Humans perceive infrasound through a unique electrical mechanism in the inner ear, rather than the mechanical processes used for normal hearing. This finding provides evidence to explain why certain environmental noises, such as the noise of ventilation systems, are felt to be physically intense and increase rapidly in volume. The results of this study were recently published in the journal scientific report.
Carlos Jurado of the Norwegian University of Science and Technology and Torsten Marquardt of University College London led a study to understand how the human body detects infrasound. Infrasound refers to sound waves with frequencies below 16 hertz. The term Hertz measures how many times a sound wave vibrates in one second. Human hearing is traditionally thought to stop at around 20 hertz, meaning that sounds below this threshold are often considered inaudible.
Despite this assumption, people can actually perceive these very low frequencies if the volume is loud enough. Normally, hearing relies on a small snail-shaped structure called the cochlea in the inner ear. Inside the cochlea, special fluids move in response to incoming sound waves. This fluid movement pushes against microscopic sensory receptors known as hair cells.
The cochlea consists of fluid-filled tubes separated by flexible septa. High tones are detected at the root of this spiral, while low tones travel to the deepest point, the apex. At the top there is a small opening that connects the fluid duct. At frequencies below 40 hertz, the sound waves simply force fluid back and forth through this opening, rather than vibrating the flexible septum.
These tiny cells have microscopic hair-like tufts on their tops that bend as fluid rushes through them. There are two main types of these sensory receptors, each of which has a different function. Inner hair cells act as the brain’s main microphones. It detects the velocity of fluid movement and sends auditory signals through the nervous system.
Outer hair cells act as mechanical amplifiers. Finds the actual distance traveled by the fluid, known as the displacement. Upon sensing this movement, the outer hair cells physically change their length to adjust the ear’s sensitivity and amplify quiet sounds.
The authors noticed two unexplained phenomena regarding how humans experience infrasound. First, the ear’s sensitivity to sound decreases at a constant mathematical rate as the pitch decreases. However, this decline has a sharp slope change below 16 hertz, meaning that the decline in hearing sensitivity quickly stops.
Second, the perceived loudness of infrasound increases very quickly despite only small increases in actual sound pressure. Small changes in the decibels that measure sound intensity can make infrasounds seem much louder. These observations suggest that a completely different biological process takes over at a very low frequency, prompting the authors to investigate the underlying biology.
To investigate this change in sensation, scientists designed a series of detailed experiments. The first test recruited 11 human participants between the ages of 25 and 50 with healthy hearing. The scientists measured participants’ minimum hearing thresholds to specific low-frequency sounds played at 5, 15, and 30 hertz.
During the same session, we measured physical movements within the participants’ inner ears. They achieved this using a highly sensitive microphone placed firmly within the ear canal. The microphone recorded the faint sound echoes that are naturally produced when outer hair cells respond to incoming noise.
By playing bass sounds and recording the returning echoes, the researchers were able to precisely track how curved the structures within the cochlea were. The scientists observed that below 15 hertz, the participants’ hearing thresholds perfectly matched the physical displacement of the outer hair cells.
Hearing thresholds did not match the fluid velocity on which inner hair cells normally depend. At these very low frequencies, the fluid moves too slowly to mechanically trigger the inner hair cells. This finding suggests that outer hair cells are responsible for catching sound, but raised the question of how the signals actually reach the brain.
The researchers then tested exactly how these outer hair cells transmit auditory signals. They hypothesized that the outer cells generate an electric field that stimulates the inner cells, circumventing the need for mechanical fluid movement. To test this, they recruited a new group of seven human participants, ranging in age from 23 to 50.
Subjects heard very low tones ranging from 4 hertz to 64 hertz. The researchers measured the lowest volume at which subjects could barely hear these sounds. The same bass tone was then played at exactly the same time as a louder, continuous 500-hertz tone.
A 500 hertz tone is played at 85 decibels, which is about as loud as a mixer or city traffic. This addition of louder sounds caused the subjects to lose the ability to hear the lowest infrasound, between 4 and 16 hertz. Participants had to play the infrasound at a louder volume to detect it.
Interestingly, the 500 hertz sound had no effect on their ability to hear slightly higher tones at 32 hertz and 64 hertz. At 500 hertz, sound waves stop passing through the ear fluid long before they reach the deepest region of the cochlea, where infrasound is processed. Since physical waves do not reach there, bass cannot be mechanically blocked.
For comparison, the researchers played a 140 hertz tone at 85 decibels. Unlike high frequencies, the 140 hertz sound masks all low frequency sounds from 4 to 64 hertz. The physical waves of the 140 Hertz sound travel deep into the cochlea and mechanically overlap the infrasound processing area. This demonstrated a clear difference between mechanical masking and the unique electrical emissions caused by the 500 Hertz tone.
The authors speculated that the 500 hertz tone depletes the voltage normally produced by outer hair cells. Depleting this voltage prevents louder sounds from causing electric fields to trigger auditory signals deep in the cochlea. This provides strong evidence that electric fields, rather than physical vibrations, mediate infrasound hearing.
To solidify their theory, the researchers built a complex computer model that simulates the electrical properties of the inner ear. They programmed the software to reproduce the physical behavior of hair cells and the voltage of the surrounding body fluid. Next, they pumped simulated infrasound into this virtual biological circuit.
The simulation created voltage changes around the virtual inner hair cells that were strong enough to trigger a neural signal. This confirmed that outer hair cells can generate an electric field that can activate inner hair cells without mechanical fluid movement. The computer model was also successful in reproducing human hearing thresholds observed in real-world acoustic experiments.
Because they cannot be measured with traditional hearing tests, people may think that infrasound is completely harmless or completely undetectable. This study provides evidence that the body actively processes these extremely low frequencies through powerful and specialized electrical mechanisms. This mechanism causes the perceived loudness of infrasound to increase rapidly, explaining why things like wind turbines and heavy machinery can cause severe discomfort to certain people.
The sample size for the human experiment was relatively small, limited to 11 participants and groups of 7 people. The study also relies heavily on computer modeling to see the biological interactions occurring deep within the skull. It is currently not possible to directly measure the electric field within the cochlea of a living human without causing permanent physical damage to the patient.
Future research will need to investigate how this electrical hearing mechanism differs from person to person. Researchers plan to investigate whether individual differences in the anatomy of the inner ear make certain people more susceptible to low-frequency environmental noise. Understanding these changes can help urban planners and engineers design better acoustic environments for the public.
The study, “Infrasound sensation is mediated by electrical potentials in the cochlea,” was authored by Carlos Jurado and Torsten Marquardt.
