Sound often com­prises numer­ous super­im­posed waves of var­i­ous wave­lengths. At cer­tain points, these con­stituent waves can all com­bine con­struc­tively to pro­duce a pulse, which moves through the medium at a veloc­ity known as the “group velocity”.

In a nor­mal dis­per­sive medium, the veloc­ity of a wave is pro­por­tional to its wave­length, result­ing in a group veloc­ity that is slower than the aver­age veloc­ity of its con­stituent waves. But in an “anom­alously” dis­per­sive medium — one that becomes highly absorb­ing or atten­u­at­ing at cer­tain fre­quen­cies — veloc­ity is inversely pro­por­tional to wave­length, mean­ing that the group veloc­ity can become much faster.

Indeed, the group veloc­ity of light has already been shown to travel faster than the speed of light in a vac­uum. But until now, super­lu­mi­nal acoustic waves have existed only in the­ory, and would require the group veloc­ity to increase almost a mil­lion times over.

William Robert­son and col­leagues from Mid­dle Ten­nessee State Uni­ver­sity in the US have man­aged to pro­duce “faster than light” sound, how­ever, by putting a sound pulse through a sur­pris­ingly sim­ple wave­guide. Inside, a loop fil­ter splits the sig­nal along two unequal length paths, and then recom­bines it to pro­duce large amounts of anom­alous dis­per­sion. As they inter­fere with each other, they repli­cate the shape of the orig­i­nal pulse, only far­ther ahead. This gives the impres­sion that the sound has trav­elled far­ther, and thus faster, in the same space of time.

Robert­son says that such split-​path inter­fer­ence can also occur nat­u­rally when a sound source is located near a hard wall: some of the sound reaches the lis­tener directly, and some reaches the lis­tener from a slightly longer path as it bounces off the wall. There­fore, he says, super­lu­mi­nal sound is an “every­day” occur­rence, although it is mostly too sub­tle to notice.

Pro­po­nents of Einstein’s spe­cial rel­a­tiv­ity need not worry, though. The under­ly­ing waves that make up the pulse remain at sub­lu­mi­nal veloc­i­ties, so no infor­ma­tion, mat­ter or energy actu­ally trav­els faster than light. (See related link: “Subluminal”.)

The effect is the same as that observed in pre­vi­ous elec­tri­cal or opti­cal exper­i­ments,” Robert­son told Physics Web. “The only some­what star­tling dif­fer­ence is that the acoustic waves mak­ing up the pulse move so much more slowly than light.”