Theory Proposes How The Ear May Detect Sound Below The Edge Of Hearing

Scientists Propose a Hidden Electrical Pathway for Sensing Ultra-Low-Frequency Sound

Most people assume that if a sound is too low to register as a tone, it simply isn’t heard. That isn’t always true. A new study in Scientific Reports proposes a biological explanation for how humans perceive infrasound, sound so low in pitch, below 16 vibrations per second, that it produces no recognizable musical note.

Rather than going undetected, this kind of sound may rely on a different pathway inside the ear once frequency drops below that threshold. Electrical signals deep within the cochlea appear to play a growing role in triggering nerve activity as tones get lower. Researchers Carlos Jurado of the Norwegian University of Science and Technology and Torsten Marquardt of University College London’s Ear Institute propose that this shift could eventually help explain why very low-frequency environmental noise provokes such different reactions in different people.

For decades, scientists knew infrasound could be felt at high volumes even without a musical pitch, but no one had explained how the ear picks it up. Jurado and Marquardt set out to solve that using non-invasive tests on volunteers, tackling two long-standing oddities in low-frequency hearing.

Below 16 Hz, the Ear’s Sensitivity Barely Fades

Inside the inner ear sits the cochlea, a spiral-shaped structure that converts vibrations into nerve signals. It holds two types of sensory cells: one responds to fluid speed, the other to how far the cochlea’s internal partition physically shifts. For everyday sounds, the speed-sensitive cells do most of the work of triggering what the brain registers as sound.

Two patterns in past research on low-frequency hearing had never been fully explained. First, as pitch drops below 16 cycles per second, the extra volume needed to make a tone perceptible does not climb as steeply as it does at slightly higher pitches. Second, once infrasound becomes perceptible, its loudness shoots up rapidly with only a small increase in volume, growing irritating fast. Both hinted that something besides the everyday detection pathway was at work.

Hearing and Cochlear Movement Track Each Other Below 15 Hz

In the first experiment, 11 participants ages 25 to 50 had hearing thresholds measured for tones at 5, 15, and 30 Hz. Researchers used changes in an otoacoustic emission, a faint sound the healthy ear naturally produces, as an indirect measure of relative cochlear-partition displacement, adjusting each tone until it suppressed that emission by a fixed amount.

Below about 15 Hz, the hearing-threshold curve and the displacement-linked measure followed nearly the same slope. Above 15 Hz, the hearing threshold became much steeper, while the displacement measure continued along roughly the same path. The researchers interpreted that split as evidence that ordinary movement-based stimulation of one type of sensory cell dominates above 15 Hz, while a displacement-linked electrical mechanism becomes more important below it.

A 500 Hz Tone Selectively Blunted Perception of the Lowest Frequencies

In the second experiment, seven participants detected tones from 4 to 64 Hz under three conditions: the target tone alone, with a 140 Hz masking tone, and with a 500 Hz masking tone, both maskers at 85 decibels.

Detection thresholds rose across the board with the 140 Hz tone, consistent with ordinary masking, where a louder nearby sound drowns out a quieter one. With the 500 Hz tone, results looked different: thresholds rose specifically for tones at 4, 8, and 16 Hz, with only small, statistically insignificant shifts at 32 and 64 Hz. Average increases for the affected frequencies ranged from roughly 3 to nearly 6 decibels. A 500 Hz tone doesn’t physically travel far enough into the cochlea to interfere with very low frequencies through ordinary masking, so researchers looked to earlier animal studies, which showed that a 500 Hz tone reduces electrical potentials the cochlea generates in response to infrasound. Human results were consistent with that electrical-suppression explanation, though the experiment did not directly measure those potentials in people.

A computer model of the cochlea’s electrical wiring tested whether the proposed mechanism could plausibly produce the observed patterns. It reproduced the flattening of the hearing-threshold curve below 16 Hz and lined up with electrical measurements from earlier animal research, supporting the idea that electrical potentials from the displacement-sensitive cells become the dominant source of nerve excitation at these very low frequencies. The exact sequence, in which those potentials might influence the other cell type and trigger nerve signaling, was illustrated by the model rather than recorded directly in the human ear.

Environmental Noise’s Mix of Frequencies May Explain Why It Bothers Some People More

Higher-frequency components within a complex sound, the authors suggest, might suppress some of the electrical activity tied to infrasound perception, offering a possible reason why the makeup of environmental noise, not just its volume, may affect how disruptive it feels. That extends naturally to a question researchers have puzzled over for years: why does a low rumble drive one person up the wall while someone standing right next to them barely notices?

The mechanism gives that question a plausible physiological angle for the first time, though the study did not test wind turbines, industrial sites, traffic noise, or complaint patterns, and the authors say confirming any link to individual noise sensitivity would take dedicated experiments of its own.

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Disclaimer: This article summarizes findings from a peer-reviewed study and is intended for general informational purposes. It is not medical advice. Anyone concerned about their hearing or their sensitivity to environmental noise should consult a qualified health professional.

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