Using the two-microphone method, acoustic properties in ducts, as, for example, reflection coefficient and acoustic impedance, can be calculated from a transfer function measurement between two microphones. In this paper, a systematic investigation of the various measurement errors that can occur and their effect on the calculated quantities is made. The input data for the calculations are the measured transfer function, the microphone separation, and the distance between one microphone and the sample. First, errors in the estimate of the transfer function are treated. Conclusions concerning the most favorable measurement configuration to avoid these errors are drawn. Next, the length measurement errors are treated. Measurements were made to study the question of microphone interference. The influence of errors on the calculated quantities has been investigated by numerical simulation. From this, conclusions are drawn on the useful frequency range for a given microphone separation and on the magnitude of errors to expect for different cases.

In an earlier work [H. Bodén and M. Åbom, J. Acoust. Soc. Am. 79, 541–549 (1986)] the influence of errors on two-microphone measurements in ducts without flow has been studied. The aim of this article is mainly to extend the earlier work to include the effects of mean flow and also of attenuation during the sound propagation. First, a short review of the various existing two-microphone methods is made. The errors in the measured input data are then analyzed and special attention is paid to the effects of neglected attenuation, nonideal microphones, and flow noise. The influence of errors on the calculated quantities has been investigated and the conclusions from the earlier work have been extended to the case with flow. It is also shown that the neglect of attenuation between the microphones leads to a low-frequency limit for the applicability of the two-microphone method. Finally, a new technique for measuring the Mach number using a two-microphone method is suggested.

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Microperforated plate (MPP) absorbers are perforated plates with holes typically in the submillimeter range and perforation ratios around 1%. The values are typical for applications in air at standard temperature and pressure (STP). The underlying acoustic principle is simple: It is to create a surface with a built in damping, which effectively absorbs sound waves. To achieve this, the specific acoustic impedance of a MPP absorber is normally tuned to be of the order of the characteristic wave impedance in the medium (∼400 Pa s/m in air at STP). The traditional application for MPP absorbers has been building acoustics often combined with a so called panel absorber to create an absorption peak at a selected frequency. However, MPP absorbers made of metal could also be used for noise control close to or at the source for noise control in ducts. In this paper, the possibility to build dissipative silencers, e.g., for use in automotive exhaust or ventilation systems, is investigated.

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