Robert D. Corsaro

Acoustic and dynamic mechanical properties of a polyurethane rubber

Acoustical and dynamic mechanical measurements were carried out on a commercial polyurethane rubber, DeSoto PR1547. The sound speed and attenuation were measured over the range from 12.5 to 75 kHz and 3.9 to 33.6 degrees C. Shear modulus was measured from 10(-4) to 2 Hz and -36 to 34 degrees C. The peak heights of the shear loss tangent varied with temperature, demonstrating thermorheological complexity. At higher temperatures, time-temperature superpositioning could be applied, with the shift factors following the Williams-Landel-Ferry equation. From the combined acoustical and mechanical measurements, values for the dynamic bulk modulus were determined. Moreover, superposition of the bulk modulus data was achieved using the shift factors determined from the dynamic mechanical shear measurements. Finally, this work illustrates the capability and the working rules of acoustical measurements in a small tank.

Strain relaxation in glass by optical correlation and pressure jump relaxation

Strain relaxation functions have been obtained for boron trioxide glass in its transformation region by digital correlation of a scattered optical beam and by pressure jump relaxation measurements utilizing a recently developed acoustic densitometer. The results confirm that the light−scattering correlation data is a measurement of the isothermal constant stress compressibility function. These measurements, together with previous ultrasonic results, cover a temperature region for B2O3 in which the structural relaxation time changes by more than 13 orders of magnitude.

Sensor—actuator tile for underwater surface impedance control studies

An actuator—sensor tile has been developed for use in studies examining the issues involved in actively controlling the reflection and transmission characteristics of generic underwater structures. The tile is 25 cm square and contains a full area actuator, acoustic pressure sensor, and (acoustic particle) velocity sensor. This paper presents the predictive models used in the tile design. Issues addressed include transducer performance, spatial sampling, near-field sensing, internal resonances, and both direct and extraneous coupling mechanisms, all of which can contribute to complicate the system transfer functions. The acoustic characteristics of the final tile were evaluated at the NRL Laboratory for Structural Acoustics (LSA) and these results are compared with predictions.

Acoustic densitometer with results for polyethylene oxide

An acoustic densitometer has been developed for the precision measurement of PVT data over a wide range of temperatures and pressures. With this device, changes in the density of a solid or liquid are measured with a precision better than 3 parts in 105. Further, a density change of as much as 15% may be accommodated with no loss in the sensitivity or accuracy of the measurement. Finally, the electronic system used is straightforward, for which components are readily available. The system was tested on the polymer, polyethylene oxide, over a temperature range 10–50 °C and at pressures up to 1 kbar. Volume derivatives were obtained from the data and used to evaluate the Broadhurst‐Mopsik model for linear polymers. The agreement is good for temperatures up to and including 30 °C. In the temperature interval 40–50 °C, changes in the degree of crystallinity of the sample are suggested by the data.

Filled rubber materials system: Application to echo absorption in waterfilled tanks

This paper describes a materials system for forming rubber composites with selectable acoustic properties. It is particularly suitable for laboratory use in molding small articles with longitudinal sound speeds and impedances less than (or not much greater than) those of water, and with sound absorption coefficients which may be varied over a wide range. The composites are formed using a common silicone rubber resin (RTV‐602) to which various fillers may be added in controlled amounts. In this study, we measured the sound speed, density, and attenuation coefficient for more than 100 samples containing various concentrations and types of these fillers. These data were then reduced to determine the best‐fit coefficients in a set of descriptive equations. Thereafter these equations could be used to calculate the filler concentrations needed to form composites with specific required properties. To demonstrate the usefulness and predictability of these materials in an application of general interest, these com...

Influence of backing compliance on transducer performance

This paper presents a simple one‐dimensional model for describing the influence of backing impedance on the performance of sensors and projectors. It uses a center‐of‐mass model, where the influence of backing can be largely extracted into a separable term. As an example, it includes consideration of the familiar underwater case of an air‐backed steel plate with an outer compliant (decoupler) coating. This approach is principally useful in large numerical structural acoustical studies, where the use of complete transducer formulations would become computationally impractical.

Active acoustic impedance modification arrangement for controlling sound interaction

An active impedance modification device or arrangement enables the interaction of sound with a structural surface to be controlled, e.g., so that, reflections from that surface (which can be the hull of a submarine) are substantially reduced or eliminated. The device comprises a coating comprising an inner driver transducer layer in contact with the structural surface an outer receiver transducer layer which receives the sound, in combination with a variable gain, variable phase shift amplifier connected between the interface of the outer layer with the sound carrying medium (e.g., water) and the interface between the second layer and the structural surface. Reflections are reduced by setting the gain and phase shift of the amplifier to simulate the input impedance of the sound carrying medium.

Double layer actuator

This paper examines the potential for dual‐layer actuators to achieve radiation efficiencies independent of the backing structure. It presents a simple center‐of‐mass representation for the device, and uses it to describe the kinematics of the dual layer actuator system with the purpose of identifying broad guidelines regarding thickness and frequency range issues. This approach has the advantage of providing a direct physical view of the device operation. In this context, it is shown that these devices work by electrically moving the center of mass of the transducer in opposition to the motion of the adjoining fluid. While such devices are shown to be generally less efficient than the simple parallel‐driven double‐layer transducer, over much of their operating range the loss in efficiency is found to be only a few decibels.

A small pressurized vessel for measuring the acoustic properties of materials

A laboratory system is described that measures the acoustic properties of materials in the frequency range from 10–100 kHz, at pressures to 150 psi (1000 kPa), and from 0°–40° C. By using transducer arrays and broadband pulses together with digital data acquisition and analysis techniques it is possible to contain this (essentially) free‐field system in a vessel with a volume of only 0.42 m3. The system provides measurements of complex reflection and transmission coefficients as a continuous function of frequency. From the data one can obtain the following material parameters: sound speed, frequency‐dependent sound attenuation coefficient, complex acoustic impedance, echo reduction, and insertion loss. Results are presented for a variety of samples to illustrate the capability of this system.

Acoustic velocity sensor for the NRL ABC research platform

A new research platform has been constructed for general underwater structural‐acoustics studies of sensor/actuator coupling mechanisms, and in particular for active acoustic boundary control (ABC) studies. It consists of an array of 15 ‘‘ABC’’ tiles arranged in a 5×3 pattern on a backing structure (an air‐backed steel plate). Tiles are 10 inches square, and each tile contains a large area actuator, pressure sensor, and (acoustic particle) velocity sensor. While the actuator and pressure sensor could be constructed of commercially available transducer material, the selection of a suitable acoustic velocity sensor proved more difficult. This paper describes the velocity sensor system selected and its impact on the resulting performance and characteristics of the ABC Platform.