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Four Audio Myths

Jun 6, 2011 2:51 PM, By Bob McCarthy

Misconceptions you need to know.


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Figure 1: The phase cycle of acoustic addition/subtraction for two sources of matched level. The level transitions gradually, but asymmetrically around the 360 degree circle. As level between the source rises, the addition/subtraction values will decrease. See larger image.

The world of audio has an air of mystery about it. Our auditory sense is a solitary experience. We hear sound waves, but we can't see, touch, smell, or taste them. Contrast this to our experience of food, which includes all five senses. The solitary nature of our auditory experience leaves it particularly prone to misunderstanding and misconception. This has led to many popular myths regarding the nature of sound and our perception thereof. This article will explore a few of the pervasive myths and unicorn quests that still lurk in our audio world. Most often, the myths spring from the foggy area between the world we can measure and the one we experience in our heads.

1. It's a Phase Problem

In the final scene of the classic movie Casablanca, the police chief orders his men to "round up the usual suspects." In our world of speaker systems, the usual suspect is phase. You don't like the sound? Blame it on phase. Don't have a clue about why it sounds so strange? Announce that it's a phase problem.

Why is it so easy to blame phase? Sound is invisible, but at least we can hear it. Phase is double encrypted: We can't see it and we can't hear it—directly, at least. What we know about phase is how it modifies our experience of amplitude, and that unfortunately is a complex issue.

Figure 2: The effect of level on acoustic addition and subtraction. The extent of the addition and and subtraction decreases as the level difference between two combined sources increases. See larger image.

You heard it here: There is no such thing as a phase problem, so please stop bullying phase. Sound ridiculous? Well, here is the caveat: There are phase + amplitude problems, and plenty of them. But there are not phase problems when there is no amplitude. A simple example: If a speaker is muted, it does not matter if it is wired reverse polarity.

Our concern about phase is relative phase, not absolute phase. Relative phase has to relate to something if it is to matter. I just listened to Abbey Road. It is 30 million degrees out of phase but still sounds fine because the amplitude from the 1968 recording sessions has long faded away.

Relative phase matters whenever two copies of the same original signal come into contact, such as direct sound and a reflection or two speakers in an array. The relevance of the phase relationships between any two sources is directly in proportion to the amplitude relationship. If they are close in level, then phase is the tiebreaker. We can gain a lot or lose a lot. If they are far apart, then the stronger signal becomes increasingly immune to the relative phase of the weaker partner. A reflection is always late, and therefore always out of phase at some frequencies and in phase at others. A reflection of equal strength to the direct sound is a worstcase scenario phase (+ amplitude) problem, but not one that we need just throw up hands and surrender to. Either reduce the level of the reflection or reduce the phase discrepancy.

So it is with speaker arrays as well. Let's take one that is wired correctly and aimed to create uniform level all over the arena. Do you think there is any seat in the house where all 16 of your main array boxes arrive at the same time? Not likely. That means the relative phase is not matched at all frequencies at any given seat. Sounds like we have a "phase problem," eh? And if you did get one seat to have all the path lengths to match, then what about the next seat?

How do we achieve success with a sound system that has inherent "phase problems"? With control of the amplitude. People at the top of the arena don't mind the lower boxes in the array being late. The boxes at the top of the array are the dominant source at the high end, which is the range that is most out of phase. The high frequencies are way out of phase, but also way out of amplitude and therefore irrelevant. The low frequencies are a shared resource, since the individual cabinets have minimal directional control in the low end. The levels are nearly equal at the low end and yet close enough in phase to add constructively. This would seem to be a recipe for a "phase problem," but we can win this one because the phase differential shrinks as we go down in frequency and the amplitude differential rises with frequency.

Is this difference between calling something a "phase problem" and a "phase + amplitude" problem just a semantic game? Not really. These issues require both parties to be involved. When we think of amplitude without phase or phase without amplitude, we are destined to make very poor choices in our quest to solve "phase problems."



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