Infrasonic sounds are generated internally in the body (by respiration, heartbeat,
coughing, etc) and by external sources, such as air conditioning systems, inside vehicles,
some industrial processes and, now becoming increasingly prevalent, wind turbines.
It is widely assumed that infrasound presented at an amplitude below what is audible
has no influence on the ear. In this review, we consider possible ways that low frequency
sounds, at levels that may or may not be heard, could influence the function of the
ear. The inner ear has elaborate mechanisms to attenuate low frequency sound components
before they are transmitted to the brain. The auditory portion of the ear, the cochlea,
has two types of sensory cells, inner hair cells (IHC) and outer hair cells (OHC),
of which the IHC are coupled to the afferent fibers that transmit "hearing" to the
brain. The sensory stereocilia ("hairs") on the IHC are "fluid coupled" to mechanical
stimuli, so their responses depend on stimulus velocity and their sensitivity decreases
as sound frequency is lowered. In contrast, the OHC are directly coupled to mechanical
stimuli, so their input remains greater than for IHC at low frequencies. At very low
frequencies the OHC are stimulated by sounds at levels below those that are heard.
Although the hair cells in other sensory structures such as the saccule may be tuned
to infrasonic frequencies, auditory stimulus coupling to these structures is inefficient
so that they are unlikely to be influenced by airborne infrasound. Structures that
are involved in endolymph volume regulation are also known to be influenced by infrasound,
but their sensitivity is also thought to be low. There are, however, abnormal states
in which the ear becomes hypersensitive to infrasound. In most cases, the inner ear's
responses to infrasound can be considered normal, but they could be associated with
unfamiliar sensations or subtle changes in physiology. This raises the possibility
that exposure to the infrasound component of wind turbine noise could influence the
physiology of the ear.
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