An Improved Apparatus for the Direct Measurement of the Absorption of Sound in Gases

An improved apparatus for the direct measurement of the absorption of sound in gases has been designed, built, and tested with the highly absorptive gases, carbon dioxide and nitrous oxide. Measurements are made of the sound pressure by a microphone as it is moved away from a piston source vibrating in a large baffle. The resulting pressure‐distance curves are recorded photographically and yield the absorption coefficient. Reflections from the walls of the cylindrical measuring chamber are reduced materially by specially designed glass baffles. The source is the end of a magnetostrictive rod driven by an improved Pierce‐type oscillator with a frequency range of 11.3 to 112 kc. A magnetic drive is employed to move the microphone, eliminating packing glands and providing a constant volume measuring chamber. The microphone was built up from a single bender‐type Rochelle salt bimorph element. A milliampere meter movement is used in the recorder, making possible comparatively rapid recordings.