The EM Poll

Tech Basics Without Tears

Mar 14, 2008 5:29 PM, By Jon Chappell With Steve Oppenheimer

           

A PAIN-FREE EXPLANATION OF FIVE CONCEPTS ALL MUSICIANS SHOULD UNDERSTAND

Phase and Polarity

Sound is made up of waves that, when allowed to interact in a reflective acoustic space, collide and combine in complex ways. When two waves of the same type meet at exactly the same time, their peaks and troughs align perfectly and the signal will be reinforced, or its amplitude increased. As we learned earlier, if an entire waveform is 180 degrees out of phase with respect to another, identical, waveform, they cancel completely, resulting in silence.

Figure 3: The top waveform (a) is a clarinet sample; the middle waveform (b) is the same sample but delayed 1.6 ms, putting it out of phase with respect to the original. When the original and out-of-phase versions mix, some frequencies are reinforced and some are attenuated. The resulting waveform (c) looks nothing like the original, and in this case, its amplitude is greatly reduced.

But if two waves are offset in time (say, because one is delayed through circuitry or by a reflection), a phase shift, or comb-filter effect, will result, where some frequencies are reinforced (when the values added together are both positive or both negative), and some are attenuated (when the plus and minus values are mixed), as shown in Fig. 3. When the fundamental is 180 degrees out of phase with respect to the other wave, significant cancellation occurs and the resultant sound often produces an extremely weak and hollow-sounding version of the original. Only your ears can tell you what's desirable and what needs correcting.

Therefore, whenever two sound waves from the same source interact, you need to be concerned with their phase.

Any time you have two mics on the same source, such as a close mic and an ambient mic on a guitar amp, you risk capturing the waves at different times in their cycle, because the different distances equate to a time displacement: the mic farther from the source will pick up the wave at a later point in its cycle. You could solve the problem by moving a mic backward or forward (changing the time factor of the phase shift) and listening to the results. But sometimes you don't have that luxury in terms of time and stage-plot mobility in a live situation. So there's another way: reverse the polarity of the signal using the “phase” or “polarity” switch on the appropriate mixer channel.

Polarity is not the same as phase, though they can produce similar results, and the terms are often used interchangeably. While phase has to do with the time relationship between identical waves or signals, polarity is concerned with the positive and negative voltage values of a signal.

If we start with two identical sine waves and we invert the polarity of one of them, the inverted sine wave will be exactly 180 degrees out of phase with respect to the original. This is the only case where the two different principles achieve the same result, but it can be a lifesaver when dealing with a phase-shift problem onstage.

If you have a signal that sounds weaker through two mics than it does through either mic by itself, you may be close to a 180-degree phase shift, because as we know, combining in-phase and 180-degree out-of-phase versions of the same signal results in the two canceling each other out. If that's so, then reversing the polarity of one mic's signal should fix the problem. Best of all, hitting a polarity-reverse switch is quick, easy, and changes nothing in the stage plot!

Here's another example of how you can use polarity to correct a phase problem. If you use two mics on a snare drum, one facing down on the upper head and the other facing up on the lower head, you risk having the signals weaken each other when they're combined at the mixer, because they are out of phase due to their position (facing each other). However, if you flip the polarity of the lower mic's signal with the channel's polarity-reverse switch, you'll produce a stronger signal.

Sound Under Pressure

Many of the concepts dealing with sound have scientific meaning and are expressed in scary-sounding terms like decibels and sound-pressure level. But you don't need to learn math to appreciate how the principles work. So while there are some scientific concepts here, we promise not to introduce any equations, and there will not be a pop quiz at the end of the session.

Let's tackle some of the most common terms and see how they can help when listening to your P.A. or guitar amp. By the time we're done, you'll know exactly what to do when the sound tech says, “Turn that guitar down by 20 dB!” (Okay, you already know the answer: turn the blasted thing down!)