AN ANALYTICAL INVESTIGATION OF THE INDIRECT MEASUREMENT METHOD OF ESTIMATING THE ACOUSTIC IMPEDANCE OF A TIME-VARYING SOURCE

This paper presents an analytical investigation into the relationship between the characteristics of a linear, time-variant acoustic source, and the effective source impedance and source strength which follow if one then assumes the source to be time invariant. It thus models the indirect measurement method of obtaining the effective source characteristics of an actual time-varying acoustic source such as the intake or exhaust system of an internal combustion engine. It is shown that the effective source impedance and source strength have no physical meaning, bear no resemblance to the real source, and are dependent on the load system used to evaluate them. It is found that the effective source resistance is frequently negative; hence time variance of the source alone is a sufficient reason for the implausible negative resistance values as observed with the indirect measurement method. It is also shown that the meaningless effective source characteristics do, nevertheless, sometimes give a fairly accurate estimate of the noise output when the acoustic source is used with a different load, again as observed by experimentation.