Air cavity modes in sound boxes recorded by TV holography

Abstract

Amplitude and phase distribution of air cavity modes in a rectangular box, in a guitar, and in a violin are measured using TV holography. The box and the top and back plates of the guitar and the violin are made transparent using 5-mm-thick and flat PMMA plates. Standing waves in the enclosed cavities are generated by a loudspeaker. Object laser light is sent through the cavities to a rigid white-painted steel block and reflected back again through the cavities to the optical unit of the TV holography interferometer. Laser light traveling through a sound pressure field will experience a phase delay, since an increased air pressure is accompanied by a higher air density as well as a higher index of refraction which in turn gives a slower speed of the laser light, which generates a phase delay compared to undisturbed air. This often small phase change is made visible by the highly sensitive phase-modulated TV holography technique. The optical system will be described and air cavity modes in a simple rectangular box, in a guitar, and in a violin will illustrate the presentation.