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

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 frequency range. The response to pure-tone excitation, generated by an open vented box, was recorded along two lines parallel to the walls. A numerical model for the radiating sound source was used to determine the strength of the source and its center. The region in the chamber where the deviation from free-field conditions was less than the allowed 1.5 dB was determined, based on the experimental data. Its was established that even above the cut-off frequency, deviations greater than the allowed 1.5 dB occurred. A door in the anechoic chamber was identified as at least one of the sources of these imperfections.