Article from The Guardian
Thursday, May 30^th, 1996

Focus on the good vibrations, Sue Nelson discovers a holographic camera that measures musicians with feeling.

Blow the note and feel the quaver

The evidence is there in black and white: some people literally have a feel for music. Photographs taken at Warwick University show that when musicians hit a perfect note they actually vibrate in tune with their instruments.

"I've gone through everything from a harpsichord and French horn to Chinese gongs and a trumpet." say Dr Peter Bryanston-Cross. "We have an orchestra here and I've been gradually working my way around. I don't think it's a random effect."

The research into musical instruments began in 1987, when Bryanston-Cross was asked to visualise the sound board of a harpsichord. "It wasn't until I took the pictures that we understood what we were seeing. The significance took a while to sink in."  The incriminating photographs were produced by a holographic camera designed and built by the university's optical engineering laboratory. "We use a double pulse ruby laser with a very short pulse duration-- around 20 nanoseconds in length. As a result, movements which wouldn't normally be seen are frozen. In this case the movement is of the flesh-- the skin (Click on thumbnails to view).

   Bryanston-Cross attaches a frequency generator to a motor beneath an aluminium plate -- "known in the trade as a shaking table" -- to explain the concept. When resonance occurs the frequency corresponds to the movement of sound across the metal plate. The resonant frequency can be heard. It can also be seen by sprinkling the plate with table salt. "When I produce resonance the salt bounces to produce patterns; where there's no salt there's little or no vibration," he says.

  "Particles of light behave in a similar way, so when I shine a laser on a surface I can create similar patterns." One minor problem had to be overcome, however, since a musician's clothing interfered with the hologram and prevented researchers viewing the vibrations. All subjects -- including Bryanston-Cross's horn-playing son, Saul -- therefore agreed to be photographed topless while playing their instruments.

   A fully clothed horn player demonstrated part of the process by playing a single middle C into a microphone. "We amplify that note and send it into a digital spectroanalyser," explains Steve Greenall, a trombone player and final-year engineering student working on the project. A large narrow peak appears on the screen.

  "This shows us the shape of a perfect note , so that when we take a holographic image of the musician we know it will be a good hologram." There is , however, one extremely small peak to the left of the note. This corresponds to a harmonic. "It's actually very difficult to play a perfect note without creating harmonics which distort the sound," says Greenall. "So what we're doing is looking at how a musician can sense if the note is perfect within himself, In some ways he's got his means of testing the note because he's actually feeling the music."

   Further research was closer to home for Bryanston-Cross. "When my son Saul hit a perfect note I asked him how he knew -- and he said he could feel it." Although Bryanston-Cross senior is a non-musician, his student's musical interest conveniently overlaps with their research. So does Greenall feel a resonant vibration when he plays the trombone?  "There are notes that you can play and feel 'yes, that's spot on.' and you can also tell instantly when you play a note that isn't right," he replies. "If the note doesn't quite settle we can correct it and that's obviously something that's coming from the interaction between the instrument and the person themselves." The resultant photographs, produced by laser light being reflected from fatty layers beneath the skin, are intriguingly dappled with light and shade. The horn resembles a giraffe's neck but, more interestingly, a similar pattern has been produced on the musician's shoulder.

  "We get classic modal vibrations on the bell of the horn," says Bryanston-Cross, pointing to the dark rings. "They're similar to the salt patterns, as the dark region is not moving while the white regions are -- so the surface is moving in and out. But on Saul's shoulder bone even the same shape of mode is apparent. "Structural engineers confirmed that bone structure did not cause the patterns. "They felt this was a mode of vibration."

  It's not yet known, however, whether sound vibrations penetrate farther and are felt within a musician's bones. So, for the moment at least, an instinctive feel for music remains skin deep.

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