A Physics Approach
Tutoring fluid physics at the ANU between 1972 and 1976, Graham became intrigued with the way soundboards vibrated and generated sound. Studying gas flows and shock waves around craft entering planetary atmospheres, Graham began to apply the gas equations to explain the way vibrating musical instruments induced flows and compression in the gas within and around them, and so generated sound.
The Australian Research Council subsequently granted equipment for frequency response analyses of musical instruments. The response graphs showed how the tops of the guitars were vibrating and how much sound they were producing at different frequencies. As the frequency of vibration sweeps steadily upwards the pen plots a series of peaks, each indicating a mode of vibration which generates sound more or less efficiently, depending on vibration geometry.
Sound waves generated by vibrating musical instrument surfaces travel through air. But at low frequencies, musical instruments act like oscillating air pumps where they pump air backwards and forwards between different parts of themselves. The sound is generated not directly from the surfaces, but from the air flowing to and fro around the instrument, when it gets compressed and decompressed during cycles of vibration. Sound waves are generated from these marvellous low frequency flow fields around guitars. At higher frequencies, the air does not have time to flow back and forth, and the compression and decompression occurs right at the vibrating surface. However, much of the guitar’s sound generation occurs at low frequencies and in the range of transition from flow fields to direct radiation from the soundboard
A person's body also affects the airflow around a guitar and that's the reason why neo-classical guitars have very thick and stiff back and sides. You don't want energy flowing into your body – you want to have the sound radiate into the audience. These new guitars will often sound louder in front of the player than behind and the player doesn't always get a full impression of the projection until he/she plays in a reverberant room. Players have often said the new guitar is not terribly loud. I tell them to play inside a room or near a door or window and then they realize how much the guitar is projecting.
Traditionally, makers spend a lot of their lives trying to make the back and sides vibrate. Players think the guitar is beautifully active and even when you speak you can feel the back and sides vibrate. But the back and sides are wasting energy. Such a small amount of energy is delivered to the plucked string. Draining it into your clothing or sound generation behind the guitar is wasteful and shortens the duration of notes.
The new guitars provide projection and sustain, and open new dimensions in performance.
Relevant articles and papers are available from the following publications:
Plate Fundamental Coupling and its Musical Importance
Catgut Acoustical Society Journal #36, 1981
The Guitar Frequency Response The "Chicago Papers"
Journal of Guitar Acoustics #6, 1982
*Background Music: Chris Daley on spruce top concert guitar.