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Humans perceive very low-frequency sounds through a unique electrical mechanism in the inner ear, rather than the mechanical process used for normal hearing. This discovery provides evidence explaining why certain environmental noises, like the hum of a ventilation system, can feel physically intense and grow rapidly in loudness. The findings were recently published in the journal Scientific Reports.
Carlos Jurado of the Norwegian University of Science and Technology and Torsten Marquardt of University College London led the research to understand how the human body detects infrasound. Infrasound refers to sound waves with a frequency below 16 hertz. The term hertz measures how many times a sound wave vibrates per second. Human hearing is conventionally thought to stop around 20 hertz, meaning 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. Hearing normally relies on a tiny, snail-shaped structure in the inner ear called the cochlea. Inside the cochlea, specialized fluid moves in response to incoming sound waves. This fluid movement pushes against microscopic sensory receptors known as hair cells.
The cochlea consists of fluid-filled ducts separated by a flexible partition. High-pitched sounds are detected at the base of this spiral, while low-pitched sounds travel all the way to the deepest point, or apex. At the very top, there is a small opening connecting the fluid ducts. When frequencies drop below 40 hertz, the sound waves simply push fluid back and forth through this opening rather than vibrating the flexible partition.
These tiny cells feature minute, hair-like bundles on top of them that bend when the fluid rushes past. There are two main types of these sensory receptors, and they perform different jobs. Inner hair cells act as the main microphones for the brain. They detect the speed of the moving fluid and send auditory signals through the nervous system.
Outer hair cells act as mechanical amplifiers. They detect the actual distance the fluid moves, which is known as displacement. When they sense this movement, the outer hair cells physically change their own length to tune 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 drops off at a steady mathematical rate as pitches get lower. However, this drop-off abruptly changes its slope below 16 hertz, meaning hearing sensitivity stops decreasing as quickly.
Second, the perceived volume of infrasound grows exceptionally fast with only tiny increases in actual sound pressure. A small bump in decibels, which measures sound intensity, makes an infrasound tone seem massively louder. These observations suggested that a completely different biological process takes over at very low frequencies, prompting the authors to investigate the underlying biology.
To investigate this sensory shift, the scientists designed a series of detailed experiments. In the first test, they recruited eleven human participants with healthy hearing, ranging in age from 25 to 50 years. The scientists measured the participants’ lowest hearing thresholds for specific low-frequency tones played at 5, 15, and 30 hertz.
During the same session, they measured the physical movement inside the participants’ inner ears. They achieved this using a highly sensitive microphone placed securely inside the ear canal. This microphone recorded faint sound echoes naturally produced by the outer hair cells when they react to incoming noise.
By playing the low tones and recording these returning echoes, the researchers could exactly track how much the structures inside the cochlea were bending. The scientists observed that below 15 hertz, the participants’ hearing thresholds perfectly matched the physical displacement of the outer hair cells.
The hearing thresholds did not match the speed of the fluid, which is what the inner hair cells normally rely on. At such extremely low frequencies, the fluid moves too slowly to mechanically trigger the inner hair cells. This finding suggests that the outer hair cells are responsible for catching the sound, but it raised the question of how the signal actually reaches the brain.
Next, the researchers tested exactly how these outer hair cells transmit the auditory signal. They hypothesized that the outer cells generate an electrical field that stimulates the inner cells, bypassing the need for mechanical fluid movement. To test this, they recruited a new group of seven human participants between the ages of 23 and 50.
The subjects listened to very low tones ranging from 4 to 64 hertz. The researchers measured the lowest volume at which the subjects could just barely hear these tones. Then, they played the same low tones at the exact same time as a louder, continuous 500-hertz tone.
The 500-hertz tone was played at 85 decibels, which is roughly as loud as a blender or heavy city traffic. The addition of this louder tone caused the subjects to lose their ability to hear the lowest infrasound tones between 4 and 16 hertz. The participants needed the infrasound tones to be played at a much louder volume to detect them at all.
Interestingly, the 500-hertz tone did not affect their ability to hear the slightly higher tones at 32 hertz and 64 hertz. A 500-hertz sound wave stops traveling through the ear fluid long before it reaches the deepest area of the cochlea where infrasound is processed. Because the physical wave does not reach that area, it cannot mechanically block the low sounds.
To serve as a comparison, the researchers also played a 140-hertz tone at 85 decibels. Unlike the higher tone, the 140-hertz tone masked all the low-frequency sounds from 4 to 64 hertz. The physical wave of the 140-hertz tone travels deep enough into the cochlea to mechanically overlap with the infrasound processing area. This demonstrated a distinct difference between mechanical masking and the unique electrical draining caused by the 500-hertz tone.
The authors deduced that the 500-hertz tone drains the electrical voltage normally generated by the outer hair cells. By depleting this voltage, the louder tone prevents the electrical field from triggering an auditory signal in the deep cochlea. This provides strong evidence that electrical fields, not physical vibrations, mediate infrasound hearing.
To solidify their theory, the researchers built a complex computer model to simulate the electrical properties of the inner ear. They programmed the software to replicate the physical behavior of the hair cells and the electrical voltage of the surrounding bodily fluids. They then fed simulated low-frequency sounds into this virtual biological circuit.
The simulation produced an electrical voltage change around the virtual inner hair cells that was strong enough to trigger a nerve signal. This confirmed that the outer hair cells can create an electrical field capable of activating the inner hair cells without any mechanical fluid movement. The computer model also successfully recreated the human hearing thresholds observed in the real-world acoustic experiments.
People might assume that infrasound is entirely harmless or completely imperceptible because traditional hearing tests do not measure it. This study provides evidence that the body actively processes these extremely low frequencies through a powerful, specialized electrical mechanism. This mechanism causes the perceived loudness of infrasound to escalate rapidly, explaining why things like wind turbines and heavy machinery cause severe annoyance for certain individuals.
The sample sizes in the human experiments were relatively small, restricted to groups of eleven and seven participants. The research also relies heavily on computer modeling to confirm the biological interactions happening deep within the skull. Direct measurements of electrical fields inside a living human cochlea are currently impossible without causing permanent physical damage to the patient.
Future studies will need to explore how this electrical hearing mechanism varies from person to person. Researchers plan to investigate if individual differences in inner ear anatomy make certain people more susceptible to low-frequency environmental noise. Understanding these variations tends to help city planners and engineers design better acoustic environments for the public.
The study, “Infrasound sensation is mediated by intracochlear electrical potentials,” was authored by Carlos Jurado and Torsten Marquardt.