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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 acoustic frequency range the handling of noise is well grounded and regulated, in the infrasound and the ultrasound ranges not only is the understanding of perception lacking but also fundamental requirements for measuring instruments as well as applicable, practice-related measurement instruc tions. In an EMRP project, it was therefore objectively investigated by means of different methods of audiology and by means of neurologic imaging procedures, how infrasound (frequency ƒ below approx. 16 Hz to 20 Hz), low-frequency sound (ƒ below 125 Hz), and ultrasound (ƒ above 16 kHz) affect human beings.

Activity in a horizontal layer in the brain, exclusively in the area of the auditory cortex; the different colors show stimulus frequencies ranging from 8 Hz to 250 Hz.

Initially, the subjective hearing thresholds of a subject group were determined from 2 Hz (infrasound) to 125 Hz (audible sound), and from 14 kHz (audible sound) to 24 kHz (ultrasound). Then, imaging procedures were used to examine whether and in which brain area the acoustic stimuli caused responses. The same test subjects were examined by means of magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). In the low-frequency and the infrasound range, an excitation in the auditory cortex could be detected down to 8 Hz. A local minimum at 20 Hz suggests that possibly the mechanism of hearing might change at about 20 Hz, meaning that infrasound (below 16 Hz) and audible sound (above 20 Hz) are perceived and/or processed differently by the brain. In the ultrasound range (above 16 kHz), neither the MEG measurements nor the fMRI measurements showed activation in the auditory cortex.

With its results, the project has provided the foundations for new and better measurement methods with which it is possible to lay down more wellfounded upper exposure limits in the long term.