The models 4670B and 4675A have 4-ohm low-
frequency sections, and the total wire resistance should
not exceed 0.276 ohms.
C. Systems for Larger Houses:
1. General Comments:
The class of houses considered here will all have
multichannel systems. Seating capacity may be as little
as 500 and as large as some of the old movie palaces of
yesterday. There are few of the latter left, except in large
metropolitan areas. For the most part, the larger theaters
have been converted into multiplex theaters.
The number of loudspeakers behind the screen will
be either three or five, and there will always be a sur-
round channel consisting of six or more smaller loud-
speakers arrayed on the side and rear walls.
These systems will be capable of playing Dolby
films, and third-octave equalization will be employed, as
part of the Dolby equipment, to adjust the system to the
room according to a standard response curve.
In general, the class of audio equipment used in
these installations will be the equal of the finest equip-
ment specified for high-quality sound reinforcement sys-
tems in auditoriums and concert halls.
DO NOT mix different systems in the multichannel
screen loudspeaker array. Not only will there be dif-
ferences in power handling, there may be polarity dif-
ferences, and these will adversely affect stereophonic
performance.
We will now cover in detail the requirements for
these systems.
2. System Recommendations for Large Houses;
500 to 2000 seats; volume 14,000 cubic meters
(500,000 cubic feet):
*Preferred
system:
Model 4675A (direct radiator
LF)
Power needed for 100 dB: 90 watts
Recommended power amplifier: JBL 6260, one
loudspeaker system per channel (300 watts into 4 ohms)
Characteristics: Broad-band sensitivity 100 dB-
SPL, one watt at one meter; JBL's most power-flat and
accurate system; uses large Bi-Radial horn for precise
90-by-40 degree pattern control down to 500 Hz; integral
power response correction; extended response down to
45 Hz
For very large houses and for the highest degree of
performance in professional dubbing theaters and
screening rooms, the 4675A-2 may be specified. We rec-
ommend that biamplification be seriously considered
with this system for optimum performance. The broad-
band sensitivity of the 4675A-2 is 103 dB, one watt at
one meter.
* Alternate vented horn systems:
Models 4676B-1 and.4676B-2
Power needed for 100 dB: 80 and 53 watts,
respectively
Recommended amplifier: JBL model 6260, one
channel per loudspeaker system (300 watts)
Characteristics: Mid-band sensitivity, 106 and 109
dB-SPL, respectively; recommended for retrofit in large,
older houses where a complete conversion to all-new
hardware is not feasible. LF response rolls off below 80
Hz
The systems discussed in this section are shown in
Figure 6A. Response curves are shown at B and C.
3. System implementation:
A. Electrical considerations:
The systems intended for larger houses may be
implemented with JBL's 3160 high-level network, with
wiring as shown in Figure 7A through B.
However, for optimum performance, we recommend
biamplification using the JBL model 5234A electronic
dividing network. The 5234A is available with a 500-Hz
card which contains high-frequency power response
correction. Implementation of biamplification is shown in
Figure 7C.
The chief value of biamplification is the reduction of
intermodulation distortion on peak program signals. Fur-
thermore, larger power amplifiers may be specified for
the LF sections than those indicated for use with the
high-level dividing network.
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