Inhomogeneous diffusion process in elongated rooms: An interpretation based on the dynamics of sound particles.

Previous studies showed that the reverberant field in elongated rooms is governed by non-homogeneous diffusion. The objective of this study is to physically interpret this phenomenon by considering the dynamics of the sound particles. Starting from the original diffusion theory, a quantity that can be interpreted as a "local" mean free path has been proposed and computed from the paths of the propagating particles. Based on the proportionality relationship between the mean free path and the diffusion coefficient, the spatial distribution of the latter could be estimated and successfully compared with a direct estimation using the Fick's law.

Dependence of the internal geometry for the calculation of the Helmholtz frequency in an axisymmetrical acoustic resonator.

The well-known formula of Helmholtz is well established and perfectly suited to predict the resonance frequency of cylindrical resonators when using neck length corrections. The potteries celled in the walls of medieval buildings are the object of archaeological studies where the knowledge of their volume is the starting point to deduce their origin. The frequency measurement and Helmholtz's formula is a clever way to get the volume without touching the building in situ. However, the internal corrections which suppose an interior abrupt connection condition are less effective because this assumption is not realistic for the complex shape of potteries. This study investigates internal corrections for two types of often observed geometries of potteries with conical or parabolic transitions between the neck and the volume. Correction formulas are proposed according to both an experimental and a numerical study.

Time-domain delay-and-sum beamforming for time-reversal detection of intermittent acoustic sources in flows.

This study focuses on the identification of intermittent aeroacoustic sources in flows by using the time-domain beamforming technique. It is first shown that this technique can be seen as a time-reversal (TR) technique, working with approximate Green functions in the case of a shear flow. Some numerical experiments investigate the case of an array measurement of a generic acoustic pulse emitted in a wind-tunnel flow, with a realistic multi-arm spiral array. The results of the time-domain beamforming successfully match those given by a numerical TR technique over a wide range of flow speeds (reaching the transonic regime). It is shown how the results should be analyzed in a focusing plane parallel to the microphone array in order to estimate the location and emission time of the pulse source. An experimental application dealing with the aeroacoustic radiation of a bluff body in a wind-tunnel flow is also considered, and shows that some intermittent events can be clearly identified in the noise radiation. Time-domain beamforming is then an efficient tool for analyzing intermittent acoustic sources in flows, and is a computationally cheaper alternative to the numerical TR technique, which should be used for complex configurations where the Green function is not available.

Experimental localization of an acoustic sound source in a wind-tunnel flow by using a numerical time-reversal technique.

The possibility of using the time-reversal technique to localize acoustic sources in a wind-tunnel flow is investigated. While the technique is widespread, it has scarcely been used in aeroacoustics up to now. The proposed method consists of two steps: in a first experimental step, the acoustic pressure fluctuations are recorded over a linear array of microphones; in a second numerical step, the experimental data are time-reversed and used as input data for a numerical code solving the linearized Euler equations. The simulation achieves the back-propagation of the waves from the array to the source and takes into account the effect of the mean flow on sound propagation. The ability of the method to localize a sound source in a typical wind-tunnel flow is first demonstrated using simulated data. A generic experiment is then set up in an anechoic wind tunnel to validate the proposed method with a flow at Mach number 0.11. Monopolar sources are first considered that are either monochromatic or have a narrow or wide-band frequency content. The source position estimation is well-achieved with an error inferior to the wavelength. An application to a dipolar sound source shows that this type of source is also very satisfactorily characterized.