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One of the most important complaints about noise in small vehicles is related to the hydraulic steering pump, which is used strongly during parking maneuvers, and produces a noise that overcomes the one coming from the engine, due to its low speed. This noise can be eliminated by the introduction of two resonators inside the high-pressure tube designed to coincide with the tube's resonance frequency. However, in some cases, as in tubes with short rubber pieces, these resonators may be not sufficient to reduce the noise to non-audible levels, due to its low ability to absorb the pressure peaks coming from the pump. In these cases it is possible to introduce orifices at each resonator to maximize its effect. Here we present the theory related to the use of these orifices and the results obtained in tests on bench.

In many cases the in situ measurement of the absorption coefficient requires an iterative method for the correct calculation of the surface impedance of a sample. This happens because when spherical waves reflect on a sample's surface the pressure field above it is a function of the sample's surface impedance. As the pressure field involves an integral term, numerical integration is required in the iterative algorithm, which can be time consuming. The aim of this paper is to present an iterative approach based on Pony's method, which instead of numerical integration uses a series expansion with a few terms. Therefore the time of processing is decreased. Besides the description of the method, simulations and measurements (with a p-u probe inside an office room and a car) are presented. A comparison, with numerical integration, regarding the accuracy and the time of processing of Pony's method is also discussed.