Optical methods of measuring loudspeaker diaphragm movement

The principles of making measurements of the vibrational behavior of lightweight diaphragms, using optical methods employing coherent light interferometric techniques, are discussed. The use of a frequency shifted reference beam (Doppler interferometry) is described, and simple methods of obtaining such a reference beam are considered. Results of measurements made on the diaphragms of loud speaker drive units are discussed and methods of presenting these results are outlined.