Surround 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
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 60
o
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)
9
Figure 1
Figure font: Helv 45, 7pt
Figure spacing: 7 x 7 mm
Figure 2
Figure 3
Figure 4
Figure 5
~40
o
~120
o
~60
o
Figure 7
Figure 6
