SURROUND LOUDSPEAKERS
Five loudspeakers are generally used with a surround-sound outfit: left main, right main, centre and a rear pair for effects, of which the first two are the most important. The centre front speaker needs to be reasonably matched, in terms of frequency response and ‘timbre’, to the L and R main ones, and where it is to be placed near a direct- view TV or monitor screen it must have a low stray magnetic field to prevent interference with tube beam-landing and consequent impurity/colour-staining effects; suitable types are made and marketed to meet this need. All the loudspeakers must have impedance and power ratings suited to the amplifier with which they are used, and correct phasing of their feeds is very important. Amplifier and speaker terminals are coded red/black or +/−, and if any speak- er(s) are incorrectly phased very strange effects can result.
Subwoofer
Low audio frequencies have virtually no directionality, and so it is not necessary to have more than one powerful subwoofer unit to reproduce the bass: so long as it is present somewhere in the listen- ing area the viewer is satisfied that the entire system is reproducing it, and perceives that the centre speaker is larger than it actually is! Surround decoders have an output terminal for a subwoofer, then, generally carrying a line-level signal for application to an ‘active’ (incorporating its own power-drive amplifier) type which can be positioned anywhere in the room – its effect is felt as well as heard.
‘Phantom’ centre sound
In domestic situations with a small audience and good seating positions the centre channel is less important than in a large cinema where some viewers may be dominated by a left, right or surround/effects loudspeaker. In home systems, then, the centre loudspeaker can be left out, and its signal simulated by the in-phase operation of L and R main speakers.