The sound you can't hear
In a lab in Alberta, researchers put people in a room, played them music, and hid subwoofers where nobody could see them. Through those subwoofers ran a tone at 18 Hz — below the floor of human hearing. Then they measured everyone's stress hormones. Not one participant could reliably say whether the tone was on. Their cortisol went up anyway.
The study that should change how you think about rooms
Published in Frontiers in Behavioral Neuroscience in April 2026, the experiment was elegantly simple. Thirty-six people sat alone in a room. Some heard calming music, some heard unsettling music. For half of them, hidden subwoofers also produced infrasound at 18 Hz — low enough to sit beneath the threshold of conscious hearing.
Participants were asked afterwards whether they thought low-frequency sound had been present. They could not tell. Their guesses were no better than chance.
Their bodies told a different story. Salivary cortisol — the standard laboratory read on the stress response — was higher when the infrasound was running. Participants also reported more irritability, more disinterest and more sadness, across both the calming and the unsettling music conditions. And critically, the researchers found no expectancy effect: this was not people talking themselves into feeling bad because they suspected a hidden speaker. The people who never flagged anything showed it too.
Why this isn't exotic — it's your office
The instinct is to file infrasound under laboratory curiosity. That would be the wrong conclusion. As Professor Rodney Schmaltz of MacEwan University put it, infrasound "is pervasive in everyday environments, appearing near ventilation systems, traffic, and industrial machinery."
Read that list again with your own life in mind:
| Source | Where you meet it |
|---|---|
| Ventilation & HVAC | Offices, hotels, hospitals, gyms, any large building — often running all night |
| Traffic | Flats on main roads, anywhere near a motorway or flyover |
| Industrial machinery | Plant rooms, lift motors, basements, buildings with commercial units below |
| Plumbing | Vibrating pipework — the study's own authors singled this out |
None of these announce themselves. That is exactly the point. A noisy road you can hear is a problem you can name and solve. A 15 Hz pressure wave from a plant room three floors down is a problem you will describe as "I just don't sleep well in this flat" for two years.
The haunted house footnote
The researchers themselves made a connection that is hard to resist. Reports of unexplained dread in particular rooms — the ones that get filed under the supernatural — may sometimes have an acoustic explanation. Frontiers headlined its own write-up with the line that the cause "might be vibrating pipes rather than restless spirits."
It is a great story. It is also a serious point wearing a costume. If a stimulus can reliably produce unease in people who cannot detect it, then "this room feels wrong" stops being a purely psychological report and starts being, sometimes, a measurement.
The other direction: sound that pulls you down
Here is the genuinely useful half. If inaudible sound can push your physiology toward stress, audible sound can be chosen to push it the other way — and that research is further along.
A 2025 cross-over study in Psychophysiology took 53 healthy participants and gave them ten minutes of either a nature-based soundscape or a reference soundscape. The nature condition produced improved heart rate variability and lower heart and respiratory rates — the signature of increased parasympathetic activity, the "rest and digest" side of the nervous system. Participants also reported less anxiety and low mood, and more comfort, enthusiasm and creativity.
Earlier work on recovery is even more pointed. After a psychological stressor, participants' sympathetic activation subsided roughly 9 to 37 percent faster while listening to nature sounds than while listening to noise. The stressor was the same. The recovery curve was not.
Broader reviews of sound interventions converge on a similar picture: music — especially classical or self-selected — reduces physiological stress markers including cortisol, heart rate variability measures and blood pressure. Nature sounds and calming voices show real potential, though that literature is thinner.
What the evidence does not say
We would rather you trusted us in five years than clicked today, so here is the unglamorous part.
The infrasound study is one study. Thirty-six participants, one frequency, a brief exposure, a single laboratory. It is well-designed and it controlled for expectancy, which many earlier attempts did not. It is not the final word, and it does not establish that everyday infrasound exposure causes lasting harm.
The nature-sound findings are promising, not settled. Reduced heart rate and increased parasympathetic activity after natural versus urban sound have been reported in several studies — and other studies have failed to replicate them. A 2025 paper found forest soundscapes improved mood, restoration and cognition relative to industrial soundscapes but did not shift physiological stress or immune markers. Both of those things are true at once.
