We have all experienced it at least once: the house is silent, everyone is asleep, and then you hear a strange hum from the walls. You lean in, only to jump at a sudden thud from the attic. Your heart races, and all the tormented spirits from horror movies turn an ordinary night into something sinister. Regardless of how rational you are, fear seeps in. But the real culprit has nothing to do with paranormal activity. In fact, you are responding to something you cannot even hear.
The Science Behind the Spook
A recent study published in Frontiers in Behavioral Neuroscience claims that ghosts and goblins are not responsible for those eerie sensations. Instead, blame old pipes and ventilation systems for things that go bump in the night. The key is infrasound, a very low frequency sound below 20 Hertz (Hz) that humans cannot hear. This sound comes from natural sources like storms or anthropogenic sources like traffic. While some animals use infrasound to communicate, others avoid it.
Humans Respond to What They Cannot Hear
Researchers investigating humans' ability to sense infrasound noted that we cannot detect it consciously, but our bodies respond to it. It is often linked to increased irritability and higher cortisol levels. Professor Rodney Schmaltz of MacEwan University, senior author of the article, explained, "Infrasound is pervasive in everyday environments, appearing near ventilation systems, traffic, and industrial machinery. 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."
Schmaltz added, "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."
How Infrasound Affects the Body
The study involved 36 participants who sat alone in a room while calming or unsettling music was played. Half of the participants were subjected to 18 Hz infrasound from hidden subwoofers. They reported their feelings, emotional rating of the music, and whether they thought infrasound was present. Saliva samples were collected before and after exposure. Results showed that participants exposed to infrasound had higher salivary cortisol levels, felt more irritable, less interested, and rated the music as sadder. However, they could not tell they were listening to infrasound.
"This study suggests that the body can respond to infrasound even when we can't consciously hear it. 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," Schmaltz said.
Kale Scatterty, first author and PhD student at the University of Alberta, added, "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. But infrasound exposure had effects on both outcomes that went beyond that natural relationship."
Felt, but Not Heard
The findings confirm that though humans can sense infrasound, they cannot identify it, and the mechanism remains unclear. Professor Trevor Hamilton of MacEwan University, corresponding author, noted, "Increased cortisol levels help the body respond to immediate stressors by inducing a state of vigilance. 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."
So, you can finally rest your case: it is not ghosts in your attic. Scatterty cautioned, "This study was in many ways a first step towards understanding the effects of infrasound on humans. 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. 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," Schmaltz added.
Now that you know, you can give the scary figures in your head some rest and get a good night's sleep.
