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The following article, Wave Phase and Cancellation - Part 2, covers the physical causees, acoustic impact and solutions to the phenomenon of audio wave phase cancellation.

Wave Phase and Cancellation - Part 2

continued from Part 1

So it is with sound – two very similar (or identical) waveshapes, perhaps from two mics covering the same source, or two identical copies of a sound played together in a digital editor (DAW), can collide unhappily when mixed together, resulting in a wide range of possible effects, from a weakening of the bass tones in the sound, to subtle, or extreme, tonal shifts, as the monitoring is switched between stereo and mono, right up to a radical loss of virtually the entire program sound of the affected tracks, even though each individual waveshape is recorded at a high level. If this happens, and what goes missing is your main rhythm guitar track, or something equally important, the results could be disastrous to your mix. Since any problems caused by acoustic phase won’t show up clearly, unless the signals are actually mixed together, the rule is: Always check your stereo mixes in mono, to show up any unknown troubles.

There are, of course, ways to deal with the problems created by phase issues before they happen, such as the X-Y pair mic technique. Another quick rule-of-thumb for using two mics is the 3:1 Rule, useful when the conditions require that the mics are spaced apart (a spaced pair), rather than close together (a coincident pair), as is the case with the X-Y. Here, place the first mic facing the sound source, at a mid-distance (say, 2 feet/60 cm), and then, place the second mic not less than three times this distance from the other one, or in this case, about six feet, or 180 cm, apart from it, at the same distance from the source as the first mic is. By doing this, there will likely be enough difference between the program content reaching each mic, that any phase issues will hopefully be minimized as a result. While this is not necessarily a perfect quick-fix, and mono checking must still be done, and adjustments made, for optimum results, it is a good starting point to work from. With a bit of effort, a professional-quality stereo recording can be achieved, with coherent phase, and a wonderfully wide, spatial aspect to the sound. If, after a few tries, the adjustment part of this is frustrating you, I suggest placing the mics so that they face outwards, away from each other a bit (even as much as 90 degrees), so that the aspects of each side of the source that each mic captures will be truly unique, giving even better phase consistency in mono.

Just to make a point about mono checks, while many folks rationalize that music nowadays is almost always heard in stereo, and that "their" mix will "never" be played in mono, they are fooling themselves. Most AM radio, some FM stations, many hotel, commercial and institutional applications, and most live-sound systems will take whatever is input, and send it out in mono, so if your mix is not mono compatible, you may get an unpleasant and public surprise, more often than you'd like.

Go back to Part 1 of this article

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Eventide Eclipse

The Eventide Eclipse 4.0 represents a world-class effects processor offering five times the processing power of the legendary Eventide H3000 (remember Steve Vai's guitar sounds in the 80's?), in a single-rackspace, hyper-flexible stereo format. Every type of Eventide algorithm is provided in the Eclipse, so your favourite scale-dependent harmonizing, including User-Definable scales, lush reverbs and incredible delay, chorus, flange, filter and phase effects are all right there, and all with the same incredible modulation possibilities as Eventide's bigger processors. Eventide's "The Knob" data entry system makes building and tweaking programs a snap.

The Eclipse 4.0 can operate its' processing in stereo, dual-mono, series or parallel, allowing the two channels of 24-bit, 96kHz processing to be used together on one audio program stream, either in tandem, or sequentially. The possibilities are virtually endless, and the quality is pure, luxurious Eventide. Programs can be stored to ordinary flash cards, when the onboard program memory gets full. The Eclipse 4.0 provides full MIDI control of its' parameters, two pedal control inputs, and both coaxial and optical S/PDIF (or S/PDIF via ADAT) connectivity, plus AES-EBU, in addition to two channels of ultra-clean analogue, via TRS/XLR multiconnector input and separate TRS and XLR outputs. Contact Rob at Rock Shop Pro Audio for more info in the Eclipse 4.0.

Contact Rock Shop Pro Audio for information and pricing on the Eclipse and Eventide products.

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Wave Phase and Cancellation - Part 2

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