Acoustic Characteristics of Space-Simulation Chambers at Low Pressures

Abstract

Three test chambers with volumes between 175 and 32 000 ft3 have been acoustically calibrated for air pressures from 760 down to 25 Torr. Such calibration presents special problems, since the normally used standard noise sources have an acoustic power output with a pressure dependence that is difficult to calculate. Therefore a special sound source was developed, using a stiff and heavy piston as radiator. This sound source, with known pressure characteristics, was used during all calibrations. Reverberation time was also measured in the chambers for all pressures. In the paper, results from the two independent calibration methods are compared to each other. The measured acoustic characteristics are also compared to theories for air damping, including effects from the humidity-dependent molecular damping. It is concluded that the chamber damping increases with decreasing chamber pressure for all tested frequencies (100–40 000 cps). At high frequencies, the measured chamber damping agrees well with the theoretically predicted air damping, thus showing that the wall damping is negligible. A general calibration curve—valid for various frequencies, air pressures, and chamber volumes—is presented.