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You can't hear it, yet this sound may explain paranormal experiences - medicalxpress

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Infrasound is very low-frequency sound, below 20 Hertz (Hz), which humans typically can't hear. It can come from natural sources like storms, or from anthropogenic sources like traffic. Some animals use it to communicate, while others avoid it. Scientists investigating humans' ability to sense infrasound determined that we can't detect it, but we do respond to it: it's linked to increased irritability and higher cortisol levels.

"Infrasound is pervasive in everyday environments, appearing near ventilation systems, traffic, and industrial machinery," said Prof Rodney Schmaltz of MacEwan University, senior author of the article in Frontiers in Behavioral Neuroscience.

"Many people are exposed to it without knowing it. 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.

"Consider visiting a supposedly haunted building. Your mood shifts, you feel agitated, but you can't see or hear anything unusual. 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.

"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."

Sound of the underground

The scientists recruited 36 participants and invited them to sit alone in a room while either calming or unsettling music was played. For half the participants, hidden subwoofers played infrasound at 18 Hz. After listening, they were asked to report their feelings, their emotional rating of the music, and whether they thought the infrasound was present. They also gave saliva samples before and after listening.

The scientists found that participants' salivary cortisol levels were higher if they had been listening to infrasound. These participants also reported feeling more irritable and less interested, and thinking the music was sadder. But they couldn't tell they were listening to infrasound.

"This study suggests that the body can respond to infrasound even when we can't consciously hear it," said Schmaltz. "Participants could not reliably identify whether infrasound was present, and their beliefs about whether it was on had no detectable effect on their cortisol or mood."

"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," said Kale Scatterty, first author and Ph.D. student at the University of Alberta. "But infrasound exposure had effects on both outcomes that went beyond that natural relationship."

Felt but not heard

These results indicate that humans can sense but not identify infrasound, though the mechanism remains unclear. They also suggest we may need to investigate whether prolonged infrasound exposure could impact health through consistently elevated cortisol levels and well-being issues related to lowered mood and increased irritability.

"Increased cortisol levels help the body respond to immediate stressors by inducing a state of vigilance," said Prof Trevor Hamilton of MacEwan University, corresponding author.

"This is an evolutionarily-adapted response that helps us in many situations. However, prolonged cortisol release is not a good thing. It can lead to a variety of physiological conditions and alter mental health."

Because the sample was comparatively small, the scientists carried out sensitivity analyses before drawing conclusions from their results.

They confirmed that their study could detect moderate to large effects of infrasound, which includes their main findings. However, more research with greater, more diverse participant samples will be needed to fully understand how infrasound influences human emotion and behavior.

"This study was in many ways a first step towards understanding the effects of infrasound on humans," cautioned Scatterty. "So far, we've only tested a specific frequency. There could be many more frequencies and combinations that have their own differential effects. We also only collected subjective reports of how the participants felt after exposure, without directly observing their responses during the trial."

"The first priority would be testing a wider range of frequencies and exposure durations," added Schmaltz. "Infrasound in real environments is rarely a single clean tone, and we don't yet know how different frequencies or combinations affect mood and physiology. If those patterns become clearer, the findings could eventually inform noise regulations or building design standards.

"As someone who studies pseudoscience and misinformation, what stands out to me is that infrasound produces real, measurable reactions without any visible or audible source. So, the next time something feels inexplicably off in a basement or old building, consider that the cause might be vibrating pipes rather than restless spirits." 

Provided by Frontiers 

by Frontiers

edited by Sadie Harley, reviewed by Robert Egan 

Source: You can't hear it, yet this sound may explain paranormal experiences