A new study suggests that very low-frequency sounds, or infrasound, which are inaudible to humans but can elevate stress hormone levels, may account for the unsettling atmosphere in so-called 'haunted' locations.
What Is Infrasound?
Infrasound refers to sound waves below 20 Hertz (Hz). These can originate from natural sources like storms or human-made sources such as traffic. Even brief exposure can increase irritability and raise cortisol levels, according to research published in Frontiers in Behavioral Neuroscience.
Lead author Rodney Schmaltz explained: 'Consider visiting a supposedly haunted building. Your mood shifts, you feel agitated, but you can't see or hear anything unusual.' In older buildings, infrasound is often present, particularly in basements where ageing pipes and ventilation systems generate low-frequency vibrations.
'Infrasound is pervasive in everyday environments, appearing near ventilation systems, traffic, and industrial machinery,' Dr Schmaltz added. '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.'
Study Details
Researchers recruited 36 participants who sat alone in a room while calming or unsettling music was played. For half of them, hidden subwoofers emitted infrasound at 18 Hz. Participants reported their feelings, rated the music emotionally, and guessed whether infrasound was present. Saliva samples were taken before and after the session.
Results showed that those exposed to infrasound had higher salivary cortisol levels and reported feeling more irritable and less interested, perceiving the music as sadder. Yet they could not reliably detect the 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 noted.
Implications
The findings confirm that humans can sense infrasound without identifying it, though the mechanism remains unclear. Co-author Trevor Hamilton from MacEwan University said: '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.'
The study only tested 18 Hz, but researchers suspect other frequencies and combinations may have distinct effects. Dr Schmaltz concluded: '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.'
