ACOUSTICAL PRINCIPLES OF A DYNAMIC COMPRESSIBILITY APPARATUS WITH INTERNAL REFERENCE

McKinney, Edelman, and Marvin [J. Appl. Phys. 27, 425 (1956)] described an apparatus for dynamic compressibility measurement consisting of a small pressure chamber containing two piezoelectric transducers, a liquid of known compressibility, and the sample. Several investigators have used this system, sometimes in slightly modified form, in studies of polymers. However, since a fully calibrated measurement requires assembling the chamber three times, results can be vitiated by change of transducer properties in pressure-temperature cycling or by failure to reproduce assembly conditions. These problems are avoided in a new apparatus, suitable for frequencies from a few hertz to more than 1000 Hz. It provides calibrated results from measurements taken quickly at any chosen pressure and temperature, without reassembly. There is a central chamber with transducers, a side chamber with sample, and a similar side chamber containing only liquid. A moveable partition is used to let the central chamber communicate acoustically with either side chamber, or both. Operating equations are derived. Also analyzed and studied is the behavior of the system with increasing frequency as the quasistatic approximation breaks down. Test data on poly(vinyl acetate) are shown.