Silent places like old mansions can be quite scary, but there’s a catch: silence isn’t actually what it seems to be. Hidden amidst the daily commotion in urban concrete, infrasound, an ultra-low frequency vibration below the range of human hearing, activates the stress hormone cortisol. The study, published in Frontiers in Behavioral Neuroscience, also mentions that people exposed to infrasounds become more irritable and less engaged. But how does a sound we don’t hear kick our instincts into overdrive?
Infrasound refers to extremely low-frequency sound waves, below 20 Hertz (Hz), that are mostly beyond the range of human hearing, which ranges from 20 Hz to around 20,000 Hz (20 kilohertz). These sounds arise from human-made sources such as traffic and industrial equipment, as well as from natural events such as storms. In fact, the loud crack of thunder subtly disperses a wave of infrasound. However, some animals also use infrasound to communicate, while others avoid it.
Are we Aware of Infrasound?
Our bodies react to infrasound, although we don’t consciously perceive it. However, even a brief exposure to infrasound can shift mood and spike cortisol, which acts as a “slow burn” stress hormone, unlike adrenaline, which produces a quick burst of energy to help avert a crisis. Cortisol’s effects are sustained for longer periods and can be linked to issues like high blood pressure during chronic stress.
Prof Rodney Schmaltz of MacEwan University, senior author of the article, describes how infrasound can sway our mood when we enter a supposedly haunted, old building:
“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 of infrasound, 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.”
Testing out How Infrasound Affects Mood
To explore the effects of infrasound, the researchers recruited 36 participants for a controlled experiment in which each person sat alone in a room and listened to either calming or unsettling music. Hidden subwoofers were placed in half of the rooms to generate infrasound at 18Hz.
Post-session, the participants described how they felt, rated the music’s emotional tone, and were asked whether they believed that infrasound was present. Researchers also collected saliva samples before and after the listening period to measure cortisol levels.
Findings reveal Clear Mood Shifts
The results revealed that participants exposed to infrasound had higher salivary cortisol levels. They also reported feeling more irritable and less engaged, and being more likely to perceive the music as sad, implying that infrasound can affect emotions. However, the participants couldn’t reliably detect the presence of ultrasound, and their beliefs about its presence had no detectable effect on their cortisol or mood.
The study suggests that the body can respond to infrasound even though we cannot hear it. The body’s normal stress response to irritation or stress increases cortisol levels. However, throwing infrasound into the mix led to outcomes that went beyond the natural relationship.
Do we feel Infrasound?
The exact biological mechanism by which our body registers sound has yet to be studied. Researchers also note that there is a possibility that long-term exposure to infrasound can contribute to low mood and ongoing irritability. Increased cortisol levels induce a state of vigilance, a response we have honed over millennia of evolution. In a way, feeling infrasound, if we can call it that, is a primal trait linked to a time when we were still primitive.
The study can be considered a first step towards understanding the effects of infrasound on humans, and so far, only a single frequency has been tested. There could be many more frequencies inducing different effects. The reports are subjective descriptions of how the participants felt after exposure, but observing their responses during the trial can provide more insights.
The scientists want to test on a larger group with a broader range of frequencies, combinations, and durations. Infrasound rarely occurs as a single, clean tone in real environments, so identifying patterns by combining frequencies can eventually inform noise regulations or building design standards.
Supernatural might just be nature trying to play tricks on you, and a little logic and science can help you get behind the trick. Don’t expect nature to reveal it; after all, a magician never reveals their trick.
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