None of this is medicine. Sound is a supportive tool. It is not a treatment for anxiety, insomnia, depression or any other condition, and nothing here should delay a conversation with a doctor.
What survives the caveats is still worth having: the acoustic environment you sit in is not neutral background. It is an input. You have more control over it than you are currently using.
What to actually do about it
Treat "this room feels bad" as information. If there is a space where you consistently feel wrung out and you have never been able to say why, look at what is mechanically near it. Plant rooms, lift shafts, ducting, a road, a basement gym. You are not being fanciful.
Notice the all-night sources. Eight hours beside a wall carrying HVAC or pipe vibration is a long exposure to something you never consciously register.
Mass beats foam. Low frequencies laugh at acoustic foam, which is designed for high-frequency reflections. Distance, dense material and moving the bed to a different wall do more than anything you can stick to a surface.
Give the room something better to be full of. This is where a deliberate soundscape earns its place — not as a cure, but as a way of choosing your acoustic environment instead of inheriting it. Steady, broadband, low-information sound gives your attention something neutral to rest on and covers the small irregular events that keep you subtly vigilant.
Pick sound that doesn't loop. A repeating track is itself a low-grade signal: the same passage returning every few minutes is a pattern, and patterns invite tracking. Sound that never repeats gives your attention nothing to latch onto — which is the entire reason EverLull generates every sound live in your browser rather than playing files.
Choose what your room is full of
Rain, ocean, wind and brown noise, generated live and never repeating — free to start, no account needed, and it works offline.
Start listening freeQuestions people ask
What is infrasound?
Sound below roughly 20 Hz — beneath the bottom edge of human hearing. You don't perceive it as a tone. It comes from ventilation systems, traffic, wind turbines, industrial machinery, large fans and vibrating pipes, which makes it ordinary rather than exotic.
Can sound you can't hear actually affect you?
A 2026 study in Frontiers in Behavioral Neuroscience found that it can. Researchers played 18 Hz infrasound through hidden subwoofers while participants listened to music. Nobody identified it above chance, but salivary cortisol was higher when it was present, and people reported more irritability, disinterest and sadness. There was no expectancy effect — it showed up in people who never suspected a thing.
Why do some rooms just feel wrong?
Sometimes it is genuinely acoustic. Low-frequency energy from plant rooms, ducts and vibrating pipes can raise stress markers without ever registering as a sound you notice. The authors of the 2026 study suggested this may explain some reports of unexplained unease in particular buildings — vibrating pipes rather than restless spirits.
What kind of sound reduces stress?
Nature-based soundscapes have the strongest support. A 2025 cross-over study found they improved heart rate variability and lowered heart and respiratory rates, indicating greater parasympathetic activity, alongside lower reported anxiety. Earlier work found recovery from a stressor ran roughly 9–37% faster during nature sounds than during noise. Promising — but not unanimous, as some studies have not replicated the physiological effects.
Does listening to calming sound lower cortisol?
Reviews of sound interventions report that music, especially classical or self-selected, can reduce physiological stress markers including cortisol, HRV measures and blood pressure. Nature sounds and calming voices show potential too, though that research is thinner. Sound is a supportive tool for stress — not a treatment for a medical condition.
- Schmaltz et al. (2026). Infrasound exposure is linked to aversive responding, negative appraisal, and elevated salivary cortisol in humans. Frontiers in Behavioral Neuroscience.
- Frontiers (2026). 'The cause might be vibrating pipes rather than restless spirits'.
- Kumpulainen et al. (2025). Enhancing Psychophysiological Well-Being Through Nature-Based Soundscapes: An Examination of Heart Rate Variability in a Cross-Over Study. Psychophysiology.
- Stress Recovery during Exposure to Nature Sound and Environmental Noise. Int. J. Environ. Res. Public Health.
- Effects of Sound Interventions on the Mental Stress Response in Adults: Scoping Review. JMIR Mental Health.
- Forest soundscapes improve mood, restoration and cognition, but not physiological stress or immunity, relative to industrial soundscapes. Scientific Reports.