The brain perceives low-frequency sounds in a completely different way than other sounds. Maybe that's why some people react more to them.
Sound below 16 Hz is what professionals like to call infrasound. This is sound that is often considered impossible to hear. But that's not the case.
"Humans can actually perceive infrasound if the sound level is high enough," says Carlos Jurado, postdoctoral fellow at the Department of Neuromedicine and Movement Science at the Norwegian University of Science and Technology (NTNU).
Some are more sensitive to low-frequency noise. For example, it can come from ventilation systems, heat pumps, wind turbines, industry, transport, generators or transformers. But this is difficult to measure, because the sound is often perceived more as a hum or physical sensation than more high-frequency sound does.
Didn't know how we perceive the sound
Scientists have long been uncertain about how we perceive infrasound. Now Jurado has investigated the case together with Torsten Marquardt from University College London.
Recently, their results were published in an article in the Nature magazine Scientific Reports.
"Our research suggests that infrasound is registered in the inner ear in a different way than normal sound does," says Marquardt.
Inside the inner ear, there are specialized sensory hair cells that are absolutely crucial for the transmission of sound signals to the brain.
"But at very low frequencies, the signals to these hair cells become too weak, and other hair cells, which normally contribute to the hearing process, can still pick them up," Jurado explains.
"These support cells, which normally receive signals from the brain to regulate hearing sensitivity, generate electric fields that are strong enough to trigger nerve signals that are sent to the brain, so that infrasound is perceived," says Marquardt.
More of a feeling than any other sound
Maybe that's why extra low-frequency sounds feel different than other sounds do.
"This may explain why infrasound is experienced differently than normal sound. Small increases in sound pressure quickly make the sound much louder. We can now easily explain this phenomenon as a natural consequence of our new findings," says Jurado.
The findings may also help to understand why some people are bothered by low-frequency noise, while others do not, as the newly discovered mechanism may vary from person to person.