D. Surround Systems:
1. General comments:
While there is little consistent practice in specifying
and implementing surround channels, we can set down
the following general requirements:
a. The total acoustical power delivered by the
ensemble of surround loudspeakers should be
equal to one of the screen channels. There are
probably not many surround systems which
meet this requirement, but it is essential if the full
impact of surround information is to be appreci-
ated. While a 15-dB headroom factor is a part of
the power calculations for the screen channels, it
may be unreasonable to demand that of the sur-
round channel. Thus, a requirement that the sur-
round channel be able to produce a level in the
house of 95 dB will suffice. In the case of distrib-
uted 200 mm (8 in) loudspeakers, a level of 92dB
is deemed sufficient.
b. The quantity of surround loudspeakers should be
sufficient so the listeners are not aware of any
one of them. This usually means a minimum of
eight: three each on the side walls and two on the
back wall.
c. The surround loudspeakers should exhibit wide
dispersion so that the entire audience area can
appreciate smooth coverage across the fre-
quency range.
d. Surround loudspeakers should be unobtrusive
and not interfere with theater decor. This require-
ment may run counter to some of the acoustical
requirements in that size is related to system
efficiency.
2. Choosing the right surround loudspeakers:
JBL has recently introduced the model 8325A three-
way system for surround applications. With input power
capacity of 80 watts and a sensitivity of 91 dB, one watt
at one meter, ten to twelve of these systems are a rea-
sonable match for a screen array of 4675A systems.
More than ten may be required for broader coverage in
larger houses.
In larger theaters, the 46120K has been success-
fully used as a surround loudspeaker. This system has a
sensitivity of 97 dB, one watt at one meter, and power
input is 400 watts. Obviously, fewer of these systems
would be required than with the 8325A.
In general, we would recommend the following
quantities of the various surround loudspeakers for rea-
sonable matching with a screen loudspeaker array
consisting of 4675A's:
8325A 10 to 12
4671 or 4671 OK 6 to 8
46120K 6 to 8
Dolby Laboratories has suggested the use of mul-
tiple 200 mm (8 in) loudspeakers for the surround chan-
nel. Upwards of 60 such loudspeakers may be used in
this configuration, mounted in groups of six or eight and
arrayed horizontally along the walls. The JBL model
8140 Co-Motional driver is ideal for this application
because of its extremely smooth and wide high-fre-
quency dispersion.
3. Implementation of the surround system:
Figure 12 shows the general mounting for surround
loudspeakers. Note that they cover the back two-thirds
of the house and that they are located one-third the dis-
tance up the walls.
A. SIDE VIEW
D
17
FIGURE 12. LOCATING THE SURROUND
LOUDSPEAKERS
B. TOP VIEW
