A recent study found that exposure to extremely low frequency sounds, which humans generally cannot hear, tends to increase stress hormones and negative moods like irritation. These findings suggest that invisible, inaudible noise pollution in everyday environments may unconsciously affect human physical and emotional well-being. The research was published in the journal Frontiers in Behavioral Neuroscience.
Sound is measured in waves, and the frequency of these acoustic waves is measured in Hertz. Human hearing generally captures sounds between twenty and twenty thousand Hertz. Infrasound refers to deep acoustic frequencies that fall below the twenty Hertz threshold. People cannot normally hear these deep, rumbling sound waves.
This low frequency sound occurs naturally through weather events like convective storms and volcanic activity. It also comes from widespread human activities and infrastructure. Everyday urban environments are often filled with infrasound generated by traffic, ventilation systems, heating units, and older building pipes.
“The main takeaway is that people appear to respond to a sound they cannot consciously hear,” said Rodney Schmaltz, a psychology professor at MacEwan University and senior author of the study.
Schmaltz noted that the research team originally became interested in infrasound because of its connection to supposedly haunted locations. “Our interest in infrasound and hauntings starts with Vic Tandy, a British engineer who worked in a lab where he reported a vague sense of unease and unusual experiences he could not account for,” Schmaltz said. “Eventually he traced a roughly 19 Hertz sound wave to a fan in the room.”
Once the fan was fixed, Tandy’s unusual experiences stopped entirely. “He later documented similar infrasound in a 14th-century cellar in Coventry that had a reputation for paranormal activity,” Schmaltz added. Other researchers previously attempted to test this haunting connection. “That idea was tested more formally in the Haunt project, a UK study by Christopher French and colleagues at Goldsmiths,” Schmaltz said.
In that previous study, the researchers manipulated the presence of infrasound and electromagnetic fields. “Neither one produced an effect on the sensations people reported,” Schmaltz told PsyPost. “The best predictor was personality traits and expectation, not the physical stimuli.”
The researchers wondered how people might react if they had something specific to focus on, particularly something frightening. “That null result is what got us curious,” Schmaltz said. “Our first study was a field study at a commercial haunted attraction here in Edmonton called Deadmonton, and we found that people moved through it more quickly when infrasound was playing. Following this, we decided to bring the research into a more controlled lab setting.”
To measure the physical response in the laboratory, the scientists focused on cortisol. Cortisol is a hormone produced by the adrenal glands that helps the body respond to stress. When a person feels threatened or agitated, their cortisol levels tend to rise to keep them alert.
The scientists recruited thirty-six undergraduate university students for the experiment. The participant pool consisted of twenty-seven females and nine males. One participant’s data was eventually excluded due to a poor saliva sample, leaving thirty-five individuals for the final statistical analysis.
Before the experiment began, participants had to follow specific rules to ensure accurate biological testing. They were instructed to refrain from consuming food, tobacco, or marijuana for at least one hour before arriving at the laboratory. They were also told to only drink water during this period to keep their saliva free of outside contaminants.
The participants were randomly divided into four groups and placed into separate testing rooms. They sat alone and listened to roughly five minutes of audio through regular computer speakers. Half of the participants listened to calming, meditative music. The other half listened to unsettling, horror-themed ambient sounds designed to evoke discomfort.
Alongside the music, half of the participants in each group were exposed to an invisible low frequency vibration. The scientists used hidden subwoofer speakers located outside the testing rooms to generate a steady eighteen Hertz frequency. This specific frequency was played at an amplitude of seventy-five to seventy-eight decibels. This volume level is similar to the acoustic energy produced by nearby heavy machinery or industrial ventilation systems.
The remaining half of the participants experienced the exact same music, but with the subwoofers turned off. This created a silent control group for comparison. The researchers running the experiment also stayed blind to which sound condition was active until right before the trial started.
To track physical stress responses, the researchers collected saliva samples from the participants immediately before the audio played. They collected a second sample twenty minutes after the audio started. This twenty minute delay provided enough time for the body to circulate noticeable changes in hormone levels. Immediately after the audio finished, the participants completed an online survey rating their emotional state and how they perceived the music.
When reviewing the results, the researchers found that the participants could not consciously detect the low frequency sound. The participants’ guesses about whether the hidden speakers were turned on were no better than random chance. Their personal expectations about the room also did not affect their physical hormone responses.
Even without conscious awareness, the physical and emotional reactions provided evidence of a noticeable physiological impact. Salivary cortisol levels increased in participants exposed to the eighteen Hertz frequency. “One thing that somewhat surprised us was about how the effect interacted with the music,” Schmaltz said. “Going in, we thought there was a chance that infrasound might simply amplify whatever people were already feeling.”
