7
D. Evolving Dynamic Range Requirements in the Cinema:
Figure 4E shows details of the headroom capabilities of cinema sound formats. The reference level of
dialog in the cinema is 85 dB-A, while added headroom is used primarily for more realistic peak levels for sound
effects and music. Depending on specific signal content, the peak level capability of Dolby SR analog tracks
can be 3 dB greater in the mid-band than with Dolby A, rising to about 9 dB at the frequency extremes. The
digital formats can provide 12 dB headroom relative to Dolby A, with overall characteristics that are flat over the
frequency band.
All digital formats are adjusted in the cinema so that a digital signal level of -20 dBFS (level relative to full
scale) will produce a sound pressure level at a distance two-thirds back in the house of 85 dB. This will then allow
a full-scale level of 105 dB, per channel two-thirds back in the house.
E. Integration of Loudspeakers into the Acoustical Environment:
The motion picture industry made the transition from old-style loudspeaker systems to the newer flat
power response systems during the early 1980s. The new systems could easily be matched to existing cinemas
inasmuch as both on-axis response and total power response were essentially flat, since they made use of
uniform coverage high frequency horns and simple ported LF enclosures. Like their predecessors, the new
systems were two-way in design.
With the coming of digital sound tracks
during the early 1990’s, the need was felt for
greater power output capability with even lower
distortion. The answer here was to be found in
newly engineered three-way systems.
F. Power Response and Power-Flat
Systems:
The discrepancy between on-axis
and reverberant room response in the older
systems was solved with the introduction
of a new family of systems based on
uniform coverage high-frequency horns
and straightforward ported low-frequency
enclosures. Figure 5A shows the horizontal
off-axis response of the JBL 4648A low-
frequency system. Note that the response is
uniform below 500 Hz over a wide angle. At 5B
we show the vertical off-axis response of the
4648A system. Note that the response begins
to narrow just below 200 Hz. The net result of
this pattern narrowing in the horizontal and
vertical planes is that they make a good match
for the pattern control of the JBL 2360B horn at
the normal crossover frequency of 500 Hz.
Figure 5C shows the off-axis response
curves for the 2360 Bi-Radial
®
horn, coupled
to a JBL 2446 high-frequency driver which has
been equalized for flat power response. Note
that the off-axis curves are essentially parallel,
indicating that the horn produces a solid
radiation angle which is uniform with respect
Figure 5: Off-axis characteristics of ported LF systems and Bi-Radial
horns. Horizontal off-axis response of 4648A (A); vertical off-axis response
of 4648A (B); horizontal off-axis response of 2360 horn/driver equalized for
flat power response (C)