Science & Environment

Shock wave from trombone filmed

Shock waves emanating from a trombone have been caught on video for the first time, researchers say.

It was first suggested in 1995 that the intense pressure waves, which can briefly exceed the speed of sound, could come from trombones.

Shock waves can form when energy is quickly put into a confined channel; weak shock waves can be formed as trains enter tunnels.

Researchers revealed the video at the Acoustical Society of America meeting.

Shock waves are just a particular kind of pressure wave - which is what sound itself comprises.

That trombones could produce such weak shock waves was first posited in a 1995 paper by Mico Hirschberg of the Eindhoven University of Technology.

Image caption Mahler's symphonies often called for dramatic "fortississimo" notes from the brass section

Now Kazuyoshi Takayama and Kiyonobu Ohtani from Tohoku University's Institute of Fluid Science worked with Professor Hirschberg to get an intimate look at the process.

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.

The shock waves are formed when the trombone is blown particularly hard - in music parlance, "fortissimo" and "fortississimo".

"Mahler and Tchaikovsky loved such dramatic specifications without knowing about shock waves," Professor Takayama told BBC News.

"Musicians sitting in front of the trombone or trumpet have suffered from these shock waves."

The team measured the pressure at the instrument's mouthpiece, in the middle of its length, and at the output, and witnessed how the train of compression waves built up to the more abrupt shock wave, travelling briefly at about 1% more than the speed of sound.

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