A whale's song may seem to travel across the ocean faster than sound... But how can sound itself be supersonic?
To understand this, we need to follow two paths. Part of the sound travels directly from the animal to a receiver. Another part first reaches the water's surface, then returns as an echo. The two signals arrive almost simultaneously when the whale or receiver is close enough to the surface.
Humpback whale and its calf beneath the ocean's surface.
Source: Elianne Dipp / Pexels
These two versions of the same sound can overlap. Depending on their offset, some parts reinforce each other while others weaken. The maximum of the received signal can therefore appear earlier than expected. If only the movement of this maximum is measured, the calculated speed is then apparently greater than the speed of sound.
Yet no information travels at this speed. Simulations by John L. Spiesberger and Eugene Terray show that information remains slower than sound in water. The phenomenon therefore results from the shape of the received signal, modified by interference between the direct path and the echo.
In other words, the observed peak is not an object that actually traveled through the water at an abnormal speed. It results from the addition of several waves arriving along different paths. This distinction avoids a paradox: the usual laws of propagation remain valid.
The researchers studied this mechanism using numerical simulations. Their work concerns acoustic wave packets in a medium where all frequencies normally propagate at the same speed.
Whales offer a particularly interesting real-world example. Their calls can travel great distances and reflect off the surface. Taking these interferences into account could change how an underwater microphone network estimates an animal's position based on the arrival time of sounds.
The study remains theoretical and is based on simulations. However, the authors indicate that the effect should be experimentally testable. Controlled measurements would make it possible in particular to verify under what conditions an echo shifts the acoustic peak enough to produce this apparent speed.