The low frequency response of orifices (slit, circular diaphragm, and perforated plate) in the presence of mean flow is well predicted by a quasisteady theory. A refinement is brought to the theory by considering a Mach number dependent vena contracta coefficient. The measurements of the vena contracta coefficient of a slit agree well with the simple analytical expression existing in the case of the Borda tube orifice. The measured scattering matrix coefficients do not depend strongly on the geometry of the element. If the frequency is increased the moduli remain relatively unaffected while the arguments exhibit a complex behavior which depends on the geometry. From these considerations an anechoic termination efficient at high mass flow is designed.

The aeroacoustic response of a diaphragm in a pipe is studied by means of an analytical model and experimental measurements. The study is restricted to quasi-two-dimensional diaphragms with a sharp-edged rectangular aperture at conditions for which the acoustic source region can be considered compact. The compactness of the source can be realized under two conditions: either a low Strouhal number and a jet Mach number of the order unity; or a low jet Mach number and a Strouhal number of order unity. In this paper, the focus is on the "rst case. The second case of low Mach number and Strouhal number of order unity is discussed in a companion paper. The results of a quasi-steady theory are compared with measurements of the re#ection and transmission coe$cients of a diaphragm. The theoretical model is based on Ronneberger's model for a step-wise expansion (D. Ronneberger 1967, Acustica 19, 222}235) and Bechert's description of an ori"ce used as an anechoic pipe termination (Bechert 1980 Journal of Sound and