Rane AC 23S, User Manual

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13
SPL Meter & Tone Generator
You can also find an accurate delay setting using an SPL meter (obtainable at most local electronics stores) and
some kind of variable tone generator. In order to exclude the effect of room acoustics and imperfect 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 backward 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 speakers 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 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 crossover 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 corrections available from the delay depends on the actual crossover 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 23S is designed
to accommodate the majority of common speaker configurations.
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 minimum and leave it
there. Frequencies below 150 Hz are omnidirectional, so that phase misalignment is virtually
inaudible below this point. Subwoofers will often 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 beginning of each alignment procedure. Once all of the crossover Delay controls are set,
then re-adjust the output Level controls.