Glossary of terms Below is a brief description of some of the technical terms found throughout this text. Experience has shown that although some of the terms may be familiar to many readers, they have often been misunderstood or misused. This glossary attempts to clarify the definitions of the terms, at least as used in British English. Acausal filter A filter applied, usually digitally, which has advanced knowledge of the signal arriving via delaying the main signal on which it is intended to act ­ effect before cause through prior knowledge of the cause. See Causal filter. Active systems Filters or loudspeaker systems, for example, where an external power source needs to be applied as well as the drive signal. A filter based on semiconductors or valves, driven from an external battery or mains supply, and placed ahead of a power amplifier in a loudspeaker system is an example of an active filter. See Passive systems. Anechoic chamber A room that is designed to simulate free-field acoustic conditions by means of the placement of highly absorbent materials on all surfaces. The absorbent materials usually are in the form of wedges pointing into the room. This arrangement ensures that sound waves arriving at the room edges are maximally absorbed for all angles of incidence. The lower frequency limit of anechoic performance is set by the length of the wedges, which are effective down to a frequency where the wedge length is equal to one-quarter wavelength. However, what little reflexions still exist will return more weakly from the distant walls of the larger anechoic chambers than from the nearer walls of a smaller chamber using similar wedges. Variously known as anechoic rooms or free-field rooms. See also Semi-anechoic chamber and Hemi-anechoic chamber.