Rane AC 23B (2003 version) - Manual (Page 12)

Manual-0
OPERATING INSTRUCTIONS
Selecting Crossover Frequencies
Most speaker manufacturers supply low and/or high frequen-
cy cut-off points for each driver, especially if these are supplied in
a system. These cut-off frequencies are based on each driver’s per-
formance at and beyond this point, with a certain safety margin
to accommodate more gentle filter roll-offs and resultant higher
output beyond the recommended performance range.
The AC 23B utilizes 41-detent crossover frequency selectors
which are precision potentiometers. The detents will assure con-
sistent accuracy from Channel to Channel and unit to unit. This
is a distinct advantage over the continuously variable designs
with low-tolerance parts, possible knob misalignment and panel
screening variations. Even with 41 choices it is possible that the
exact recommended crossover frequency may not fall on one of
the detents on the selector. Not to panic, for these sound reasons:
1. The AC 23B possesses 24 dB/octave roll-off, so the crossover
points may be set to the nearest detent above or below the
recommended limit with virtually no hazard to the driver or
degradation in sound quality. If extremely high power levels
are expected, it is safer to defer to the high frequency driv-
ers and shift the crossover point up in frequency rather than
down.
2. Detents do not rely on knob alignment, silk-screen accuracy,
parallax and other variables which erode the accuracy of
continuously variable designs. Chances are that even careful
visual alignment on these will often yield a frequency error
greater than a full detent on the AC 23B.
3. If it is absolutely critical to obtain the exact crossover fre-
quency (Mil Spec., P.A., etc.), the selector can be positioned
between detents if necessary. This of course will require the aid
of a precision signal generator and other equipment to verify
the exact setting.
For best overall system results, try to choose the speaker
components so that each operates well within its recommended
limits. This will provide valuable leeway so that you may move
crossover points in order to fine-tune the system, and will also
yield higher system reliability. If at all possible, beg, borrow or
best yet always use some kind of realtime analyzer to tune your
crossover and fine-tune the system for each different location
with an equalizer. Refer to the following pages for further align-
ment details.
Time Delay Adjustment
Before jumping feet first into the realm of time delay and
how to adjust it, realize why on earth this delay is really neces-
sary. For a short course on time delay, Linkwitz-Riley and other
mouth-watering details, we urge you to read the “Linkwitz-Riley
Crossovers” RaneNote from Rane’s website.
Problems pop up when two different speakers emit the same
frequency as occurs in the crossover regions of two, three, four
and five way systems. Because the two drivers are displaced verti-
cally, cancellation occurs somewhere off-axis because the sound
waves have to travel different distances from the two speakers
and hence, will arrive out of phase. This forms a “lobe” or radia-
tion pattern, bounded on either side by cancellation lines or axes,
which narrow the dispersion or listening area of the speaker.
Fine. So we put up with it. But to make matters worse, when
the two drivers are horizontally displaced – that is, one is in the
front of or behind the other, this “lobe” or dispersion pattern
gets tilted (usually upward) toward the driver that is further
behind. This gets hard to put up with, because the end result is
that your speaker system will have two, three, four or more tilted
radiation patterns and only two or three people in the house will
have decent seats. This rampant lobing error can make a sound
system a real headache to listener and operator alike.
The idea is to be sure that all drivers are vertically aligned
and that all components are always in phase. Then all the main
lobes are on-axis, well behaved, and the system enjoys the widest
possible dispersion pattern so that everyone gets good sound.
The one catch is that in many cases it is physically or otherwise
impossible to get all the drivers vertically lined up at the sound
source. This is where time delay comes in.
By electronically delaying the signal going to the front driver,
enough time allows the sound from the rear driver to literally
catch up to the forward driver’s voice coil, so that signal from
both drivers is emitted in phase—and it works! Time delay
makes an appreciable improvement in overall sound. The trick is
finding the proper time delay amount: hence this manual.
In-Phase Axis Response Without Time Delay
Corrected In-Phase Axis with Time Delay on the Low Driver
Unfortunately
the amount of time
delay is a function
of two factors: the
amount of horizon-
tal displacement
between driver voice
coils, and the actual
crossover frequency
involved. Setting de-
lay controls by ear is
supposedly possible,
but very tricky and
unreliable. The fol-
lowing methods are a
couple of (but by no
means all) means of
setting time delay.