For spaces of 2700
or greater,
JBL recommends that the model 4675C be specified in
biampiified mode.
cu. ft.)
111
cu. ft.)
. _
Table Maximum Reverberant
for JBL
Systems in Large Cinemas
mode).
B. Advantages of Biamplification
The importance of biampiification in large cinemas cannot be overestimated. Even though the
systems detailed in Table 1 B use the same amplifier model as the systems detailed in Table
1
A,
the
reallocation of the power through biampiification has important beneficial effects. Specifically,
intermodulation distortion is reduced at-high operating levels, and available power can be more
directly matched to the
HF or LF
C. Cinema Playback Level Calibration
The actual level requirements in the film
dubbing cinema are established by relating
them directly with modulation level on the recorded medium. For magnetic media, this is established
as 85
in the house when the modulation on the tape is so-called ‘zero level,’ or 185
This last quantity has to do with
technology, and we need not concern
ourselves with it further, except to note that modulation peaks often exceed zero level by 8 to 10
Thus, the peak output per loudspeaker may be only 95
Good engineering practice allows
additional headroom of 6 to 8
above this, so it is clear that the values we have listed in Tables
and B are not excessive in the cases of the larger houses. in the smaller houses, we can certainly
make do with smaller amplifiers than indicated in the table; but even then, the cost of the added power
is very slight, and the benefit substantial. The powers recommended in Tables
and B are in
accordance with the suggestions made by Lucasfilm Limited (6) in the specification of THX systems.
amplifier
with appropriate front-end frequency division and power
equalization, is
recommended for these applications. The LF loudspeaker section
a
load, to
can
deliver 400 watts; the HF section presents an S-ohm load, for whii the ampliier can deliver 275 watts.
17