Attenuation of the (1,0) and (2,0) modes in rectangular ducts

The attenuation constant of the (1,0) and the (2,0) modes of a rectangular duct have been studied in the frequency range 1180…3400 c/s. A rectangular duct, confining a constant volume between a source and a reflector, is constrained to move between fixed rails. The various modes are excited by the ‘hole’ method and the sound field within the duct is surveyed by a probe microphone small enough not to disturb the sound field. The standing-wave ratio is measured by a resonance peak method which has the advantage of avoiding the need to delineate the complete standing-wave pattern along the tube and it is suitable for determining small attenuations which is the case in this work. The experimental attenuation constants for the (1,0) and the (2,0) modes agree with the theory of Beatty within 1 %.