From the roar of a crowd to the quiet of a library, sound and silence might seem like polar opposites. However, according to new research, our brains perceive them in the same way. Silence may not be a sound, but scientists say we can truly hear it.
In a new study, published Monday in the Proceedings of the National Academy of Sciences, researchers examined how people experience silence using well-known auditory illusions. The illusions are meant to test the perception of noise, but for the study, the team adapted them to measure people’s response to silence, instead.
“If you can get the same illusions with silences as you get with sounds, then that may be evidence that we literally hear silence after all,” Chaz Firestone, a co-author of the study and cognitive scientist at Johns Hopkins University, says in a statement.
In the study, participants were tricked by these “silence illusions” in a similar way to how people are typically fooled by the sound versions of the experiments.
“This gives reason to suppose that silences are treated by the auditory system in the same way sounds are treated,” Nico Orlandi, a philosopher at the University of California, Santa Cruz, who was not involved in the research, tells Science’s Claudia Lopez Lloreda.
We experience noise when sound waves travel from our outer ear through our ear canal and rattle our eardrum. Silences, however, don’t do this. But philosophers and cognitive scientists alike have wondered whether we actually perceive silence or merely note the absence of noise.
“Silence, whatever it is, is not a sound,” Firestone tells Scientific American’s Shayla Love. “It’s the absence of sound. And yet it often feels like we can hear it. If silence isn’t really a sound, and yet it turns out that we can hear it, then hearing is more than just sound.”
To get at this question about the nature of hearing, the researchers prepared seven experiments with three different perceptual illusions and tested them on 1,000 study participants, according to Scientific American.
In one experiment, researchers played a recording that sounded like ambient noise in a crowded place. In the first half of the recording, the background noise is interrupted by two blips of silence. In the second half, one continuous period of silence cuts in.
Researchers asked participants which silence felt longer—the combination of the first two periods of silence, or the longer, uninterrupted one. Most participants thought the continuous silence was longer, but it was actually the same length as the two shorter silences combined.
These results aligned with previous research that examined a similar illusion, which used two short beeps and one continuous beep instead of periods of silence, according to the statement. With that illusion, people also perceived the continuous interval as longer than the two short ones together.
In another experiment, participants listened to a recording that contained two sounds, such as a high-pitched organ and a low engine rumble. In the first four trials, the same one of the two sounds would go silent—for example, the organ would cease playing, leaving just the running engine. In the fifth trial, the other sound would go silent—in this case, the engine would shut off and leave the organ. Participants then had to decide whether the period of time with just one sound playing was longer or shorter in the fifth trial compared to the other four.
Again, the periods of time were the same length, but listeners perceived the period in the fifth trial as being longer, according to Science.
Similar findings across the seven experiments suggested that humans experience silence and sound in much the same way: Both sound and silence can distort our perception of time.
Firestone tells the New York Times’ Bethany Brookshire that the silence illusions had just as strong an effect on people as the sound illusions did. “It’s not even like, ‘Oh, it kind of works with silences, but it’s just a lot weaker,’” he tells the publication. “No, you get the same effect.”