Human hearing does not end at a hard wall marked 16 or 20 hertz. Sensitivity falls sharply as frequency drops, but sufficiently intense infrasound can still be perceived, and experiments have repeatedly shown that very-low-frequency signals reach the human auditory system.
One of the clearest demonstrations came from research involving Germany’s Physikalisch-Technische Bundesanstalt (PTB). A multimodal experiment using individually calibrated tones, magnetoencephalography and functional magnetic resonance imaging examined frequencies reaching down to 8 Hz and found activity associated with auditory processing even in that infrasonic range.
More recent work has gone further. A 2026 Scientific Reports study by Carlos Jurado of the Norwegian University of Science and Technology and Torsten Marquardt of University College London describes a physiological mechanism that may explain how sounds below 16 Hz reach auditory sensation even when the cochlea is operating differently from the way it does for ordinary tones.
What the PTB-linked experiments actually measured
The experiments were designed to separate genuine responses to infrasound from the higher-frequency harmonics that can contaminate very-low-frequency sound sources. A 2015 MEG and fMRI study presented by PTB and collaborating researchers used sinusoidal stimuli at 8, 12, 20, 40, 63, 125 and 250 Hz, individually calibrated for 15 volunteers.
That work placed the response inside the auditory system rather than treating infrasound only as vibration felt elsewhere in the body. Its authors reported brain activation in the auditory-cortex region during the low-frequency and infrasonic stimulation.
A later PLOS ONE experiment tested 20 normal-hearing participants with 32 Hz and 8 Hz tones while measuring loudness, unpleasantness and fMRI responses. The estimated average detection threshold for the 8 Hz stimulus was about 104 dB SPL, with individual thresholds ranging from roughly 90 to almost 120 dB SPL.
The brain response depended strongly on level. At an average 8 Hz level of about 111 dB SPL, only small clusters appeared under the study’s more permissive statistical threshold; at the high-intensity condition, averaging about 138 dB SPL, large regions of primary and secondary auditory cortex were activated.
How the cochlea may register sound below 16 hertz
For ordinary sound, inner hair cells in the cochlea are central to converting mechanical motion into neural signals. Their mechanical input is strongly related to velocity, and that route becomes progressively less effective as frequency falls toward the infrasonic range.
The 2026 Jurado–Marquardt study found evidence that another part of the cochlear machinery becomes increasingly important at those frequencies. Their human measurements indicated that displacement-coupled outer hair cells begin contributing to neural excitation as the velocity component that normally drives inner hair cells becomes very small.
The authors then proposed an electrical model for the next step. In that model, voltage changes generated by outer hair cells alter the membrane potential of nearby inner hair cells strongly enough to plausibly cause synaptic release and ultimately auditory-nerve activity.
That distinction matters. The study supports the involvement of outer hair cells experimentally, but the complete path from those electrical potentials to synaptic release is presented as a physiological model rather than a directly observed chain of events.
Why infrasound does not feel like an ordinary note
The conventional lower edge of hearing is partly a question of perception rather than the point at which the auditory system becomes absolutely unresponsive. The Scientific Reports paper notes that sound below 16 Hz lacks normal tonality, and below roughly 12 Hz the cycles of a steady tone can be perceived as separate events rather than as one continuous pitch.
The threshold curve changes there as well. Between roughly 40 and 16 Hz, the sound pressure required for sensation rises very steeply as frequency decreases, while below 16 Hz the slope becomes shallower, a pattern the authors connect to a change in the cochlear transduction mechanism.
Once infrasonic sound becomes audible, however, loudness can rise unusually fast. Both the 2026 work and the earlier PLOS experiment found that comparatively small increases in sound pressure above threshold can create much larger increases in perceived intensity than listeners expect from ordinary-frequency sound.
There is also substantial person-to-person variation. In the PLOS study, listeners differed markedly in 8 Hz detection threshold, loudness growth and unpleasantness, but the researchers did not identify a population-level neural marker that explained why one participant was more sensitive than another.
Why environmental complaints are a separate question
The laboratory result therefore does not mean that every low-frequency environmental source produces an audible 8 Hz signal. Infrasound perception requires high sound pressure, and the relationship between a calibrated pure tone delivered in an experiment and the complex spectrum produced by machinery outdoors is not one-to-one.
That distinction is especially important in the wind-turbine debate. One field investigation summarized in an open-access review of wind-turbine infrasound measured indoor levels of about 58 to 60 dB(G) in three homes and placed them roughly 25 dB below the perception level used in that analysis.
Other experiments nevertheless confirm that people can hear infrasound when its level is high enough. A 2009 fMRI study, for example, found auditory-cortex activation during 12 Hz stimulation at high sound-pressure levels, adding to the evidence that infrasonic frequencies are not categorically excluded from human hearing.
Instruments can detect such waves at levels and distances that say nothing by themselves about human audibility. The Comprehensive Nuclear-Test-Ban Treaty Organization’s global infrasound network uses arrays of pressure sensors to detect ultra-low-frequency waves travelling through the atmosphere, including signals from explosions and natural events far beyond the range of ordinary human perception.
What brain activation does not prove
A lit-up auditory cortex is evidence that the nervous system is processing a stimulus. It is not by itself evidence that the stimulus has damaged tissue, disturbed sleep, produced a headache or caused any other clinical effect.
The distinction has been visible throughout the PTB-linked research program. A 2017 PLOS ONE study found changes in brain connectivity during near-threshold 12 Hz exposure, including regions associated with auditory, emotional and autonomic processing, but its authors described possible health implications as speculative and explicitly called for further research.
The 2026 cochlear study is similarly careful. Its mechanism may help researchers understand why very-low-frequency environmental sounds generate complaints, but it did not test whether heat pumps, ventilation systems, industrial machinery or wind turbines cause disease through that mechanism.
Nor has the work yet shown that the large differences between sensitive and insensitive listeners arise from a particular anatomical difference in their outer hair cells. The experiments establish variation in perception; the biological source of that variation remains an open problem.
The old hearing boundary is becoming a transition zone
The useful change in the science is therefore narrower and more interesting than saying that textbooks were simply wrong. The lower boundary of hearing is not a switch that turns off at one frequency. It is a region where sensitivity falls, tonality disappears and the cochlea appears to rely increasingly on a different physical route.
That fits a broader pattern in hearing, where the route taken by a signal matters as much as its frequency. Bone conduction, for example, helps explain why a recorded voice sounds different from the voice heard inside one’s own head.
Low frequencies also occupy sensory worlds far beyond human speech. Elephants use extremely low-frequency signals and ground vibration, while whale calls exploit the long-distance propagation of sound underwater. Human infrasound sensitivity is far poorer, but it belongs on the same physical continuum.
An 8 Hz wave completes one cycle every 125 milliseconds. At ordinary levels it may pass without becoming a conscious sound, but raise the pressure far enough and the cochlea begins to register it, the cortex begins to respond, and a frequency once drawn outside the neat border of hearing is no longer quite outside at all.