PROPAGANT PHASE IN REVERBERANT ENVIRONMENTS

The phase of the transfer function between two points in an extended s ystem can be easily measured if it is taken to be the accumulated phas e obtained by smoothly raising the measurement frequency from zero to the reference frequency. Lyon et al., in an extended series of papers [most recently J. Acoust. Soc. Am. 95, 286-296 (1994)], have examined the behavior of this accumulated phase in systems of two and three dim ensions and have elucidated the concept of a reverberant phase which i s independent of the separation between the two measurement points, pr ovided they are far enough apart, but which rises sharply with increas ing frequency. In some applications, for example, in nondestructive te sting of extended structures, it Is important to be able to observe si mple wave-propagation behavior and in particular to measure the propag ant phase as a function of frequency and position. The conditions unde r which this is possible are investigated, and are shown to impose con straints On the ratio between the propagation distance and the size of the structure under test, and either the material damping coefficient or the reflection coefficient at the domain boundaries. These results , which represent an extension of those of Lyon et al., are discussed in terms of the distribution of zeros of the transfer function in the complex frequency plane. Many platelike structures of practical intere st are found to satisfy these conditions, so that measurement of propa gant phase behavior can provide the basis for a useful technique of no ndestructive examination. (C) 1996 Acoustical Society of America.