ABSTRACT
This paper describes studies leading to the development of an acoustic instrument for measuring properties of micrometeoroids and other dust particles in space. The instrument uses a pair of easily penetrated membranes separated by a known distance. Sensors located on these films detect the transient acoustic signals produced by particle impacts. The arrival times of these signals at the sensor locations are used in a simple multilateration calculation to measure the impact coordinates on each film. Particle direction and speed are found using these impact coordinates and the known membrane separations. This ability to determine particle speed, direction, and time of impact provides the information needed to assign the particle's orbit and identify its likely origin. In many cases additional particle properties can be estimated from the signal amplitudes, including approximate diameter and (for small particles) some indication of composition/morphology. Two versions of this instrument were evaluated in this study. Fiber optic displacement sensors are found advantageous when very thin membranes can be maintained in tension (solar sails, lunar surface). Piezoelectric strain sensors are preferred for thicker films without tension (long duration free flyers). The latter was selected for an upcoming installation on the International Space Station.
ABSTRACT
Corprene has long been used in underwater projectors and receivers as an acoustic isolation material. Based on data initially reported by Higgs and Eriksson for type DC-100 Corprene [J. Acoust. Soc. Am. 46, 1254-1258 (1969)], a simple set of empirical relations is formulated that allow its density, sound speed, and acoustic attenuation to be estimated over a wide range of pressures. Laboratory measurements of the compressibility of recently manufactured material are also reported. One of the primary motivations behind this present study is to accurately determine the compressibility of Corprene. This information is needed to ascertain the variable ballast in a submersible as it goes to depth. This newly reported data, which additionally include measurement of hysteresis, agree with the results published by the aforementioned authors, indicating that this common material has remained unchanged over the past four decades.
ABSTRACT
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.
