A recent study found that exposure to infrasound, which humans cannot normally hear, tends to increase stress hormones and negative moods such as irritability. These findings suggest that invisible and inaudible noise pollution in everyday environments can unconsciously affect human physical and mental health. The study was published in the journal Frontiers of behavioral neuroscience.
Sound is measured in waves, and the frequency of these sound waves is measured in Hertz. Human hearing generally picks up sounds between 20 and 20,000 hertz. Infrasound refers to deep sound frequencies below the 20 Hertz threshold. Normally, people cannot hear these deep sound waves.
This infrasound is naturally produced by weather phenomena such as convective storms and volcanic activity. They also arise from a wide range of human activities and infrastructure. Everyday urban environments are often filled with infrasound generated by traffic, ventilation systems, heating equipment, and plumbing in old buildings.
“The main takeaway is that people seem to respond to sounds that they can’t consciously hear,” said Rodney Schmaltz, a professor of psychology at MacEwan University and lead author of the study.
Schmalz said the research team was originally interested in infrasound because of its association with places believed to be haunted. “Our interest in infrasound and ghosts began with Vic Tandy, a British engineer who worked in the lab, where he reported a vague sense of anxiety and unexplained anomalous experiences,” Schmalz said. “Ultimately, he tracked a sound wave at approximately 19 hertz to a fan in the room.”
Once the fan was repaired, Tandy’s paranormal experiences stopped completely. “He then recorded similar infrasounds in a 14th-century crypt in Coventry, famous for paranormal phenomena,” added Schmalz. Other researchers have previously attempted to test this haunting relationship. “The idea was more formally tested in the Haunt Project, a British study by Christopher French and colleagues at Goldsmiths,” Schmalz said.
In previous studies, researchers manipulated the presence of infrasound and electromagnetic fields. “Neither had any effect on the sensations people reported,” Schmalz told SciPost. “The best predictors were personality traits and expectations, not physical stimuli.”
Researchers wondered how people would react if they had something special to focus on, especially something frightening. “That invalid result is what got us interested,” Schmalz said. “Our first study was a field study at a commercial haunted attraction called Deadmonton here in Edmonton, and we found that when infrasound was being played, people moved faster through it. Following this, we decided to take our research into a more controlled laboratory environment.”
To measure physical responses in the lab, scientists turned to cortisol. Cortisol is a hormone produced by the adrenal glands that helps the body respond to stress. When people feel threatened or excited, cortisol levels tend to increase to keep them alert.
Scientists gathered 36 university students for an experiment. Participants consisted of 27 women and 9 men. Data from one participant were ultimately excluded due to poor quality of the saliva sample, leaving 35 participants for the final statistical analysis.
Before starting the experiment, participants had to follow certain rules to ensure accurate biological testing. They were instructed to refrain from eating food, tobacco, or marijuana for at least 1 hour before arriving at the laboratory. They were also told to drink only water during this period to prevent foreign contaminants from entering their saliva.
Participants were randomly divided into four groups and placed in separate testing rooms. They sat alone and listened to about five minutes of audio through regular computer speakers. Half of the participants listened to calming, meditative music. The other half listened to anxiety-provoking, horror-themed environmental sounds designed to evoke feelings of discomfort.
Along with the music, half of the participants in each group were exposed to invisible low-frequency vibrations. The scientists used subwoofer speakers hidden outside the test chamber to generate a stable 18 hertz frequency. This particular frequency was played at an amplitude of 75 to 78 decibels. This sound level is similar to the acoustic energy produced by nearby heavy machinery or industrial ventilation systems.
The other half of the participants experienced the exact same music with the subwoofer turned off. This created a silent control group for comparison. The researchers who conducted the experiment did not know what acoustic conditions were active until just before the experiment began.
To track physical stress responses, the researchers collected saliva samples from participants just before the audio was played. They collected a second sample 20 minutes after the onset of the audio. This 20-minute delay allowed the body enough time to cycle through noticeable changes in hormone levels. Immediately after the audio ended, participants completed an online survey assessing their emotional state and how they perceived the music.
When the researchers examined the results, they found that the participants were unable to consciously detect low-frequency sounds. Participants’ guesses as to whether the hidden speaker was on or not were based on chance. Their personal expectations for the room also had no effect on their physical hormonal responses.
Even without conscious awareness, physical and emotional responses provided evidence of significant physiological effects. Participants exposed to a frequency of 18 hertz had increased cortisol levels in their saliva. “What surprised us somewhat was how the effects interacted with the music,” Schmalz said. “In fact, we thought infrasound might simply amplify what people are already feeling.”
