Acoustical Behavior of the Large Anechoic Chamber at the Laboratoire de Mécanique et d'Acoustique in the Low Frequency Range

Abstract

It is of vital importance to have detailed information about the characteristics of an anechoic chamber,since any imperfections lead to deviations from free-field conditions and will perturb measurements. In the present paper the behavior of the large anechoic chamber at the LMA is studied in the 20 to 200 Hz frequencyr ange. The response to pure-tone excitation, generated by an open vented box, wasrecorded along twolines parallel to the walls. An umerical model for the radiating sound source wasu sed to determine the strength of the source and its center.T he region in the chamber where the deviation from free-field conditions wasl ess than the allowed 1.5dBwas determined, based on the experimental data. Its wasestablished that even above the cut-off frequency, deviations greater than the allowed 1.5d Bo ccurred. Ad oor in the anechoic chamber wasi dentified as at least one of the sources of these imperfections.

Active Control in an Anechoic Room: Theory and First Simulations

“…The precalibration step is based on the essential fact that H does not depend on the source S. It can therefore be evaluated with a set of sources denoted here identification sources which are chosen such that their sound radiation patterns are known. It is determined by minimizing the difference between the diffracted pressure at point M and its expression (1). Once a numerical approximation of H is obtained, the approximation of the integral relation can be used to compute the diffracted pressure for any other source.…”

“…3 The integral relation between the total pressure and the diffracted pressure By using conventional Green's representations it is possible to relate the sound pressure inside a volume to the values of this pressure and of its first normal derivative on the boundary of the volume [37]. Writing a relation such as (1) with no normal derivative term is a bit more challenging.…”

“…A simple idea is to choose the specific Green's function defined in the volume inside Γ which satisfies a homogeneous Dirichlet boundary condition on Γ. This leads to a relation of the expected form (1). However this Green's function is not uniquely defined for frequencies equal to the Dirichlet resonance frequencies of the volume.…”

“…Conversely, existing measurement facilities are barely adapted to the measurement of sources below 70-100 Hz, as conventional lining depth cannot be increased enough for technical and economical reasons. It is thus well known that, at lower frequencies, some unwanted modes of an anechoic room may appear [1]. As an example, measurement of usual audio loudspeakers is a real concern [2].…”

Fast Solution Methods

“…The 1.5 dB region in the 20 to 200 Hz frequency range was obtained by measuring the sound pressure radiated by a bass-reflex box. For a detailed description of the experiment we refer to [58]. The anechoic chamber at the LMA has inner dimensions of 5. admittance matrix Y (ω) ∈ C 324×36 is needed.…”