Seymour Edelman

Measurement of Correlation Coefficients in Reverberant Sound Fields

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 recorders servomechanism to measure the ratio of two dc voltages. The results of correlation measurements in reverberant sound fields are given.

Piezoelectric polymer probe for ultrasonic applications

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.

Opto-acoustic transducer and telephone receiver

An optical fiber element of low density, low heat capacity, a large coefficient of thermal expansion, and a large Youngs modulus varies in light transmissivity gradually between its ends from high transmissivity to opacity, whereby power modulated light transmitted through the fiber element is absorbed to cause a change in temperature of the fiber element and a resultant thermal expansion and contraction thereof. As a transducer in a telephone receiver, a light absorbing fiber element or group of such elements is coupled between the optical fiber waveguide in the receiver and a resiliently mounted acoustical diaphragm which is caused to respond over the audible range.

APPARATUS FOR THE DIRECT DETERMINATION OF THE DYNAMIC BULK MODULUS

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.

Piezoelectric polymer receiving arrays for ultrasonic applications

Broadband ultrasonic detector elements each approximately 2 mm in diameter and multielement linear arrays have been constructed using commercially available polyvinylidene fluoride sheet 25 μm thick. Electrode and electrical lead patterns were deposited on both surfaces by vacuum evaporation. Poling was achieved by applying ∼2000 V across the electroded parts of the sheet while the temperature was cycled from room temperature to ∼120°C over a period of ∼1 h. The open‐circuit responsivity of the samples to ultrasound was measured in the frequency range of 300 kHz to 1 MHz with the samples freely immersed in water, with the array backed by a steel ref1ector plate, and with the array a distance away from the reflector plate. The measurements were performed with the elements connected to a preamplifier by means of a 50 Ω coaxial tube about 30 cm long. Since the capacitance of an element is about 8 pF, the measured responsivity of about −230 dB re 1 V/μPa was degraded considerably by the cable capacitance. The...

Opto-acoustic transducer for a telephone receiver

A sound producing diaphragm of low density, low heat capacity, a large coefficient of thermal expansion and a large Youngs modulus is located adjacent the end of an optical fiber transmission line element which is adapted to propagate modulated light and transmit the light outwardly therefrom particularly from the end region thereof to the surface of the diaphragm. The diaphragm absorbs light transmitted from the end region of the fiber optical element which heats and cools in response to the light energy absorbed thereby to expand and contract and produce a sound field. As a transducer in a telephone receiver, the diaphragm is mounted in the earpiece with the end of the optical fiber transmission line located in close proximity thereto and having an enlarged end which includes an anti-reflective coating so that light propagated along the fiber is transmitted to the diaphragm and not reflected back down the fiber.

Piezoelectric Effect in Oriented Polyvinylchloride and Polyvinylflouride

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.

Some Developments in Vibration Measurement

This paper describes cemented barium titanate accelerometers of two simple types, an electromagnetic shaker with a symmetrically mounted shake table, a set of barium titanate shakers, and two optical methods of accelerometer calibration, one using a microscope with a stroboscope of wide frequency range and one using a Fizeau type interferometer. Calibration results from 50 to 11 000 cps are given.

Improved Piezoelectric Effect in Polymers

Improved poling techniques have led to improvement in the piezoelectric activity of bulk polymers. Results now achieved approach the activity of conventional piezoelectric materials such as quartz, and further improvement is anticipated. The measured value of the piezoelectric strain modulus d31 of a length expander bar of polyvinylchloride was 0.6×10−12 m V−1 and the electromechanical coupling factor k31 was 0.6%. The response of a sample as an underwater sound receiver was −112 dB re 1 V/μbar.

Optical Calibration of Vibration Pickups at Small Amplitudes

A photometric device for accurately measuring the amplitudes of sinusoidal vibrations is described. The device is designed for calibrating vibration pickups. The apparatus employs a Fizeau‐type interferometer with a photomultiplier observing the fringe pattern. The method is applied to the measurement of vibrations throughout the audio and near ultrasonic frequency range with amplitudes ranging from 72 to 4400 A. Extension to amplitudes as low as 5 A can be made with some modification to the apparatus.