“The idea was that people who listen to spooky sounds might have a greater stress response, whereas calming music might not have that effect,” Schmalz said. But the results showed something different. “That’s not what we found,” Schmaltz added. “Cortisol was elevated in the infrasound condition in both groups.”
“Even people who listened to calming music showed an increase in cortisol,” says Schmalz. This shows that the hidden vibrations are acting independently. “This shows that infrasound is not simply reinforcing the mood already set by the music, but is itself triggering a stress response,” Schmalz explained.
Self-report findings were consistent with this physical stress response. Participants exposed to hidden vibrations reported feeling higher levels of irritation during short music clips. They also felt less interested in the overall listening experience than the control group.
Exposure to low frequencies even changed the way participants evaluated the music itself. Those in the active vibration group were more likely to rate the audio track as sad and uninteresting than the silent control group. “The key is convergence,” Schmalz said. “The individual measures in our study are modest.”
“Irritability, decreased interest, appraisals of sadness, none of that is that dramatic in and of itself,” Schmaltz said. “But they all moved in the same direction, and self-reports changed in line with physiological markers in the cortisol data.” He noted that seeing multiple independent indicators pointing in the same direction gives the overall pattern more weight than a single number.
“The way you read this is not that infrasound is having an alarming effect,” Schmalz says. “The implication is that the infrasound produced a small, consistent boost in mood and stress physiology at a level below the level of consciousness.” These biological and emotional changes may explain why certain places feel unnatural. “This is how I always describe the infrasound experience,” Schmalz said.
“Think about going to a concert,” Schmaltz said. “When the bass hits, there’s a tightening sensation in my chest and the hairs on the back of my neck stand up.” At a concert, the music becomes the obvious source of that physical sensation, so the listener doesn’t think anything of it.
“Now imagine a gentler version of that sensation, without an auditory source to explain it,” Schmaltz said. “You just feel a little unwell.” This invisible source of discomfort is often misunderstood in certain circumstances. “Infrasound can irritate someone for no apparent reason, and if that person is standing in a dimly lit old building said to be haunted, that discomfort suddenly has a story to tell,” Schmalz says.
“The feeling is real, but the explanations people use may lean more toward the paranormal than the low roar of old pipes and ventilation systems,” Schmaltz says. Schmalz stressed that infrasound doesn’t magically make people believe in ghosts. Instead, when people try to understand vague physical discomfort, most of the work is done through expectations and suggestion. Infrasound is just one piece of the puzzle when it comes to explaining why people experience ghosts.
As with all research, this study has some limitations. “There are a few other things to keep in mind,” Schmaltz said. “Our participants were only exposed for five minutes, so we can’t say what happens with hours or weeks of exposure. It might accumulate, or people might develop a habit. We don’t know.”
He also noted that the sample consisted of a small group of primarily young undergraduates, limiting how generalizable the results are to the broader general public. “I don’t want you to read this as evidence that infrasound is a serious environmental pollutant, but it is a signal that this issue is worth further investigation,” Schmalz said. “Another implication is that infrasound is worthy of further investigation as a low-level environmental irritant.”
“We’re already doing a follow-up investigation,” Schmaltz said. “In the lab, we are measuring whether buildings that are said to be haunted contain more infrasound than comparable buildings that are actually said to be haunted.”
The discovery of higher levels of infrasound in these spaces would strengthen the case that infrasound contributes to reports of paranormal phenomena. Beyond their connection to haunted houses, Schmalz is interested in investigating how these vibrations act as everyday environmental stimulants. “Low frequency sounds are often heard in everyday life,” says Schmalz. “Ventilation systems, furnaces, traffic jams, large appliances, and industrial equipment all generate it.”
Future studies will need to test longer exposure times and a wider range of acoustic frequencies to better understand real-world effects. Real-world infrasound covers a much wider range than the single 18 Hertz tone used in this lab. “The second one is the urban environment,” Schmalz said of future research directions. “Cities are full of infrasound sources, from subways to traffic to building ventilation.”
People living in dense urban environments often report general irritability and stress. These feelings are usually caused by noise, crowding, and poor air quality.
“We don’t expect infrasound to be the main culprit, but if low-level exposure can cause people to feel depressed without them realizing it, it would be interesting to investigate whether some of our everyday urban irritability comes from infrasound,” Schmalz said.
The study, “Exposure to infrasound is associated with aversive responses, negative appraisals, and elevated salivary cortisol in humans,” was authored by Cale R. Scatterti, Dawson Fonstein, Lisa B. Pritchard, Brian C. Franczak, Trevor J. Hamilton, and Rodney M. Schmalz.
