Rane AC 23B (2003 version) - Manual (Page 14)

Manual-
Time Delay Adjustment Using SPL
Meter & Tone Generator
Now that good quality realtime analyzers are becoming more
affordable and easier to use, there are few reasons why one of
these should not be regularly used in any sound system. If an
analyzer is simply not available, an accurate delay setting can
be obtained by using an SPL meter (obtainable at most local
electronics stores, the best is the Rane RA 30) and some kind of
variable tone generator.
In order to exclude the effect of room acoustics and imper-
fect driver response, only the crossover frequencies are to be
emitted (one at a time) by the tone generator. First the highest
crossover frequency is run through the crossover and each of the
two speakers sharing the crossover point is set separately to an
arbitrary 0 dB level on the SPL meter. When both drivers emit
the crossover tone simultaneously, the combined response should
read +3 dB higher on the meter. If the drivers are not phase
aligned, some cancellation will occur on-axis, resulting in a
combined response less than +3 dB. Turning the delay control up
causes the lower frequency driver to electronically move back-
ward until the SPL meter reads +3 dB; then the two drivers are
electronically aligned and the on-axis cancellation is eliminated.
This procedure is repeated for the next lower crossover point(s).
STEP BY STEP PROCEDURE
A 3-Way mode consisting of high, mid and low drivers is
used here as an example. For other configurations, use the same
procedure starting with the highest crossover point and repeating
steps 2 through 5 for each lower crossover point.
1. Set the tone generator to the highest crossover frequency and
plug it into the INPUT of the crossover. Turn all crossover
LEVEL controls fully down.
2. Position the SPL meter mic about 15 feet in front of the speak-
ers and at a height about midway between the high and mid
drivers. It is very important that the meter remain in exactly
the same position throughout the test, so affix it to a mic
stand, small tree or other stable object. Set the switches on
the SPL meter to “C-weighting”, “Slow” if available. Be sure
to minimize background noise (air conditioners, fans, traffic,
wild animals, etc.) as these will effect the meter reading.
3. Slowly turn up both the crossover MASTER LEVEL control
and the MID LEVEL control until the tone is heard through
the mid driver. Adjust the SPL meter control and/or the cross-
over LEVEL controls until you obtain a 0 dB reading on the
meter. Verify that no sound is coming from any other speakers
except the mid driver.
4. Now press in the MID MUTE switch on the crossover so the
tone is removed from the mid driver. Without re-adjusting
either the meter or the crossover Input or Mid frequency Level
controls, turn up the HIGH LEVEL until the tone coming
from only the high driver reads 0 dB on the SPL meter.
5. Release the MID MUTE switch so the tone is emitted from
both the high and mid drivers. Check the SPL meter reading:
i. If the meter reads +3 dB, then the drivers are properly phase
aligned and no delay is necessary; leave the MID DELAY
control at full minimum.
ii. If the meter reads less than +3 dB, slowly turn up the MID
DELAY control until the meter just reads +3 dB. Now the
drivers are electronically phase aligned and the delay control
should be left in this position at all times, unless the speaker
system is physically altered.
iii. If you have turned the MID DELAY control all the way
up and still do not obtain a +3 dB reading, you will have
to physically move the high driver farther forward until the
SPL meter reads +3 dB. The amount of displacement correc-
tions available from the delay depends on the actual cross-
over frequency: the higher the frequency the less amount
of correction capability. If the drivers are built into a single
cabinet and/or it is impossible to change relative positions,
then you will have to obtain additional delay to achieve
proper phase alignment.
iv. If turning the MID DELAY control up makes the SPL
reading decrease instead of increase, this means that the high
driver is actually in front of the mid driver; adding delay to
the mid driver then only worsens the situation. There are a
couple of ways to deal with this:
a. Try to move the high driver back as far as possible without
losing stability in balancing the speaker stack. You may
want to raise it up as well to restore dispersion close to
the stack. If you cannot move the high driver, then you
will have to obtain an additional external delay source to
align the high and mid drivers. The built-in delay system
in the AC 23B is designed to accommodate the majority
of common speaker configurations; if you encounter con-
fusion or difficulty with your particular system, it is best
to consult your dealer or the Rane factory for assistance.
b. If this decrease in the display due to the LOW DELAY
control occurs at a low frequency crossover point below
about 150 Hz, set the LOW DELAY control to mini-
mum and leave it there. Frequencies below 150 Hz are
actually omnidirectional, so that phase misalignment is
virtually inaudible below this point. Subwoofers will of-
ten possess long folded or straight horns, resulting in the
diaphragm being well behind the rest of the stack. Most
authorities agree that phase alignment of subwoofers is
unnecessary. Otherwise you will have to obtain additional
delay equipment to align these to the rest of the system.
6. Tune the tone generator to the next lower crossover frequency
and then repeat steps 2 through 5, using the appropriate level
and delay controls. Once the DELAY control is set, you may
re-adjust any of the crossover LEVEL controls at the begin-
ning of each alignment procedure. Once all of the crossover
DELAY controls are set, then re-adjust the output LEVEL
controls as outlined on page Manual-14.