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Binaural perception of infrasound


The intense discussions on noise disturbance, particularly in the vicinity of wind turbines, raise the public awareness to infrasound (sound with frequencies < 20 Hz). Of major importance, thereby, is the knowledge about infrasound perception under realistic conditions, e.g. with binaural hearing. The monaural and binaural loudness perception of infrasound was investigated in a study with listeners, which was conducted in collaboration with the University College, London.

In general, hearing with both ears is important for sound localization, which is based on the evaluation of phase- or level differences between both ears. In addition, a binaural stimulus is perceived louder than a monaural stimulus. The amount of this loudness increase, i. e., the psychoacoustic effect of binaural loudness summation, depends strongly on the processing in the brain. Hence, this study allows an indirect investigation of the underlying physiological processes.

At the perception threshold, i. e., at a sound level, at which the ear barely perceives a tone with a given frequency, the binaural loudness gain for both, infrasound and audio sound (20 Hz to 20 kHz) is about 3 dB [1, 2]. An audio stimulus presented above the perception threshold is perceived even 6 dB louder [1]. Until now, no such data for the above-threshold infrasound case existed.

In a study, conducted in collaboration with the University College, London, the binaural loudness summation for above-threshold infrasound was determined by means of experiments with 15 listeners. In addition, a potential influence of the phase difference between both ears (interaural phase difference; IPD) on the loudness perception was investigated.

During a hearing process, the interaural phase difference is due to the anatomical location of the ears at the side of the head and the resulting positions with respect to the arriving sound wave. It applies: the lower the frequency of the sound wave, the greater is its wavelength and the smaller is the interaural phase difference (e. g. a wave with an infrasound frequency of 10 Hz has a wavelength of about 34 m). Although the interaural phase differences of infrasound are insignificant under real-life conditions, an interaural phase difference influence is highly interesting for understanding infrasound perception.


Figure 1. The binaural loudness gain for an above-threshold stimulus presentation is shown as boxplot for three different interaural phase difference (IPDs) and two stimulus frequencies (400 Hz represent audio sound, 8 Hz represent infrasound). The applied sound pressure levels (+40 dB above threshold for 400 Hz and +12 dB for 8 Hz) led to a similar loudness perception of both stimuli. A box contains half of the results of all listeners, the horizontal line within the box provides the median value. The whiskers on both sides of a box indicate minimum and maximum values. Any outliers (distance to the box of more than 1.5 times the length of the box) are shown as circles.

The results of our measurements are given in Figure 1. For audio sound (400 Hz stimulus), a phase difference between both ears has no statistically significant influence on the loudness perception. The binaural loudness gain of the 400 Hz tone is 6 dB, on average.

The 8 Hz stimulus presented with an interaural phase difference of 0°, i. e., no phase difference between the sound arriving at both ears, led to a similar gain of 5 dB. This value is, statistically, not significantly different from the value at 400 Hz. The binaural loudness gain, however, reduces significantly with increasing interaural phase difference. At the maximal different interaural phase difference of 180°, the binaural stimulus is still perceived louder as a monaural stimulus, but the loudness gain is reduced to about 2.4 dB.

These results, together with the well-known physiological hearing processes for audible sound, allow the derivation of hypotheses for the hearing process of infrasound. For the effect described above, the time sequence of processes appears to be important. A processing step for the loudness perception requires a time interval of 100 ms to 200 ms. Within this time interval, several cycles of an audio stimulus are contained, while for the “slower” infrasound a single cycle is just covered (the 8 Hz stimulus has a cycle duration of 125 ms). This presumably leads to a reduced loudness perception with increasing interaural phase difference.



[1] B. C. J. Moore, B. R. Glasberg. Modeling binaural loudness. J. Acoust. Soc. Am. 121 1604 (2007). Opens external link in new windowLink

[2] N. S. Yeowart, M. J. Evans. Thresholds of audibility for very low-frequency pure tones. J. Acoust. Soc. Am. 55 814 (1974)



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last edited: 16.05.2022