Rane AC 22B (2003 version) - Manual (Page 8)

Manual-
Time Delay Adjustment
Before jumping feet first into the realm of time delay and
how to adjust it, it might help to spend a moment here to re-af-
firm why on earth this Delay is really necessary. For a detailed
and enjoyable short course on time delay, Linkwitz-Riley and
other mouth-watering details, we urge you to read the RaneNote
“Linkwitz-Riley Crossovers” available in the Library section of
the Rane website. In the way of summary, a few words are in
order here to outline the basic effects of time delay in crossovers.
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 shifted in phase. This forms a “lobe” or
radiation pattern, bounded on either side by cancellation lines or
axes, which narrow the dispersion pattern 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 (see Figure 1). 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. And we’re not talking trivial
pursuits here—this rampant lobing error can make a sound
system a real headache, to listener and operator alike.
The idea, then, 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 driver up
front, enough time is allowed for 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 (See Fig. 2). And
it works! Time delay can make an appreciable improvement in
overall sound. The trick is finding the proper amount of time
delay: hence the rest of this section.
Unfortunately the amount of time delay is a function of two
factors (life ceased to be simple after age 9, right?): the amount
of horizontal displacement between driver voice coils, and the
actual crossover frequency involved. Setting Delay controls by
ear is supposedly possible, but very tricky and unreliable. The
following methods are a couple of (but by no means all) means
of setting time Delay.
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 22B utilizes 41-detent crossover Frequency selectors
which are precision potentiometers. The detents assure consis-
tent accuracy from Channel to Channel and unit to unit. This
is a distinct advantage over the continuously variable designs
using 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 22B 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 drivers
and shift the Frequency up 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 22B.
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. Keep reading for further alignment details.