Which animals can hear low-frequency noises?
Perception and health risks
Perception of infrasound
The quality and type of perception change even below 100 Hz: sounds are significantly worse and below 50 Hz they are no longer heard. However, some people can also hear tones with significantly lower frequencies. For them, therefore, a sound that is inaudible to most people can seem unbearably loud. It is becoming apparent that the human ear is much more sensitive to infrasound and low-frequency sound than previously thought.
Audible and inaudible low-frequency noise emissions are often described as ear pressure, vibrations, or feelings of fear or insecurity. In general, the lower the frequency, the higher the sound pressure level (measured in decibels) has to be so that people can hear something of the sound. However, the transition from hearing to exclusive feeling is fluid.
Locating infrasound sources is difficult because the distance between the ears is significantly smaller than the wavelength. Localization is possible with special "microphone arrays" (a network of microphones at a large mutual distance).
If the sound is no longer audible, people can still hear low-frequency sound. Mechanoreceptors convey pressure, touch, tickling and vibration stimuli. Mechanoreceptors are certain nerve cells that are distributed throughout the body and convert mechanical forces into nerve impulses.
An impressive illustration of the perception and generation of infrasound by animals can be found at http://www.birds.cornell.edu/brp/elephant/cyclotis/language/infrasound.html
Infrasound health risks
Basically, all people are exposed to low-frequency sound, since low-frequency sound is ubiquitous and part of modern life. Below a sound pressure level of 170 decibels (dB) (which in the audible range would be louder than an aircraft taking off), no harmful effects on human health could be demonstrated. Complaints such as a decrease in the ability to concentrate are more likely to be seen as a result of annoyance, which also disappear again after exposure.
There are a few laboratory tests especially for infrasound (below 20 Hz). They show that infrasound can have a tiring and concentration-reducing effect and can influence performance. The equilibrium system was also impaired in the laboratory tests. The body's best-documented response to infrasound is increasing tiredness after exposure for several hours.
Some people react particularly attentively and sensitively to low-frequency sound. Those affected suffer from an obsessional alert, which means that they always have to concentrate on the low-frequency sound. This can lead to chronic fatigue and insomnia, among other things. This phenomenon is a factor that cannot be neglected in environmental medicine.
The primary effect of low-frequency sound on humans appears to be annoyance. The resulting symptoms cover a wide spectrum: headache, tension, irritation, mental and physical exhaustion, dissatisfaction, difficulty concentrating, disturbance of night sleep. In a study of citizen complaints, most people could not hear the sound. But almost all of them described sensory perception in the form of body or object vibrations (Møller & Morten, 2002).
The population affected by infrasound
An impact analysis was carried out on behalf of the Federal Environment Agency. There was a tendency in southern Germany, the Ruhr area and Berlin to report more complaints about infrasound and low-frequency noises.
The main reasons for the complaints were energy generation and transport systems (33.0%) and ventilation and air conditioning systems (22.8%). Of the energy generation and transport systems, the most common source types were biogas systems (8.4%), combined heat and power units (6.5%) and wind energy systems (3.3%). The most frequent cause of complaints were heat pumps (9.3%).
The distribution to the source groups is illustrated in the figure below.
Further information on the "feasibility study on the effects of infrasound" by the Federal Environment Agency can be found here.
Environmental medicine paradox
Often it is older people in a quiet environment who complain about infrasound and low-frequency sound. As a rule, women are more often affected than men. Research shows that they do Not have a particularly low hearing threshold.
The calm environment evidently leads to an increased perception of infrasound, because paradoxically those affected like to open the windows "to let in normal outside noise" and in this way to reduce the perception / effect of infrasound.
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