Measuring hybrid passive-active sound absorption of a microperforated liner at oblique incidence

The performance of hybrid passive-active absorbers at normal incidence has been previously demonstrated. However, their feasibility in realistic environments remains to be confirmed. This paper proposes a hybrid passive-active liner absorbing at oblique incidence. The passive absorber is a 2.44x2.44x1 mm(3) microperforated panel backed by a 5 cm deep air layer and a rigid ending. An active multichannel cell, composed of five single-input single-output controllers, releases the sound pressure at the entrance of the air cavity in a reduced area of the microperforated panel absorber. Hybrid passive/active absorption of the prototype is measured in an anechoic room up to a maximum incidence angle of 21 degrees. For instance, at 20 degrees such a system is able to provide an average absorption of 94% between 266 and 1500 Hz. In addition, the performance that would provide such a hybrid absorber in a reverberant room has been quantified theoretically by an index describing the active-to-passive reverberation time ratio in terms of the active-to-passive absorption area and absorption coefficient ratios. The proposed prototype, implementing 7% of the active proportion area, would afford a reverberation time reduction between 77.5% and 12% from 100 to 500 Hz.