# Thread: theory on website not correct

1. ## theory on website not correct

I've read some explanations on balance audio and sound waves etc. on this website which are actually incorrect:

Sound waves which are exactly 180 degrees out of phase cancel each other out and produce silence (this is how many noise-cancellation devices work).
This only works for pure sine waves and not for sound waves in general! If you shift a square wave like this -__-__-__-__ over 180 degress , you aint gonna get silence at all.

What all balanced audio devices do is INVERT one signal. This is, make positive negative and negative positiv. In the mixer the inverted signal is inverted again (inverting opamp) and the noise is cancelled. (the positiv noise cancels with the negativ noice, which is the same)

2. This is a confusing issue but here's my take on it....

I think it's a problem of terminology. When we were doing our "apprenticeships", the standard way to refer to an inverted audio signal was to say it was 180 degrees out of phase. This is still very common, in fact a quick Google search for "noise cancellation" showed that most sites use the same terminology we have. I have also included below a screenshot from Sennheiser's NoiseGard Introduction which explains their noise cancellation technology using both the terms "180 degrees out of phase" and "inverted".

The term "out of phase" is confusing because it can imply a linear phase relationship i.e. delaying the signal until it becomes out of phase. As you point out, this would only work with a sine wave (which would look the same as if it was inverted).

I'm really not sure why "out of phase" became the usual way to describe the way a wave is inverted. In any case I think you're right in saying that "inverted" is a better way to look at it. I have updated our page to try and clarify this: http://www.mediacollege.com/audio/ba...-balanced.html

I'd be keen to hear more discussion about this issue. Does the page above make sense? Is there a better way to explain it? Does anyone have any other ideas about this whole thing?

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