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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
Figure 3
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
o
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.
8
8
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)
Figure 5
Figure 7
Figure 6
~60
o
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