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Auditory perception of infrasonic signals

The more components an infrasound complex tone is composed of, the better it can be heard

PTBnews 1.2021
07.01.2021
Especially interesting for

environmental and occupational health medical practitioners

scientists conducting research on noise and the impact of noise

The growing number of infrasound sources in our environment is increasingly attracting society’s attention. Together with the Otto von Guericke University Magdeburg, PTB has investigated how humans perceive infrasound with their ears and whether the detection threshold of an infrasonic signal depends on whether it is composed of one or of several tones. This allows conclusions to be drawn as to the perception of complex signals such as those we encounter in our real environment. These investigations revealed that the detection threshold is not determined by a dominant component within the complex tone, but that spectral integration takes place in the auditory system.

The infrasound playback system consists of two sound generators (earphone transducers in an air-tight metallic housing). Sound is guided to a joint insert earphone via a flexible tube.

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.

Contact

Holger Joost
Department 1.6
Sound
Phone: +49 531 592-1519
Opens local program for sending emailholger.joost(at)ptb.de

Scientific publication

B. Friedrich, H. Joost, T. Fedtke, J. L. Verhey: Spectral integration of infrasound at threshold. JASA 147, EL259–EL263 (2020)

Opens external link in new windowDOI: 10.1121/10.0000897