Towards an active acoustic anechoic chamber

An acoustic anechoic chamber is a space in which no reflections occur at the walls. The acoustic anechoic chamber therefore approaches free-field conditions, in which no reflections occur either. Acoustic free-field conditions allow measurements to be conducted without contamination from any boundary. The free-field conditions are realized with special laboratory facilities known as acoustic anechoic chambers. This allows for standardization, product testing, research and development and calibration, all under free-field conditions. The sound absorption in an acoustic anechoic chamber is typically realized by using absorption material, which is placed at the boundaries of the room. However, because the thickness of the absorption material is related to the wavelength of the acoustic waves, the performance of the absorption material deteriorates at lower frequencies. Consequently, a typical acoustic anechoic chamber has a lower cut-off frequency of up to 200 Hz, meaning that free-field conditions are not guaranteed below this frequency.

Active noise control is effective at lower frequencies, which therefore makes it a promising technique to complement the passive absorption and to lower the cut-off frequency of an acoustic anechoic chamber. To implement an active noise control system in an acoustic anechoic chamber, an accurate estimation of the reflected sound field is necessary. The solution to this problem is found in the application of the Kirchhoff-Helmholtz integral, which in two dimensions uses several microphones distributed along a circle. The microphones measure the acoustic pressure and the particle velocity, which are the input to the Kirchhoff-Helmholtz integral. The output of the Kirchhoff-Helmholtz integral is the reflected sound field due to any source located within the circle, while for any source located outside the circle it outputs the total sound field.