Sensorless Electroacoustic Absorbers Through Synthesized Impedance Control for Damping Low-Frequency Modes in Cavities

This paper presents a concept of sensorless electroacoustic absorber for damping the low-frequency modes in a cavity such as a duct or a room. Taking advantage of the reciprocity of the voice coil transducer, it is shown that a synthetic electrical admittance can be designed so that the loudspeaker diaphragm is matched to a target specific acoustic impedance. This electroacoustic device provides a relatively broadband sound absorption that can be used to dampen room modes regardless of the sound field in which the loudspeaker is located. A digital filter is used to replicate the frequency response of the synthetic load, and a voltage-controlled current source is needed so that the filter is seen as an electrical admittance. Unlike previous attempts to implement the synthetic load using an electrical network, greater flexibility and accuracy can be obtained. Experimental results confirmed the validity of this sensorless electroacoustic absorber (SEA) in a 1D sound field, showing that the dynamic range of the sound pressure level in a duct can be reduced by 15 dB from 50 Hz to 300 Hz compared to a hard surface panel. A discussion on the strengths and limitations of this concept is provided, in particular with a view to employing SEAs for modal equalization in actual listening rooms.