Tube method of sound-absorption measurement extended to frequencies far above cutoff

Equipment had been constructed for measuring sound absorption in the frequency range from 4 to 100 kHz in a large tube 25.4 cm in diameter and 4.8-m long. The technique employs a large moveable solid-dielectric capacitance transducer that completely fills the tube cross section and generates pulses of plane waves. An identical transducer terminates the other end of the tube and serves as a microphone to detect and reflect the sound pulses. Measurements in argon, nitrogen, and air indicate that the attenuation of the sound pulses differs by less than 1% from values calculated for the zero-order mode for frequencies up to 44 times the cutoff frequency for the first 'nonplane' mode. Above that frequency, the measured values are less than those predicted by theory by an amount that is approximately proportional to the wavelength to the -3.1 power. In a smaller tube of similar construction, the high-frequency deviation from theory is absent.