11
Operation
Selecting Crossover Frequencies
Most speaker manufacturers supply low and/or high frequency cut-off points for each driver, especially if these are
supplied in a system. These cut-off frequencies are based on each driver’s performance at and beyond this point,
with a certain safety margin to accommodate gentler filter roll-offs and resultant higher output beyond the
recommended performance range. The AC 23S utilizes 41-detent crossover frequency selectors which are precision
potentiometers. The detents will assure consistent 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. These factors should mitigate any concerns, though:
1.
The AC 23S 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
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 23S.
3.
If it is absolutely critical to obtain the exact crossover frequency (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 some freedom to move crossover points in order to fine-tune the system, and
will also yield higher system reliability. If at all possible, use some kind of realtime analyzer to tune your crossover
and fine-tune the system for each different location with an equalizer.
For additional information on time delay, see the
Linkwitz-Riley Crossovers
RaneNote at
rane.com/note160
.
Problems occur 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 vertically, 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 radiation pattern, bounded on either side by cancellation lines or axes, which
narrow the dispersion or listening area of the speaker.
Furthermore, 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
tolerate because the result is that your speaker system will have two, three, four or more tilted radiation patterns
and will sound acceptable in only a few places in the venue.
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 issue is that, in many cases, it is nearly impossible to line up all drivers vertically at
the sound source. Time delay can help this.
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.
Unfortunately the amount of time delay is a function of
two
factors: the amount of horizontal displacement between
driver voice coils,
and
the actual crossover
frequency involved.
Setting delay controls by ear is theoretically possible
but very unreliable.
The following
methods are a couple
of (but by no means
all) means of setting time delay.
Summary of Contents for AC 23S
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