29
G. Subwoofers:
The general rule here is one amplifier section to one subwoofer driver, as in the case of the 4641 and
4645C models. The model 4642A contains two 2441H drivers connected in parallel resulting in a 4-ohm load. This
system is intended for use in those installations that require multiple drivers to attain the necessary acoustical
level. They should be used with amplifiers capable of delivering continuous signals of 1200 watts into a 4-ohm
load.
H. Wire Gauges and Line Losses:
The following table lists the recommended AWG and metric wire gauges to be used for the indicated load
impedances and distances between amplifiers and their loads. These values are predicated on a maximum power
loss of 5%, or approximately 0.25 dB:
(Note: Metric gauge values indicate the actual cross-sectional area of copper wire in square millimeters)
I. System Setup and Checkout:
The vast majority of system performance problems can be avoided through proper design procedures
and proper assembly. If everything has gone well, the system will work, and the field crew can proceed with final
calibration and equalization of the system. The following points need to be addressed:
1. When a loudspeaker has been assembled, either in the shop or in the field, it should be tested with an
oscillator-amplifier combination to ensure that there are no buzzes or rattles.
2. As pairs of loudspeaker lines are installed, the ends at the loudspeaker should be shorted and a
resistance check made at the booth to assure line continuity. Any discrepancies should be corrected.
3. Set up a gain-loss diagram for the system prior to making any adjustments on the system. An example
is given in Figure 39. Here we have shown the gain structure for the model 5674. We show here only the LF
system section at full output; the MF and HF, sections operate at lower power amplifier levels so that the individual
acoustical outputs of LF, MF, and HF sections will all be proportional.
All aspects of A-chain calibration should be performed according to the methods laid down in the various
manuals supplied by the manufacturers of the cinema processing equipment. Since most cinema systems have
the same basic architecture, it should be necessary to establish the operating norms only once.
The gain-loss diagram for this system indicates clearly maximum output levels of each component in
the system as well as the noise floor of each component. The goal in proper systems engineering is to ensure
that the widest possible dynamic range is preserved through the chain. No electronic device ahead of the power
amplifier should be driven into distortion before the power amplifier itself has reached its maximum output
capability. Additionally, the noise floor of the system, once it has been established at the preamp, should not
be compromised by allowing the signal level to fall too low at any subsequent point in the chain. The gain-loss
diagram is a convenient means of ensuring all these points. It is customary to carry the gain structure diagram into
the acoustical domain in the cinema, indicating relevant levels of both signal and noise at some reference distance
from the loudspeakers.
AWG
25 ft
50 ft
100 ft
250 ft
500 ft
16 ohms
21
18
15
11
8
8 ohms
18
15
12
8
5
4 ohms
15
12
9
6
3
Metric
7.5 m:
15 m:
30 m:
75 m:
150 m:
16 ohms
0.7
1.0
1.5
2.5
3.5
8 ohms
1.0
1.5
2.0
3.5
4.5
4 ohms
1.5
2.0
3.0
4.0
6.0
