Researchers at Cardiff University reporting at the Acoustical Society of America meeting have presented striking images of stringed instruments made using "holographic interferometry", which allows them to visualise precisely how and where the soundboards of instruments vibrate. (Photos courtesy Bernard Richardson)

First, a laser beam is scattered off the instrument when it is still. That pattern, along with another laser beam, are cast back onto the instrument as it is actively vibrated. The contour lines appear where tiny movements of the soundboard - less than a millionth of a metre - cause interference between the two beams.

For centuries, instrument makers have used the "Chladni method" to find the "nodal lines" of a soundboard - the points where no movement occurs. Black powder is scattered on the soundboard, and as it is vibrated, the powder preferentially moves to the places where the soundboard is still.

The holographic method shows the same nodal lines, as well as bending of the soundboard. Some instrument makers use mode visualisation to make “tonal copies” of master instruments, copying their vibration patterns instead of simply their dimensions, resulting in a more faithful reproduction of the sound quality.

One of the predecessors to the modern guitar was the vihuela, comprising six pairs of matched strings.

Dr Richardson said that despite being smaller and of a different shape, the vihuela produces vibration patterns broadly similar in shape to the guitar - so the two instruments share similar acoustics. Because of its lighter soundboard, it can achieve similar volume even though its strings are thinner.

"Guitar-making is a compromise between stiffness and flexibility," Dr Richardson said. "The struts on the underside of the soundboard help to create a lightweight but strong plate which creates lots of sound in response to the vibrations of the strings."

"At low frequencies, the guitar plate moves back and forth not unlike the piston of a loudspeaker," Dr Richardson explained. "The bright line near the ribs is a nodal, or non-moving, line. This mode of vibration creates large volume changes in the air and produces a lot of sound radiation."

"At higher frequencies, the plate starts to divide into distinct patches separated by nodal lines. Sound radiation from these modes is not as strong, but these modes 'colour' the sound to give each instrument a unique voice."