The Sound We Don’t Hear, but the Body Does

When the Ear Is Not the Only Thing Listening

In 1998, British engineer Vic Tandy was working alone at night in a medical equipment laboratory in Warwick. He felt cold, sensed an uncomfortable presence, and saw a grey figure out of the corner of his eye. The next day, while examining a fencing sword clamped in a vice, the blade began to vibrate on its own, without anything touching it. What Tandy discovered was not a ghost: it was an air extractor emitting a standing wave of 18.98 hertz, almost exactly the resonant frequency of the human eyeball. The “haunted laboratory” was basic physics. And the feeling of presence was a bodily response to a frequency that the ear does not encode as sound.

That anecdote, later published in a scientific journal, opened up a question: how much of the acoustic environment affects us without us really knowing at what level? Almost three decades later, the answer is beginning to take shape.

The 20-Hertz Boundary

Below 20 hertz lies infrasound: sound waves that the human ear does not normally encode as audible sound. Alongside it, low-frequency noise, which ranges from 20 to 200 hertz, forms a part of the spectrum with particular properties. It travels farther, passes through walls and windows more easily than mid or high frequencies, and is also transmitted as structural vibration. That is why someone may feel a persistent low hum in their bedroom, even if the decibel meter does not register much.

The relevant scientific question is not whether we “hear” these frequencies, but whether the body registers them when conscious awareness does not.

The Body’s Response

In April 2026, Scatterty and colleagues published an experiment in Frontiers in Behavioral Neuroscience that refined this question considerably. They gathered 36 people in two groups: half listened to a five-minute piece of music with an added 18-hertz infrasound, the same frequency as Tandy’s extractor; the other half listened to it without the added frequency. The participants could not detect the infrasound. But when it was present, measurable changes appeared: they were more irritable, rated the sound experience more negatively, and their saliva samples contained higher cortisol levels, the hormone the body releases in response to demands or threats. Measuring it in saliva is a standard method in stress research: it avoids the needle prick — which can itself trigger cortisol and alter the measurement — and reflects the biologically active fraction of the hormone.

The authors then carried out an intelligent check. When someone feels irritated or uncomfortable, their cortisol can rise for that reason alone. So the question was whether infrasound affected the body directly or only through conscious experience. To separate the two, they compared cortisol levels among people who had reported the same degree of irritability. Even then, those exposed to infrasound had higher levels than those in the group without it. The same pattern appeared when comparing people with the same degree of fear or discomfort. In other words, infrasound added its own effect on the body, not only through the filter of “this bothers me.” The sample is small, but the direction is clear: an inaudible frequency modulates physiology without passing through conscious awareness.

Another Route to the Brain

Two years earlier, a study published in Scientific Reports in October 2024 had already approached the same phenomenon from another angle. Thirty-eight people slept for four weeks, eight hours per night, next to a device: half had one emitting inaudible 6-hertz infrasound; the other half had an identical device that was turned off, without knowing which one they had been assigned. Before and after, their brains were scanned. The infrasound altered several brain networks — those involved in bodily sensation, attention and internal control — but not the auditory network, the one that processes what we hear.

And that detail is highly relevant because, if infrasound produces effects, it is not through hearing, but through another pathway. The ear lets those frequencies pass as if nothing were happening; the rest of the brain does not.

Low Frequencies at a High Cost

Pure infrasound, outside the laboratory, is relatively rare. What is more common in everyday life is low-frequency noise: traffic, air conditioning, machinery, trains, planes, lifts, generators. And here, the evidence is no longer subtle.

The European Environment Agency’s Environmental Noise in Europe 2025 report, published in June 2025, quantifies the problem bluntly. More than 110 million Europeans, over 20% of the population, are exposed to harmful levels of transport noise according to the thresholds set by the European Directive; nearly 150 million, more than 30%, if the stricter WHO recommendations are applied. Prolonged exposure is associated with 66,000 premature deaths each year, 50,000 new cases of cardiovascular disease and 22,000 new cases of type 2 diabetes. The economic cost is around 95.6 billion euros per year: 0.6% of European GDP.

One figure challenges the popular assumption that “if it is not loud, it does not affect us”: adverse cardiovascular effects already appear at 45 dB Lden, far below current regulatory thresholds. Commenting on these findings, cardiologist Thomas Münzel, head of the European Society of Cardiology’s environmental sustainability group, has gone so far as to describe it as a “medical emergency”. This is not a minor nuisance, but a modifiable risk factor, comparable in epidemiological weight to passive smoking or environmental lead exposure.

When the Brain Is on Guard

A review published in Environmental Disease in March 2025 summarises what happens in the body. Night-time noise repeatedly activates the autonomic nervous system, triggers cortisol cascades and promotes low-grade chronic inflammation. Sleep disruption is not a side effect: it is the main bridge between chronic acoustic exposure and cardiovascular disease.

The brain does not switch off during sleep; it filters. A repetitive low frequency that is difficult to locate can fragment rest without fully waking the person. That is why many complaints associated with low-frequency noise are not expressed as “I heard something”, but as “I didn’t rest”, “I woke up tense”, or “there was a strange feeling in the room”. It follows the same logic as Tandy’s laboratory: the body reports first, before the brain manages to name the source.

Questions Still to Be Answered

That said, there are limits. Sensitivity to infrasound varies greatly from person to person, and the effects detected under experimental conditions remain, so far, modest. Some previous studies on prolonged exposure have not found clear cognitive or psychological changes. The open question is not whether infrasound “does something” in general terms, but at what dose, for how long, in which people, and in combination with which other acoustic factors.

Changing the Metric

The practical implication is direct. Hospitals, care homes, schools, offices and residential buildings are still designed and assessed according to audible noise, measured in weighted decibels that systematically underestimate low frequencies. European regulation continues to operate within those parameters, even though the evidence already points to effects below existing thresholds.

Silence is not only the absence of perceived noise. It also means that the nervous system is not responding, in the background, to signals that the mind never processes as sound. Tandy discovered that idea by accident, with a sword vibrating on its own in 1998. Three decades later, physiology and neuroimaging are proving him right.

The question that remains is not whether we build silent buildings. It is whether we build them silent for the body too. And whether our current metrics, designed for what we hear, are enough to address the problem.