Rane AC 22S Operation Manual - Page 9

OPERATING INSTRUCTIONS 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 more gentle filter roll‑offs and resultant higher output beyond the recommended performance range. The AC 22S utilizes 41‑detent crossover Frequency selectors which are precision potentiometers. The detents assure consistent 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 22S 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 22S. 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 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. 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‑affirm 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 vertically, 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 Figure 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. Manual-6