Abstract
The human perception of sound at frequencies below 200 Hz is reviewed.
Knowledge about our perception of this frequency range is important,
since much of the sound we are exposed to in our everyday environment
contains significant energy in this range. Sound at 20-200 Hz is
called low-frequency sound, while for sound below 20 Hz the term
infrasound is used. The hearing becomes gradually less sensitive
for decreasing frequency, but despite the general understanding that
infrasound is inaudible, humans can perceive infrasound, if the level
is sufficiently high. The ear is the primary organ for sensing infrasound,
but at levels somewhat above the hearing threshold it is possible
to feel vibrations in various parts of the body. The threshold of
hearing is standardized for frequencies down to 20 Hz, but there
is a reasonably good agreement between investigations below this
frequency. It is not only the sensitivity but also the perceived
character of a sound that changes with decreasing frequency. Pure
tones become gradually less continuous, the tonal sensation ceases
around 20 Hz, and below 10 Hz it is possible to perceive the single
cycles of the sound. A sensation of pressure at the eardrums also
occurs. The dynamic range of the auditory system decreases with decreasing
frequency. This compression can be seen in the equal-loudness-level
contours, and it implies that a slight increase in level can change
the perceived loudness from barely audible to loud. Combined with
the natural spread in thresholds, it may have the effect that a sound,
which is inaudible to some people, may be loud to others. Some investigations
give evidence of persons with an extraordinary sensitivity in the
low and infrasonic frequency range, but further research is needed
in order to confirm and explain this phenomenon.
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