Visualizing sound: scientists reveal hidden behaviors of sound waves
City St George’s, University of London
An international team of scientists has developed a new analysis of how sound waves behave, revealing surprising effects that have largely been overlooked for decades.
In the new paper in Scientific Reports, which was led by researchers from City St George’s, University of London, the team explored how sound waves move through air and how those movements might be perceived visually.
Sound travels as a longitudinal wave, meaning air molecules vibrate back and forth rather than moving up and down like waves in a violin string. These vibrations are usually assumed to be smooth and regular, and as a physical phenomenon they form the basis of acoustics and some forms of seismic transmission. However, the new theoretical analysis of physical longitudinal wave motion reveals that the behaviour of sound waves changes dramatically when they become stronger (i.e. above 160 dB at 10 kHz, which is similar to the noise level inside a high-pitched jet engine), and the prior assumptions are only true for moderate sounds.
Using computer simulations, the researchers – namely Professor Christopher Tyler and Professor Joshua Solomon at City St George’s and Professor Stuart M. Anstis from the University of California, San Diego – created animations where each dot represents an air molecule. Each dot moves back and forth in place, slightly out of step with its neighbours. This tiny delay between dots creates the appearance of a wave travelling through space as the dots move back and forth in place, just as sound does in real life.
At low sound levels, the wave looks smooth and simple. But as the sound gets louder, the shape of the wave becomes progressively distorted. At extremely high levels, far beyond what we experience in everyday life, the wave no longer looks like a smooth curve at all. Instead, wave crests compress into very narrow “spikes”, which eventually split into pairs of “spikes” as the intensity increases further.
What surprised the researchers most was how the motion is perceived. Rather than seeing one uniform travelling wave, observers see two motions at the same time: the crests of the wave appear to move forwards, while the troughs appear to move backwards. The brain combines these opposing motions into a transparent motion percept (a visual effect where two overlapping surfaces move transparently over each other).
This challenges a common assumption about motion perception. Normally, when people watch something moving in one direction for a long time, their brain adapts, causing a “motion aftereffect” (for example, a waterfall seeming to flow upward after staring at it). But the simulated sound waves did not produce this effect, showing that the brain processes wave motion very differently from ordinary movement.
Professor Tyler, lead author of the study and Professor of Visual Science at City St George’s, said:
“Our findings suggest that sound waves involve more complex, non-linear behaviour than is usually taught in physics. These effects only become obvious when the waves are visualised or reach very high intensities, which may explain why they have been largely ignored until now. Although the sounds modelled in the study are much louder than those we encounter in daily life, the work offers important insights into the fundamentals of acoustics, seismic waves, and how the brain interprets complex motion.”
The researchers believe their approach opens up new ways to study wave behaviour, not just through equations, but by making it visible. By turning sound into something we can see, they hope to deepen our understanding of both physics and human perception.
Journal
Scientific Reports
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