Beyond Human Hearing: Exploring Audible, Inaudible, and Harmful Noise

In physics, the range of sound frequencies is much wider than what the human ear can hear (humans can hear from 20 Hz to 20 kHz). These inaudible frequencies are essential in modern technology, scientific research, and security systems in current human societies. Infrasound (below 20 Hz) exists in the natural environment (ocean waves, storms, earthquakes, and volcanoes). Ultrasound (above 20 kHz) can be found in natural creatures or environments, such as the sounds made by bats and dolphins or the ultrasound produced by lightning discharges.

These frequencies, which cannot be heard, have many uses in the earth sciences or industry. Infrasound is often used in meteorological monitoring and is particularly adapted to monitor large natural or engineering phenomena over long distances. Ultrasound is, however, also applied in industry for high-precision detection, small-scale imaging, and medical treatment. The practical applications show that sounds outside the human hearing range can be an effective tool for research projects.

However, when the level of inaudible frequencies or sound pressure level is too high, it may be harmful to human health. Excessively high intensity levels of infrasound below 20 Hz can cause physical discomfort (headache, nausea, etc.). High pressure levels of ultrasound above 20 kHz can cause damage to human cells and tissues. Controlling these frequencies that cannot be heard by the human ear is vital to reduce harmful social effects and to ensure safe operation in sectors such as aerospace, transportation, manufacturing, and healthcare centers. Therefore, developing more efficient noise control strategies, more sophisticated monitoring systems, and safer living environments for the societies are essential.

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