A broadband passive–active sound absorption system

Abstract

In recent years, hybrid absorption systems have been implemented which achieve high sound absorption over a broad frequency range. This work is an experimental study of a broadband hybrid absorption system which is comprised of a layer of sound-absorbing material ͑the passive component͒ positioned at a distance from a movable wall ͑the active component͒ inside an impedance tube. The movable wall is used to impose desired boundary conditions in the cavity behind the passive layer, thereby increasing the absorption of the system at frequencies where the passive material is not independently effective. Both pressure-release ͑i.e., minimizing the pressure at the back surface of the layer͒ and impedance-matching ͑i.e., minimizing the reflected wave from the layer͒ boundary conditions are studied. The performance of the hybrid system for these two boundary conditions is compared for broadband disturbances over a frequency range of 100-1000 Hz. The unmodified passive system showed absorption coefficients greater than 0.7 only above 500 Hz, while the impedance-matching condition yielded absorption coefficients of 0.8 to 1.0 over the 100-1000-Hz range. The impedance-matching condition provided significantly better absorption than that achieved with the pressure-release condition. The sensitivity of these control approaches to system parameters is also investigated.

Low-frequency absorption using a two-layer system with active control of input impedance

“…To implement the impedance-matching condition a wave deconvolution circuit was used, similar to the standing wave separator used by Guicking et al ͑1985͒, together with two microphones in the air gap. The experimental results of Beyene and Burdisso, further corroborated by Smith et al ͑1999͒, showed the superiority of the impedance-matching condition as compared to the pressurerelease condition on the hybrid passive-active absorption for broadband noise. Nevertheless, the authors recognize that a a͒ Electronic mail: iacpc24@ia.cetef.csic.es confirmation of this experimental result would require an analytical model that includes the physical properties of the absorption material.…”

Active Control in an Anechoic Room: Theory and First Simulations

“…So-called "hybrid" absorbers have thus been studied, combining absorbing materials and transducers driven so that the resulting device exhibits the performances of a conventional passive lining at higher frequencies and suitable performances at lower frequencies. Such systems have also been thoroughly investigated, with varying combinations of passive material, actuator, and control strategies [14,15,16,17,18,19,20].…”

Active diffusers: some prototypes and 2D measurements

“…This concept was first suggested by Olsen and May 4 and Furstoss, Thenail and Galland 10 showed that better wideband absorption characteristics could be gained from this approach than using superposition. A concise summary of the development of the hybrid approach is given by Smith et al 16 . Furstoss et al 10 showed that a somewhat better termination criteria is obtained by considering the optimal backing impedance more completely in a transfer matrix approach.…”