Here’s what you’ll learn when you read this story:

• Infrasound waves are usually too low for the human ear to actually hear, but they can still have a negative effect on mood—possibly an explanation for places that feel haunted
• An experiment that tested subjects’ reactions to music found that infrasound made them more irritable and raised stress levels.
• Because infrasound is generated by many urban sources, not just haunted locales, infrasound pollution has become a mental health concern.

Something usually feels off about places rumored to be haunted. Shutters tremble, doors open themselves with a creak of invitation, and shadows appear to move on their own—but at least some of the dread might be caused by a phenomenon that has nothing to do with ghosts.

Whether that haunted feeling happens in an abandoned old mansion or a cemetery that’s been overgrown for the last two centuries, something nameless and oppressive seems to hover over places where the dead supposedly linger. But the real source of that eeriness may not be ghosts at all—according to new research, it’s coming from low-frequency noises known as infrasound. Infrasound is below the decibel range that the human ear can pick up, but these imperceptible sound waves are thought to have an adverse effect on us nevertheless. Psychologists Rodney Schmaltz and Kale Scatterty of MacEwan University in Edmonton, Alberta, Canada, wanted to see whether infrasound is what’s actually been haunting us all along.

“Infrasound has been postulated to cause aversion and feelings of fear in supposedly haunted locations and to contribute to anxiety, distress, and reduced well-being in the vicinity of energy infrastructures such as wind turbines,” they said in a study recently published in Frontiers in Behavioral Neuroscience. “Infrasound also occurs naturally…[and is] prevalent in urban areas.”

Paranormal suspicions aside, the sources of infrasound all exist in the physical world. Volcanoes, hurricanes, and earthquakes (already nightmarish in themselves) generate infrasound waves, which also emerge from traffic, pipes, ventilation, power systems, heaters, and air conditioners. Infrasound may even be lurking beneath live music blaring at concerts. Some animals are already known to have an aversion to infrasound, as Scatterty observed during an earlier experiment with zebrafish that actively avoided it. He and Schmaltz recruited human subjects who listened to ambient music with and without infrasound. Unaware of when the infrasound was introduced, they would describe the differences in how they felt.

The researchers had an idea of what to expect. Hearing infrasound has been said to cause spikes in discomfort and irritability, with some studies suggesting it can induce anxiety, sleep disturbances, and even nausea. Participants would randomly hear either calming instrumentals or horror audio through computer speakers. Music clips would be played alone, then with infrasound being pumped in from hidden subwoofers. Any sounds with infrasonic frequencies had been removed from the music files before the experiment. The listener then rated the intensity of their emotions right after a clip was played. Schmaltz and Scatterty also sampled saliva before and after each clip to test for cortisol, a well-known hormonal indicator of stress.

What they found was that when infrasound was turned on, participants became more irritable, less interested, and perceived the music as sadder than those who were hearing it without infrasound. The results were the same regardless of whether the specific piece of music was a zen track that belonged in a yoga studio or an ominous dirge suitable for the main theme of a slasher film. Levels of cortisol were also found to be higher post-infrasound. While infrasound made both the zen and horror listening experiences more uncomfortable, infrasound reactions were not as intense with calmer music. They researchers also found that unsettling music raised cortisol levels on its own, but adding infrasound amplified the effect. Further analysis revealed that the effects of infrasound on cortisol levels persisted after exposure.

To the relief of horror fans and ghost hunters, infrasound actually evoked annoyance and irritability (at least, in this experiment) more than anything else. There was no proof of it being linked to anxiety, so there must still be something else at play as well—for instance, at least part of the negative aura of a purportedly haunted house may actually come from the rumbling of hidden pipes or the gasping of an ancient heater. But the researchers aren’t focusing their attention on the occasional infrasound anomalies that might make an old house or isolated castle feel haunted. They’re much more concerned about the large number of urban sources of infrasound, since infrasound pollution in such settings is widely detrimental to mental health.

“Infrasound is associated with negatively affective states,” they said. “Considering the prevalence of infrasound in and around human habitats, these findings emphasize the potential value of identifying and mitigating sources of infrasound pollution within our environments.”