An error inherent in the use of the two-microphone method for gas pulsation measurement in a reflective environment

Abstract

It is standard practice to provide anechoic termination in order to obtain consistent gas pulsation measurement data for the positive displacement machinery in the compressor industry [1]. The same holds for the measurement of acoustic power generated by fans or compressors. The underlying idea is to eliminate reflected wave. This prompted several researchers to make use of the so-called two-microphone method or the transfer function method to decompose the standing wave into the two constituent progressive waves [2] in order to meet the exigencies of the industrial applications where an anechoic termination may not be available. This is particularly true of the refrigeration compressor industry where it is di$cult to control the terminal impedance in the suction or discharge lines of the regular testing facility in the factory environment [3]. However, it is generally not known or appreciated that the forward wave pressure or the associated power in a reflective environment is not equal to that in an anechoic environment. Therefore, extraction of the forward wave pressure from the standing wave pressure field by means of the two-microphone method is inherently falacious. This has been demonstrated by Lai through several tests on a simplified model [3]. The same is sought to be established here in a general way.