LOW FREQUENCY ACTIVE ACOUSTIC ABSORBER BY ACOUSTIC VELOCITY CONTROL THROUGH POROUS RESISTIVE LAYERS

摘要

A device for wide band low frequency absorption comprises at least one electroacoustic transducer 1, mounted on an acoustic baffle 2, separating a closed rear volume 3 and a front volume 4. The front volume is closed by an acoustic fabric 5 of determined acoustic air-flow resistance. A power amplification electronic 6 with feedback control 7 is configured to obtain a transducer membrane velocity proportional to an input voltage, coming from a microphone 8 located in front of the acoustic fabric on a side opposite from the front volume. The microphone is connected to a preamplifier 9 and a feedforward control 10, with adjustable gain and band-pass filter, taking a first pressure signal coming from the microphone preamplifier and driving the power amplifier input. The feedforward control gain is equal to where Af is the fabric area, Als the projected transducer membrane area, R the fabric air-flow resistance and G1 the preamplifier gain, minimizing the acoustic pressure in the front volume, thus having a specific impedance, defined as pressure/velocity ratio, in front of the acoustic fabric equal to the determined air-flow resistance of the acoustic fabric.