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Author Topic: Mixing "coherent" vs "incoherent" sound.  (Read 3830 times)

Randall Hyde

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Mixing "coherent" vs "incoherent" sound.
« on: March 11, 2011, 12:04:20 PM »

Hi All,
I've been reading some articles and posts (elsewhere) about mixing waveforms. The claim I'm reading is that when you mix two in-phase identical ("coherent") waveforms you get a +6 dB boost, when you mix two different waveforms, the overall boost is +3 dB.

Question one: I've read this in a couple of places, but I'd ask for verification that this is actually true.
Question two: All of the examples and calculations I've seen have provided sine waves as their example; if this statement is true is it an artifact of sine waves, or is it true for all waveforms?
Question three: In the examples I've seen, as the two (combined) sine waves are brought out of phase, the boost drops (of course) hitting +3db at 45 degrees (IIRC, might have been 90 degrees, I wasn't paying real close attention). The implication is that if the two signals are less than "x" degrees out of phase, you get a +3dB or better boost.

Now the real question: when coupling subs, they're obviously slightly out of phase because of the distance between the centers of the drivers. Do you get better than +3dB boost? Or is this a fantasy.  It seems like "money for nothin'" do me (and, therefore, a bit suspicious).
Thanks,
Randy Hyde
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Doug Fowler

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Re: Mixing "coherent" vs "incoherent" sound.
« Reply #1 on: March 11, 2011, 12:39:15 PM »

Hi All,
I've been reading some articles and posts (elsewhere) about mixing waveforms. The claim I'm reading is that when you mix two in-phase identical ("coherent") waveforms you get a +6 dB boost, when you mix two different waveforms, the overall boost is +3 dB.

Question one: I've read this in a couple of places, but I'd ask for verification that this is actually true.
Question two: All of the examples and calculations I've seen have provided sine waves as their example; if this statement is true is it an artifact of sine waves, or is it true for all waveforms?
Question three: In the examples I've seen, as the two (combined) sine waves are brought out of phase, the boost drops (of course) hitting +3db at 45 degrees (IIRC, might have been 90 degrees, I wasn't paying real close attention). The implication is that if the two signals are less than "x" degrees out of phase, you get a +3dB or better boost.

Now the real question: when coupling subs, they're obviously slightly out of phase because of the distance between the centers of the drivers. Do you get better than +3dB boost? Or is this a fantasy.  It seems like "money for nothin'" do me (and, therefore, a bit suspicious).
Thanks,
Randy Hyde


Randy -

Play around with this applet.  I use it in SysTune class as part of "phase".


http://www.udel.edu/idsardi/sinewave/sinewave.html


Of course it's just math and not real world, but you should get the idea.

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John Roberts {JR}

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Re: Mixing "coherent" vs "incoherent" sound.
« Reply #2 on: March 11, 2011, 01:50:22 PM »

Hi All,
I've been reading some articles and posts (elsewhere) about mixing waveforms. The claim I'm reading is that when you mix two in-phase identical ("coherent") waveforms you get a +6 dB boost, when you mix two different waveforms, the overall boost is +3 dB.

Question one: I've read this in a couple of places, but I'd ask for verification that this is actually true.
True, but 3dB gain is typical and varies with sources.
Quote
Question two: All of the examples and calculations I've seen have provided sine waves as their example; if this statement is true is it an artifact of sine waves, or is it true for all waveforms?
Only true if waveforms are identical, like two mics picking up same waveform. 
Quote
Question three: In the examples I've seen, as the two (combined) sine waves are brought out of phase, the boost drops (of course) hitting +3db at 45 degrees (IIRC, might have been 90 degrees, I wasn't paying real close attention). The implication is that if the two signals are less than "x" degrees out of phase, you get a +3dB or better boost.
This relationship is well explored in loudspeaker crossover design where multiple bandpasses overlap in the transition region and relative phase impacts summing.
Quote
Now the real question: when coupling subs, they're obviously slightly out of phase because of the distance between the centers of the drivers. Do you get better than +3dB boost? Or is this a fantasy.  It seems like "money for nothin'" do me (and, therefore, a bit suspicious).
Thanks,
Randy Hyde

Not something for nothing but trading on axis response for total room response. The gain is real depending on where you are standing.  This too is well explored and the subs combining deserve their own chapter in the book.

JR
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Re: Mixing "coherent" vs "incoherent" sound.
« Reply #2 on: March 11, 2011, 01:50:22 PM »


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