Screen Losses - JBL CINEMA SOUND SET UP Manual

For digital cinemas with two (" split" ) surround channels, we recommend that the minimum
number of loudspeakers per channel be set at 8, making a total of 18 loudspeakers. A series-parallel
hook-up will be useful for each surround channel. Specific power requirements for the surround
channels will be discussed in Section VI-C.
H. Screen Losses
Through-the-screen losses are complex to analyze in detail. The on-axis loss appears to be
a 6 dB/octave rolloff commencing at about 5 kHz. However, off-axis response is quite different. At
certain angles, high frequencies are transmitted through the screen with relatively little loss. When an
on-axis HF boost is applied to the signal for proper system response on-axis, patrons seated toward
the sides (off-axis) will hear more HF than those listeners on-axis. This, coupled with the normal off-
axis fall-off of the horn' s response, tends to maintain a good balance of high and mid frequency
program and enables patrons seated to the sides to enjoy good dialog intelligibility.
With the newer high frequency hardware, the overall required system equalization is
substantially the requirement for flat system power response. When this is provided, the diffuse field
response measured in the house at a distance one-half to two-thirds back often fits the IS0 2969 X-
curve rather closely. Details of this are shown in Figure 24.
with and without
From a design viewpoint, the engineer must ensure that there is adequate electrical headroom
in the high frequency drivers to attain flat power response above 3 kHz. This usually requires that the
signal be boosted 6 dB/octave above 3 kHz, and this means that the drive level at 12 kHz will be 12
and at the same time be able to provide a good match with the low frequency system. All JBL cinema
systems have been engineered with this requirement in mind.
B.
Figure 24. Screen Asses and house equalization