Is silence a sound?

"Is silence a sound, a noise?""What is the reader question of the week?" asks Jordi Campoy on our Facebook page. Thank you all for your participation.

"Philosophers have long debated whether silence is something we can literally perceive, but no scientific study had directly addressed this question."Chaz Firestone, assistant professor of psychology at Johns Hopkins University (USA), explained to us in an article published in 2023 entitled " Silence produces sound we can hear, study finds“.

"Silence, whatever its form, is not a sound; it is the absence of sound."

To move beyond the abstract debate, Chaz Firestone and his team chose an original approach: using auditory illusions to test the perception… of the absence of sound.

Their study, published in 2023 in the journal Proceedings of the National Academy of Sciences (PNAS) does not claim to redefine silence as sound. On the contrary, the researchers are clear: "Silence, whatever its form, is not a sound; it is the absence of sound."Rui Zhe Goh, co-author of the study, pointed out in 2023. But then, what exactly does our brain perceive when there is "nothing" to hear?

To answer this question, this research team repurposed well-known auditory illusions based on segmentation: our auditory system constantly breaks down the continuous flow of sound waves into distinct events (a note, a word, a beep). The challenge was audacious: to replace sounds with silences. If the same illusions appeared, it would mean that the brain also processes silences as distinct auditory events.

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"We hear the silence"

The experiment involved nearly 1,000 participants. They first listened to soundscapes simulating the noise of public places. Then they listened to the same soundscapes, this time interrupted by brief periods of silence. The result: as with sounds, a long silence is perceived as longer than two short silences, even when their total duration is identical. "This suggests that the auditory system processes moments of silence in the same way as sounds. In other words, we hear silence.", summarized Ian Phillips, co-author of the study, in 2023.

In this experiment, researchers at Johns Hopkins University replaced sounds with silences in a well-known auditory illusion. Credits: Johns Hopkins University

"There is at least one thing we hear that is not a sound."

However, beware of jumping to conclusions: hearing silence does not mean it becomes noise. Researchers emphasize this nuance. Silence remains an absence of auditory stimulation, but this absence is actively processed by the auditory system, just like the presence of sound.

"There is at least one thing we hear that is not a sound: the silence that occurs when sounds disappear.", explains Ian Phillips. In other words, our ear – and especially our brain – does not just record vibrations: it also detects their interruptions, their contrasts, their disappearances.

Does absolute silence exist?

This study doesn't close the philosophical debate, but it enriches it. What are the limits of our perception of silence? Can we hear very long silences? And above all, is there such a thing as absolute silence, a total absence of sound, or does our brain always end up "filling the void"? These are all questions that open up a new field of research: the perception of absence. A way, perhaps, to understand why silence can sometimes be as oppressive as a cacophony.