A note on the acoustic character of time-variant one-port sources

초록

One-port acoustical source in a duct system can be characterized by the acoustical source impedance and strength, usually using the linear frequency-domain modeling [1], [2]. It is known that the direct method employing an array sensor layout yields different results from the indirect method, i.e., load method, in measuring the source parameters of various fluid machines. Although the real part of the source impedance is expected to be non-negative in physical sense, the indirect method sometimes comes up with negative source resistance values at some frequencies [3], [4]. It is reported that the time-varying nature of the source is the major cause of a negative resistance [4], [5], [6] although the nonlinearity also contributes to the phenomenon to some extent [7]. In this paper, the acoustical characteristics of fluid machines are studied analytically, in which the sources are considered to be time-variant. For this purpose, a simple fluid machine consisting of a cylinder with a reciprocating piston and an exhaust (or, in general, connected) pipe system is taken as a simplified model representing a typical periodic, time-varying duct system. In addition, the equivalent circuit corresponding to such a simple model is analyzed.

키워드