You can hear silence, claim researchers, settling an old debate

To test this proposition that silence can, in fact, be perceived, the scientists sought evidence of whether our brains treat silences the way they treat sounds when subjected to auditory illusions.

Auditory illusions

The illusions used by the research team for the study were well-known but changed to achieve the effects they were hoping to explore. In particular, the researchers created versions of auditory illusions in which sounds from original illusions instead featured moments of silence. One such illusion had a sound that felt longer than it was and in the new version, a silence appeared to be longer than it truly was.

In another example, a sound-based illusion referred to as one-is-more was utilized by the researchers. This usually involves playing a sound consisting of one long beep, which appears to be longer than two short consecutive beeps, even though the two sequences are actually of the same length. In the version revised for the experiment, the sounds in the one-is-more illusion were replaced with moments of silence — an auditory illusion they called the one-silence-is-more illusion.

Interestingly, what the researchers found was that the amended illusions produced the same effects on the brain as the sound-based versions. One long moment of silence seemed longer to participants than two short ones, even though they were the same exact length. This led the scientists to conclude that people hear silence similarly to how they hear sounds.

A similar conclusion was drawn when the researchers made study subjects listen to soundscapes from busy restaurants, markets, or train stations. The participants had to listen for periods when all sound dropped out, as the audio files briefly went silent. What was being tested was not only the participants' ability to experience illusions containing silences, but the fact that the silences triggered illusions that could presumably be only caused by sounds. 

The study’s co-author, Ian Phillips, Professor of Philosophy and Psychological and Brain Sciences at Johns Hopkins University, suggested that this proves silence can be heard.

"There's at least one thing that we hear that isn't a sound, and that's the silence that happens when sounds go away," he shared in the press release, adding, "The kinds of illusions and effects that look like they are unique to the auditory processing of a sound, we also get them with silences, suggesting we really do hear absences of sound too."

A deeper insight

Interesting Engineering communicated with the study’s lead author, Rui Zhe Goh, a Johns Hopkins University graduate student in philosophy and psychology, for greater insight on the study.

For starters, Goh confirmed that silence should be defined as the absence of sound. 

Asked whether, when listening to silence, people actually hear the silence or do their brains function in a way that is similar to when they hear sounds, Goh replied that, “Our results show that the auditory system segments periods of silence into discrete events, just as it segments continuous acoustic input into discrete sounds.”

What this likely means, according to the researcher, is that, “this process of event segmentation underlies both experiences of sounds and experiences of silence.”

As far as why the researchers chose to use auditory illusions for their study, Goh explained: “We chose these three auditory illusions because they are all are thought to be caused by the same mechanism – auditory event segmentation. We wanted to find out if moments of silence also trigger this important auditory process. Our reasoning was that if moments of silence cause these illusions, that’s good evidence that moments of silence are also segmented into discrete events by our auditory system."

Goh considers the implications of the study on our understanding of hearing to be that, “sounds are not the only objects of hearing.” He proposed that “Empty periods of time can also be segmented by the auditory system into discrete events, and this allows us to perceive silences, as well as sounds."

Read the study “The perception of silence,” published in the journal Proceedings of the National Academy of Sciences.

Abstract

Auditory perception is traditionally conceived as the perception of sounds—a friend’s voice, a clap of thunder, a minor chord. However, daily life also seems to present us with experiences characterized by the absence of sound—a moment of silence, a gap between thunderclaps, the hush after a musical performance. In these cases, do we positively hear silence? Or do we just fail to hear, and merely judge or infer that it is silent? This longstanding question remains controversial in both the philosophy and science of perception, with prominent theories holding that sounds are the only objects of auditory experience and thus that our encounter with silence is cognitive, not perceptual. However, this debate has largely remained theoretical, without a key empirical test. Here, we introduce an empirical approach to this theoretical dispute, presenting experimental evidence that silence can be genuinely perceived (not just cognitively inferred). We ask whether silences can “substitute” for sounds in event-based auditory illusions—empirical signatures of auditory event representation in which auditory events distort perceived duration. Seven experiments introduce three “silence illusions”—the one-silence-is-more illusion, silence-based warping, and the oddball-silence illusion—each adapted from a prominent perceptual illusion previously thought to arise only from sounds. Subjects were immersed in ambient noise interrupted by silences structurally identical to the sounds in the original illusions. In all cases, silences elicited temporal distortions perfectly analogous to the illusions produced by sounds. Our results suggest that silence is truly heard, not merely inferred, introducing a general approach for studying the perception of absence.