June 2, 2011
When I think of a shock wave, I think “explosion,” like the ones on “Mythbusters” (where you can often see the resulting shock wave when the hi-speed video is played back in slow motion). I don’t think of musical instruments. But perhaps I should. In a 1996 paper from the Journal of the Acoustical Society of America, Mico Hirschberg of the Eindhoven University of Technology in the Netherlands and colleagues documented shock waves emanating from trombones played at fortissimo (very loud) levels and predicted that similarly shaped “bright” instruments, like trumpets, that have a segment of cylindrical pipe after the mouthpiece would also produce these shock waves.
These shock waves might seem far too weak to catch on film, but now Hirschberg (along with Kazuyoshi Takayama and Kiyonobu Ohtani from Tohoku University in Japan) have done exactly that, revealing the footage (below) at the recent meeting of the Acoustical Society of America. BBC News explains:
They used what is known as schlieren photography to catch the shock wave. The technique can image variations in what is known as the refractive index of air—in essence, the speed of light in a given medium. Because shock waves represent a stark and sudden change in refractive index, they show up clearly in schlieren photographs.
These trombone-generated shock waves briefly travel at speeds of about 1 percent greater than the speed of sound, fast enough that anyone sitting in front of a trombone player might actually feel it. “Musicians sitting in front of the trombone or trumpet have suffered from these shock waves,” Takayama told BBC News.
It’s a reminder that listening to my brother practice his trombone might best be done from the room next door.
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