Have you ever entered a place that doesn't feel quite right? While a supernatural explanation may seem more exciting, there could be another reason for that chill down your spine.
It's called infrasound, and a new study reveals how this phenomenon has a very real impact on human stress levels and irritability – at least in the short-term.
"Consider visiting a supposedly haunted building. Your mood shifts, you feel agitated, but you can't see or hear anything unusual," says the study's senior author Rodney Schmaltz, a psychologist at MacEwan University in Canada.
"In an old building, there is a good chance that infrasound is present, particularly in basements where aging pipes and ventilation systems produce low-frequency vibrations," Schmaltz continues.
"If you were told the building was haunted, you might attribute that agitation to something supernatural. In reality, you may simply have been exposed to infrasound."
Any sounds below the threshold at which humans can consciously perceive them – around 20 hertz or lower – are considered infrasound. These sounds have a remarkable ability to travel past obstacles without dissipating, so their influence is wide.
They can be generated by old pipes and machinery – hence the haunted building link – but also emanate from natural sources like storms, earthquakes, volcanoes, and aurorae. Some animals even incorporate infrasound into their communication and navigation.
Infrasound is widespread in newer built environments, too. It can be produced by traffic, industrial machinery, and ventilation systems, all of which are pervasive in modern life. Not all infrasound is necessarily a problem, however: it's a broad category, and nuance is important.
But its ubiquity does require scientists take infrasound seriously. We're exposed to it quite often, so if certain frequencies do indeed put our bodies into a state of high-alert, it's worth investigating any potential consequences over the long-term.
Schmaltz and the team of researchers from MacEwan University enlisted 36 undergraduate students into an experiment to test the effects of infrasound on human stress levels.
These participants sat alone in a room, listening to either calming or unsettling music. For half of these participants, hidden subwoofers also played infrasound at 18 hertz, unbeknownst to them.
After the listening session, participants filled out a survey about how they felt the music affected them. They also gave swabs of their saliva before and after the session, which the researchers used to test levels of cortisol, a biomarker for stress in the human body.
It's a pretty small sample size, and most of the participants were women. While a larger study with more participants is needed to generalize the results further, they are interesting nonetheless.
"Our findings suggest that even a brief exposure may shift mood and raise cortisol, which highlights the importance of understanding how infrasound affects people in real-world settings," says Schmaltz.
Participants exposed to infrasound reported feeling more irritable, uncomfortable, and perceived the music as sadder – even if they'd listened to the calming music. They were also unable to detect whether the sub-sonic vibrations were present or not.
Previous experiments have already suggested infrasound could have this bone-chilling effect, but what the new study adds is the relationship with cortisol, which increased significantly in the presence of infrasound.
"Increased irritability and higher cortisol are naturally related, because when people feel more irritated or stressed, cortisol tends to rise as part of the body's normal stress response," says University of Alberta behavioral neuroscientist Kale Scatterty, the study's first author.
"But infrasound exposure had effects on both outcomes that went beyond that natural relationship."
It's normal for cortisol levels to become elevated from time to time – as a stress hormone, it's meant to warn us of potential danger so we can react.
There are theories that some animals react to the infrasound from natural events like earthquakes and tsunamis before real disaster strikes.
Perhaps there's a similar reason why humans have such negative reactions to infrasound: it could be something we're hardwired to avoid for our own safety.
The findings may help us better understand the causes behind seemingly paranormal activity, but could also inform noise regulations and building design standards.
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"This study was in many ways a first step towards understanding the effects of infrasound on humans," cautions Scatterty.
"So far, we've only tested a specific frequency. There could be many more frequencies and combinations that have their own differential effects."
The research is published in Frontiers in Behavioral Neuroscience.