JBL CINEMA SOUND SET UP - B. Advantages of Biamplification; C. Cinema Playback Level Calibration

For spaces of 2700 m3 or greater, JBL recommends that the model 4675C be specified in
biampiified mode.
d
2700m3
103dB 113dB 500
tt%z
(100,ooo cu. ft.)
HF:25OW
S400m3
106
111
loo0
LF:4oow
(200,ooo cu. ft.)
HF:2!5OW
. _ Table 1C. Maximum Reverberant Levels’ for JBL 4675C Systems in Large Cinemas (biimpliied 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 specific HF or LF load.
C. Cinema Playback Level Calibration
The actual level requirements in the film makel’s dubbing cinema are established by relating
them directly with modulation level on the recorded medium. For magnetic media, this is established
as 85 dB-SPL in the house when the modulation on the tape is so-called ‘zero level,’ or 185
nanowebers/meter. This last quantity has to do with Mording technology, and we need not concern
ourselves with it further, except to note that modulation peaks often exceed zero level by 8 to 10 dB.
Thus, the peak output per loudspeaker may be only 95 dB. Good engineering practice allows
additional headroom of 6 to 8 dB above this, so it is clear that the values we have listed in Tables 1A
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 1A and B are in
accordance with the suggestions made by Lucasfilm Limited (6) in the specification of THX systems.
2JBL amplifier model MPA400 with appropriate front-end frequency division and power msponse equalization, is
recommended for these applications. The LF loudspeaker section presents a 4-ohm load, to which the amplifier can
deliver 400 watts; the HF section presents an S-ohm load, for whii the ampliier can deliver 275 watts.
lm
:
page 17