Bowers & Wilkins CT800 installation manual ~60 o, Surround speaker dispersion mode

Figure 5Surround speaker dispersion mode

	Surround speakers generally fall into two main types – those that one might describe as ‘normal’ speakers - so-
	called monopoles, where the sound comes from a set of drive units mounted on the front of the enclosure - and
	those that give a more diffuse sound field, such as dipoles. Each type has its advantages.
	Most multi-channel music is recorded with home entertainment in mind and is monitored using monopole surround
	speakers, whatever the multi-channel recording format. This enables better location of side and rear images,
	although the formation of such images is never quite as precise as it is between the front speakers.
	Most films are originally balanced for cinemas, where a large number of speakers spread around the auditorium
~60o	are used to create the surround sound field. In that case there are more surround speakers than there are discrete
	channels of information and a less precise image is created that gives an all-enveloping effect. Dipoles and similar
	diffuse speakers are better at recreating this type of sound field in the home, but using fewer speakers to do it. Image
	positioning with these types of speaker is never as precise as it can be with monopoles. However, they do have the
	advantage of making it easier to balance the system for a larger listening area.
	You may well receive conflicting advice from different sources on the best type of surround speaker to use. The truth
	is that there is no one perfect solution for all situations and the final choice for any given application will be influenced
	by personal preference.
	The CT8 DS is a specialist surround speaker and has the advantage of offering a choice of both monopole and
	dipole operation. The mode of operation is set by a relay inside the speaker, the state of which is controlled from the
	CT8 XO crossover/equaliser assigned to the speaker via a cable connecting the 3.5mm sockets on both products.
	If this cable is not fitted, the speaker will only operate in monopole mode. On the CT8 XO is a switch used to
	select the mode. In the monopole position, this switch may be overridden by a 12V trigger signal from the surround
	processor. Some surround processors can recognise the type of programme being played from a code on the disc
	and allow you to assign a 12V trigger to this code. Some other processors allow you to assign a 12V trigger to
	the input selection and you may, for example, use a different disc player for audio and movies and set the trigger
	accordingly. In both these cases, you may choose whichever type of operation best suits the conditions of the
	listening room, the size of audience and the type of programme being played. Indeed, you may even change the
	characteristic for different types of programme and, as the total energy into the room is the same in both modes, no
	recalibration of the installation is necessary when switching between them.
	The two bass drivers operate in both modes. In monopole mode, they cross over to the midrange and tweeter drive
	units on the front face and the speaker is acoustically similar to the CT8 LR, but without the swivelling head. In dipole
	mode, the bass drivers cross over to the six side-firing drivers. The drive units on opposing sides are connected
	out of phase with one another, which creates a wedge-shaped null zone, approximately 60o wide, at right angles to
	the wall. If the listeners sit within this zone, they become less aware of the location of the speakers and hear more
	reflected sound; hence the diffuse nature of the sound field. (figure 5)

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