Boxoffice-July.1999

j
THEORETICRL "H" CURUE
Note: For purposes of
illustration only, I have
included the "X" curve
display shown in Figure 1
in Figures 2 and 3. Because
the center channel is off
when setting the subwoofer
levels, the analyzer
will obviously show only
the frequencies of the subwoofer.
The subwoofer
frequencies are indicated
by the blue-yellow bars.
3. Place the processor in
the optical format. Now
adjust the optical subwoofer
level so that the
subwoofer frequencies are
even with the reference
line established by the center
channel. See Figure 3.
4. It should go without
saying that listening tests
should be done with familiar
material, but not trailers.
It might be necessary to reduce the
optical subwoofer level if the three main
screen speakers are providing enough
bass (or almost enough) on their own.
Maybe. But not far enough, I think. ]
practice, I have found this to be err(
neous, no matter what kind of sound sy
tern is involved. For one thing, there is ri
such thing as a "far field" in a good su^
round array. The microphone should geij
erally see a fairly flat near-field respon:?
when walked around the area bound^j
by the surround speakers. To place tl
measurement microphone off cent
could result in the two surround channe
playing out of balance even though the
measurements were equal. If one wan
the surround channels to play in equ
balance, they must be measured equall}
A symmetrical microphone placemei
is also beneficial for the screen speakei
258 588 1888 2888 4888 8888 16888
So I would recommend that techniciai
Frequencies on octaue centers
set sound levels with the measuremei
microphone simply placed in the cent'
Figure 1
of the seatir
area. Try
DIGITRL SUBWOOFER RESPONSE SUPERIMPOSED OUER THE THEORETICRL "K" CURUE and see if tl
balance isr
I i I i li 1; t! tlEr^::
63 125 258
I
588 1888
I !
2888 4888 8888 16088
Frequencies on octaue centers
Figure 2
better.
©1999 Jo)
F. Allen, a
Rights Reserve
John F. All
\
is the found
and president
High Perforr\
once Stereo I
Newton, Mai
He is also r.
inventor of f.
HPS-4000 cirl
ma sound 5>j
tern and in 19<
was the first
bring dii^ii
sound to tn
cinema.
OPTICRL SUBUJOOFER RESPONSE SUPERIMPOSED OUER THE THEORETICRL "K" CURUE
For
future reference, one may
record the resulting subwoofer's
SPL meter readings in both formats.
Only using these recorded levels,
as referenced to an analyzer, would an
SPL meter be useful for checking subwoofer
levels at some future time.
The RP200 recommended practice
covers the adjustment of sound levels in
all the channels of multi-channel motion
picture sound systems. It specifies that
the measurement microphone be located
about two-thirds back from the screen
and one-third the width of the theatre
away from a side wall. In other words,
off center. The argument goes that the
microphone is in the far field and thus
258 58B 1888 2888 4888 8888 16888
Frequencies an octane centers
Figure 9
36 BOXOFFICE