Reverberation chambers used for acoustical measurements should have completely random sound fields. We denote by R the cross-correlation coefficient for the sound pressures at two points a distance r apart. R = 〈p1p2〉Av/(〈p12〉Av〈p22〉Av)12, where p1 is the sound pressure at one point, p2 that at the other, and the angular brackets denote long time averages. In a random sound field, R = (sinkr)/kr, where k = 2π/(the wavelength of the sound). An instrument for measuring and recording R as a function of time is described. A feature of this instrument is the use of a recorder's servomechanism to measure the ratio of two dc voltages. The results of correlation measurements in reverberant sound fields are given.
This paper describes a piezoelectric shaker consisting of a combination of damped resonant cylindrical elements. Material characteristics and design parameters are chosen so that the resonances of the combined elements overlap to provide “good” motion over a wide frequency range. Data from three shakers are presented to show how a suitable set of shakers can be used for the calibration of vibration pickups at frequencies up to 100 kHz.
An apparatus has been developed for the direct measurement of the real and imaginary parts of the dynamic bulk modulus of solid and liquid materials over the frequency range of 50 to 10 000 cps. Piezoelectric crystals serving as driver and detector, together with the sample and a confining liquid, are contained in a cavity small compared with the wavelength of sound at these frequencies. Static pressure is superposed to eliminate the effect of small air bubbles. The complex compliances of the sample, confining liquid, and the cavity, are additive in this region, where the compliance is pure dilatation. The dynamic compliances of several natural rubber-sulfur mixtures were obtained in a preliminary evaluation of the behavior of the apparatus.
Miniature piezoelectric polymer hydrophones for ultrasonic field characterization in the low megahertz region have been developed and tested. The principal advantages of these devices over conventional hydrophones are their uniform frequency response and minimal perturbation of the field. These characteristics are achieved by rendering a small central region of a thin sheet of the polymer polyvinylidene fluoride locally piezoelectric, and then supporting the sheet in the field by holding it taut in a metal hoop having dimensions larger than the field being probed. Both single elements having diameters less than 1 mm and multielement arrays have been formed on the polymer. Methods of construction, signal amplification, and, in one design, rf shielding are discussed, and data are presented on insertion loss, sensitivity, frequency response, and immunity to rf interference.
The direct piezoelectric effect has been observed in roll-elongated films of polyvinylchloride and polyvinylfluoride. The effect was produced by applying tensile stress to a clamped specimen at a fixed frequency of 20 Hz. The piezoelectric modulus g31 was determined to be 0.2 to 0.7 V m−1/N m−2 in polyvinylchloride; in polyvinylfluoride g31 ≃ 0.2 V m−1/N m−2 and g32 ≃ 1 V m−1/N m−2. The piezoelectric effect in these films is believed to be due to mechanical distortion of oriented dipoles, resulting from the tensile stress, and the orientation appears to differ in the two materials.
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