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Are infrasound and ultrasound audible?


Noise is one of the fundamental environmental factors impairing health and well-being. Whereas in the hearing frequency range, the handling of noise is well grounded and regulated, in the infrasound and ultrasound range, not only the understanding of perception is missing but also fundamental requirements for measuring instruments as well as applicable, practice-related measurement instructions.

 Numerous sources of noise at workplaces, in public, in buildings and homes emit sound not only within the so-called "hearing frequency range" of about 16 Hz to 16 kHz, but also below (infrasound) and above (ultrasound). The indication of this "hearing frequency range" with the above-mentioned limits has become established. This is, however, misleading as human beings can still perceive sounds and noises also in the neighbouring frequency ranges. This perception is often felt to be disturbing; little is known, however, about the basic mechanisms playing a role in this matter. This lack of knowledge is also the reason why – especially for ultrasound in air – there are hardly any regulations and standards for requirements made on measuring instruments, measurement methods and upper limits. But also in the infrasound range, there are no recognized upper limits, and the measurement regulations are again and again the subject of controversy. 

In a project supported by the European Union within the "European Metrology Research Programme" (EMRP), it was therefore objectively investigated, by means of different methods of audiology and by means of the imaging procedures of neurology, how infrasound and ultrasound affect human beings. Initially, the subjective hearing thresholds of a group of test persons were determined in the frequency ranges 2 Hz to 125 Hz and 14 kHz to 24 kHz. In the infrared range, it was also possible to determine the curves of the same subjective loudness level individually for each test person by means of newly developed methods and thus also for the group as a whole (see Figure 1). By means of these audiological methods, individual audiological perception could be quantitatively described.

Figure 1: Average equal loudness contours (ELC) between 8 Hz and 125 Hz (black lines with symbols) for an insert earphone. The blue lines represent free-field equal loudness contours from DIN ISO 226, the red lines are estimates from literature.

In a further step, imaging procedures were used to examine whether, and in which range, the acoustic stimuli outside the hearing frequency range cause reactions in the brain. In this way, objective measures of modern brain research were to be compared to the subjective perception of infrasound and ultrasound. The same volunteers for whom an audiological characterization was available, were examined by means of magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). In the infrasound range, an excitation in the auditory cortex could be detected down to a frequency of 8 Hz. As an example, the activation measured by means of fMRI in various sectional planes through the brain at stimulus frequencies between 8 Hz and 250 Hz is shown in Figure 2. The results also suggest the conclusion that the mechanism of hearing may change at about 20 Hz, i.e. that infrasound (below 20 Hz) and 'audible' sound (above 20 Hz) are perceived and/or are processed by the brain differently. In the ultrasound range, the MEG measurements also showed an activation in the auditive cortex, but only for signals above the hearing threshold. In contrast to this, brain reactions could be detected in the infrasound range in some experiments also for stimuli 2 dB below the hearing threshold, which suggests an unconscious perception of infrasound.

Figure 2: Activity areas within a horizontal layer in the brain exclusively within the region of the auditory cortex; the different colours represent different stimulus frequencies between 8 Hz and 250 Hz.

With its results, the project has provided the foundations for new and better measurement methods, by means of which better-grounded upper exposition limits can be determined in the long term. Until then, many open questions still have to be answered and in many cases, the extent of the measurements carried out so far is not yet sufficient for statistically validated statements. Therefore, further research projects are envisaged.



Christian Koch, FB 1.6, Opens window for sending emailchristian.koch(at)ptb.de