ABSTRACT
The absorber and diffuser technologies discussed in previous chapters have difficulty with low frequency sound. Low frequency waves have long wavelengths, which means the absorbers and diffusers have to be large to perturb or absorb the wavefronts. This can be overcome to a certain extent by the use of resonant structures, most often used in bass absorber design, but in the recent years there has been growing interest in the use of active control technologies to absorb or diffuse low frequency sound. Active control offers the possibility of bass absorption or diffuse reflections from
relatively shallow surfaces, as well as the possibility of variable acoustics. An example application for active absorption is the control of modes in small rooms. The cost and difficulties of implementation are, however, considerable, and this is the major reason why this technology has not been more widely applied. Active absorption has much in common with active noise control, indeed in many
ways it is the same concept just re-organized behind a slightly different philosophy. Olson and May carried out pioneering active control experiments and they suggested an active noise control method based on interference [1]. In their method, an electroacoustic feedback loop was used to drive the acoustic pressure to zero near an error microphone placed close to a secondary loudspeaker. This is illustrated in Figure 13.1.
This was the first active absorber. More sophisticated active absorption systems specifically alter the surface impedance of the control loudspeaker, towards a userspecified target value. They may be configured as feedforward or feedback devices and often constructed around single channel filtered-x LMS adaptive filter algorithms. Recent developments have moved away from the use of superposition (interference), and resistive material is used in combination with active controllers to gain actual dissipation. For instance, by using the active controller to maximize the particle velocity through the resistive material and so maximize absorption, this concept was also first suggested by Olsen and May [1].
