Human beings have always been producers of sound. While vocal cords are the primary instruments of sound production, our bodies also generate sonic signals through muscles and other organs. These signals often have frequencies lower than what the human ear can detect.
When you clench your fists and place your thumbs in your ear canals, you might hear a distant rumbling, akin to distant thunder. This low hum is the result of muscle contractions. Such sounds, with frequencies below 20 Hz, are termed infrasonic vibrations. While these frequencies are generally inaudible, their vibrational energy can be sensed.
Origins of Infrasonic Vibrations
The human body’s production of infrasonic vibrations can be attributed to various physiological processes. These include heartbeats, blood flow, and the rhythmic movements associated with breathing. Among these, the infrasonic vibrations produced by muscles have been studied most extensively. Pioneering work in the 1950s by G. Oster and J. S. Jaffe, using transistorized stethoscopes, revealed the effects of different factors on the sonic signals produced by muscles. For instance, increasing the load on muscles led to more intense sounds. Interestingly, the blood supply to the muscles didn’t significantly influence the sound’s tonal qualities, suggesting that sonic signal generation is an inherent property of skeletal muscles.
Historically, the phenomenon of muscles producing infrasonic waves was first documented by F. M Grimaldi in 1665. Later studies by Herroun and Yeo found that electrical stimulation of muscles generated sound waves similar to those produced by voluntary movement.
Other Sources of Infrasonic Vibrations
Muscles aren’t the only source of infrasonic vibrations in humans. Given the complexity of the human body, with its myriad of active micro-units, various organs produce these vibrations. For instance, the heart resonates at a frequency close to 1 Hz, while the resonance frequency of blood circulation is even lower. The human brain, with its intense neuronal activity, has a resonance frequency estimated around 10 Hz.
Applications and Implications
Modern science has enabled the transformation of brain oscillations into audible sound. Instruments like the “brain stethoscope” can convert brain waves, typically recorded on an EEG strip, into sound waves. Such tools can detect potential seizures, especially the silent ones that lack the usual violent manifestations.
In the animal kingdom, infrasonic signals are a well-established communication method. Birds like pigeons use these signals for orientation and migration, while mammals like elephants and whales use them for long-distance communication.
Environmental Infrasound
Both natural and man-made sources contribute to environmental infrasound. Natural sources include wind, marine storms, geomagnetic activity, and seismic phenomena. Man-made sources encompass diesel engines, airplanes, wind turbines, and more. The interference of external infrasonic signals with the body’s inherent signals can lead to various medical conditions. Prolonged exposure to low-frequency noise has been linked to symptoms like nausea, mood changes, and even a condition called vibroacoustic disease.
However, not all effects of external infrasonic vibrations are negative. Low-frequency sound signals have therapeutic applications, especially in treating muscular disorders and chronic pain.
While the study of sound has been extensive, the realm of infrasonic vibrations remains relatively unexplored. These vibrations offer a promising research area, potentially revealing unknown facets of our body’s workings. Sound therapy aims to harness these vibrations, providing protection against the potential harms of prolonged exposure to environmental low-frequency noise.
References:
Gonzalez-Gonzalez, S. (2017). [The role of mitochondrial oxidative stress in hearing loss](https://dx.doi.org/10.15761/NDT.1000117). This study discusses the role of oxidative stress in hearing loss and the impact of infrasonic and ultrasonic frequencies on the human body.
