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 behavior 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 neighbors. This tiny delay between dots creates the appearance of a wave traveling through space as the dots move back and forth in place, just as sound does in real life.