“The idea was that the people hearing the creepy audio might show a bigger stress response, while the calming music would not have this effect,” Schmaltz said. However, the results showed something different. “That is not what we found,” Schmaltz added. “Cortisol rose in the infrasound condition across both groups.”
“Even people listening to calming music showed an increase in cortisol,” Schmaltz said. This indicated that the hidden vibration acted independently. “That tells us infrasound was driving the stress response on its own, rather than just intensifying whatever mood the music had already set,” Schmaltz explained.
The self-reported survey results matched this physical stress response. Participants exposed to the hidden vibrations reported feeling higher levels of irritation during the short music clip. They also felt less interested in the overall listening experience compared to the control group.
The low frequency exposure even altered how participants appraised the music itself. Those in the active vibration group tended to rate the audio tracks as sadder and less interesting than those in the silent control group. “The key is convergence,” Schmaltz said. “Each individual measure in our study is modest.”
“The irritability, the drop in interest, the sadness rating, none of them are overly dramatic on their own,” Schmaltz said. “But they all moved the same direction, and the self-report shifts lined up with a physiological marker in the cortisol data.” He noted that seeing multiple independent measures pointing the same way gives the overall pattern much more weight than any single number would.
“The way to read this is not that infrasound has an alarming effect,” Schmaltz said. “It is that infrasound produced a small, consistent push that showed up across mood and stress physiology at the same time, and did it below the level of awareness.” These biological and emotional shifts might explain why certain locations feel unnatural. “Here is how I usually explain the experience of infrasound,” Schmaltz said.
“Think about being at a concert,” Schmaltz said. “When the bass hits, you feel a tightening in your chest and the hair on the back of your neck stands up.” At a concert, the music provides an obvious source for that bodily sensation, so listeners think nothing of it.
“Now imagine a milder version of that sensation with no audible source to explain it,” Schmaltz said. “You just feel a bit off.” This invisible source of discomfort often gets misinterpreted in certain settings. “If infrasound can make someone feel irritated without any obvious source, and that person is standing in a dim old building they were told is haunted, the discomfort suddenly has a story to attach to,” Schmaltz said.
“The feeling is real, but the explanation people use may lean towards the paranormal rather than the low rumble of old pipes or a ventilation system,” Schmaltz said. Schmaltz stressed that infrasound does not magically make people believe in ghosts. Instead, expectation and suggestion do most of the work when a person tries to make sense of vague bodily discomfort. In terms of explaining why people experience hauntings, infrasound serves as just one piece of the puzzle.
As with all research, the study does have some limitations. “A couple of other caveats,” Schmaltz said. “Our participants were exposed for only about five minutes, so I cannot say anything about what hours or weeks of exposure would do. It might build, or people might habituate. We do not know.”
He also pointed out that the sample consisted of a small group of mostly young undergraduates, which limits how far the results might generalize to the broader public. “I would not want anyone reading this as proof that infrasound is a serious environmental pollutant, but it is a signal that the question is worth investigating further,” Schmaltz said. “Another implication is that infrasound is worth further investigation as a low-level environmental irritant.”
“We already have a follow-up running,” Schmaltz said. “The lab is measuring whether supposedly haunted buildings actually contain more infrasound than comparable buildings that are not reputed to be haunted.”
Finding higher levels of low frequency noise in these spaces would strengthen the case that infrasound contributes to paranormal reports. Beyond the haunted house connection, Schmaltz is interested in exploring how these vibrations act as an everyday environmental irritant. “Low frequency sound is common in everyday life,” Schmaltz said. “Ventilation systems, furnaces, heavy traffic, large appliances, and industrial equipment all produce it.”
To better understand real world impacts, future studies will need to test longer exposure times and a wider band of acoustic frequencies. Real world infrasound covers a much broader range than the single eighteen Hertz tone used in this laboratory experiment. “The second is urban environments,” Schmaltz said, regarding future research directions. “Cities are full of infrasound sources, from subways to traffic to building ventilation.”
People living in dense urban settings often report general irritation and stress. These feelings are typically blamed on noise, crowding, and poor air quality.
“I would not expect infrasound to be the main driver, but if low-level exposure can shift mood downward without people noticing, it would be interesting to investigate whether some small slice of that everyday urban irritability comes from infrasound,” Schmaltz said.
The study, “Infrasound exposure is linked to aversive responding, negative appraisal, and elevated salivary cortisol in humans,” was authored by Kale R. Scatterty, Dawson VonStein, Lisa B. Prichard, Brian C. Franczak, Trevor J. Hamilton, and Rodney M. Schmaltz.