Testing the body’s stress response

A chance conversation with neuroscientist colleague (and co-author) Kale Scatterty inspired this latest study. Scatterty co-authored a 2023 paper demonstrating an aversion in zebrafish to infrasound, specifically an anxiety response that caused the fish to avoid certain tank areas. This suggested a physiological response to infrasound, and Schmaltz wanted to see if this was also true in humans. So they designed a lab-based experiment to test the hypothesis that cortisol levels in people’s saliva—part of the body’s normal stress response—would increase in response to infrasound.

Thirty-six participants sat alone in a room and were exposed either to calming music similar to what one might hear in a yoga setting, or “more unsettling ambient music,” per Schmaltz, with half of them also being exposed to infrasound emitted from hidden subwoofers. “What we thought might happen was when the infrasound was on, people would find the calming music even more relaxing, while the scarier music would be scarier,” he said.

Instead, the results showed that, across the board, participants felt more irritated and unsettled when the infrasound was turned on, regardless of which kind of music was playing, and their cortisol levels increased significantly. None of the participants could reliably tell when infrasound was present. This suggests that human beings can have a physiological response to infrasound even when we can’t consciously hear it.

While this is a promising result, infrasound is unlikely to be the sole factor behind our perceptions of hauntings; it’s probably one of several, including Wiseman’s earlier findings on suggestibility. “It’s not that infrasound is ‘causing’ hauntings,” said Schmaltz. “I want to be very clear on that. We’re definitely not saying we’ve solved hauntings. But in some of these older buildings, there could be low rumbling pipes [producing infrasound], and if somebody already has the expectation that something spooky might happen, the infrasound might drive that a bit. So infrasound doesn’t explain all of it, but it could certainly be a piece of the puzzle for some of these haunting experiences.”