Vibratory Patterns Observed in Real Time by Interferometric Holography

Abstract

Interferometric holography [K. Stetson, J. Acoust. Soc. Amer. 39, 1215 (1966)] provided detailed photographic records of the vibrational modes of given structures at known amplitudes ranging from approximately 10−5 to 10−4 cm. This method is currently being adapted for a study of the vibrational modes of the tympanic membrane. However, the approach is handicapped by the fact that nothing can be observed until the exposed holographic plate has been processed, and that each plate records only the response to one given frequency/amplitude combination. In an effort to alleviate these shortcomings, a real-time technique was developed. Although of lesser quality than Stetson's original method, it facilitates viewing the complex dynamics of vibratory events directly as they occur with changes in signal parameters. Interesting mode patterns may then be selected for later registration in the standard manner and for more detailed analysis. A motion picture is presented showing real-time interferometric holography of the vibrating diaphragm of an earphone with mode patterns changing continuously as signal parameters are being varied. [This work was supported by NIH grants.]