Very low-frequency sounds, which humans cannot hear but can lead to high stress hormone levels in the body, could be an explanation for “haunted” locations, a new study finds.
Such sounds below 20 Hertz (Hz) come from natural sources like storms or from human-made sources like traffic, and can cause increased irritability and higher cortisol hormone levels in the body.
Now, a new study has found that even brief exposure to such infrasound may shift mood and raise cortisol levels.
“Consider visiting a supposedly haunted building. Your mood shifts, you feel agitated, but you can't see or hear anything unusual,” said Rodney Schmaltz, an author of the research published in the journal Frontiers in Behavioral Neuroscience.
In old buildings, infrasound is likely present, especially in basements where ageing pipes and ventilation systems produce low-frequency vibrations, researchers say.
“Infrasound is pervasive in everyday environments, appearing near ventilation systems, traffic, and industrial machinery,” Dr Schmaltz said.
“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,” he explained.
In the latest study, researchers recruited 36 participants and invited them to sit alone in a room while either calming or unsettling music was played.
For half of the participants, hidden subwoofers played infrasound at 18 Hz.
Participants were asked to report their feelings, their emotional rating of the music, and whether they thought the infrasound was present.
Their saliva samples were also collected before and after listening.
Participants’ salivary cortisol levels were found to be higher if they had been listening to infrasound.
The same people also reported feeling more irritable and less interested, and thinking the music was sadder, although they couldn’t tell they were listening to 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,” Dr Schmaltz said.
“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,” he said.
The findings confirm that humans can sense but not identify infrasound, though the mechanism remains unclear.
Scientists hope to conduct further research involving more participants to probe whether prolonged infrasound exposure could impact health through consistently elevated cortisol levels.
“Increased cortisol levels help the body respond to immediate stressors by inducing a state of vigilance,” said Trevor Hamilton, another author of the study from MacEwan University.
“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,” Dr Hamilton said.
The latest research only assessed people’s response to 18 Hz.
Researchers suspect that more low frequencies and combinations might have their own differential effects.
“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,” Dr Schmaltz said.
“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,” he said.
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