Studying a guitar's radiation properties with nearfield holography

Abstract

Recent studies of an intact string-excited guitar have shown that the motion of the air in the rose is quite significant over the first two octaves of the instrument's range. Our nearfield holography technique has revealed that the rose is a dominant source of the radiated energy. We have found that modeling the guitar as a rigid cavity with flexible top plate and a sound hole is not accurate. The motion of the back plate in the guitar studied was quite significant, with amplitudes across it 2–3 times greater than at the top for the lower frequencies, and therefore should not be ignored in subsequent studies. Additionally, accepted plate modes seem to occur at more than one frequency in the string-excited guitar. The recurrence of these modes may tend to smooth out the frequency response curve, and a better understanding of this effect may lead to a more well balanced instrument.

Scattered ultrasound fields measured by scanning laser vibrometry

“…The quality of such recordings were however quite low. Acoustic holography using an array of microphones was reported for instance by Williams et al 24 and applied to guitar research by Strong Jr. et al 25 . Similar measurements using force transducers or accelerometers have been performed to detect ore and oil in the ground.…”

Four decades of near-field acoustic holography

“…The technique has enabled to address a large array of problems, ranging from fuselage vibration to tire-road interaction, design of loudspeakers, and radiation of classical guitars and of legendary historical violins. 9,11,12 Presently, NAH is enabling us to better characterize the acoustic properties of materials, analyze complex sound fields in rooms, understand principles of metamaterial design, or determine elastic properties of submerged bodies. 13,14 Furthermore, the formalisms developed in acoustic holography have been instrumental for advancing areas such as sound field control, sound field analysis and reproduction.…”

Acoustic radiation from bowed violins

“…Solving for the coefficients of the expansion essentially solves for the whole sound field, although concerns were expressed at the time with convergence at small radii. This technique is analogous to that of planar near-field acoustical holography ͑NAH͒, 10,11 which Strong and co-workers used on guitars 12 and violins. 13 NAH, which is the basis of measurements taken in this work as well, reconstructs the three-dimensional sound field, including particle velocity and acoustic intensity, from one two-dimensional set of complex pressure measurements.…”