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post #1 of 23 Old 06-20-2004, 04:51 AM - Thread Starter
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A dealer in my area wrote:

Quote:
Rear channels should be as follows for theatre application: dipole sides, mono pole rears, however there are no real rules here. I'm the THX certified guy here and I use bipolar sides and rears, using dipole sides and rears can be done but you have to be careful with polarity!
So, if you put up four dipole surrounds, two side and two rear, what should the polarity be? What would the polarity issues be?

I understand polarity in the fronts (it's audible if you botch it), but when we move to the four surrounds, each of which is dipole, I'm now unsure about the effects.
With a single dipole speaker the idea is to have two speakers in the same phase (polarity), so I would have thought that all four would have to be the same polarity. Is this all there is to it -- just getting four red wires to connect to eight red terminals and four black wires connected to eight black terminals?

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post #2 of 23 Old 06-20-2004, 07:42 AM
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Bob,

> Is this all there is to it -- just getting four red wires to connect to eight red terminals <

Yes. Even though dipole speakers have two radiating surfaces, I believe they both push out and pull in together. So that should happen with the same polarity as all the other speakers. However, depending on the program material, having the surround and rear speakers reversed from the mains may not be as noticeable as if the mains were reversed from each other.

--Ethan
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post #3 of 23 Old 06-20-2004, 04:31 PM
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Bob,
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So, if you put up four dipole surrounds, two side and two rear, what should the polarity be? What would the polarity issues be?
Keep in mind how dipoles work: there is an in-phase driver that pushes out while the anti-phase driver pulls in at the same time.

Dipole surround speakers are typically labeled Left & Right so that, when you set them up at your sides, the in-phase drivers are pointing towards the front of the room. This allows them to be in phase with your front speakers and avoid cancellation problems that can occur when out of phase drivers are facing each other.

Now, if you're going to place another pair of these same surrounds on the back wall as the rear speakers of a 7.1 set-up, you'll run into a problem. The anti-phase drivers of the side speakers will be pointing towards the rear of the room; in fact they will be facing the in-phase drivers of the rear speakers. So you'll have to swap the rear Left & Right speakers, which will make sure that like-phased drivers are always facing each other in your set-up.

Drawing this out on a piece of paper helps to visualize what I'm talking about.

Best,
Sanjay

Sanjay
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post #4 of 23 Old 06-20-2004, 08:21 PM
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Quote:
Originally posted by Ethan Winer
Even though dipole speakers have two radiating surfaces, I believe they both push out and pull in together.
Sorry, Ethan, that's how BI-polar speakers work. Di-poles have the drivers electrically out of phase.

I agree with Sanjay; the spaces between speakers should have the two speaker faces (one from each speaker) facing each other also matching phase, so the sounds they share seem to come from between them.

Sorry for the tongue-twister!

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post #5 of 23 Old 06-20-2004, 10:19 PM
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Please keep in mind Phase is not equal to polarity. polarity is when you have the wires reversed. Something not in phase is a time difference. The 2 are not =
This is all explained here

http://www.prosoundweb.com/studyhall...rity_and_phase
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post #6 of 23 Old 06-21-2004, 08:29 AM
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Larry,

Thanks for setting me straight. Though I admit I'm really surprised to hear they wire them that way!

--Ethan
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post #7 of 23 Old 06-21-2004, 08:55 AM
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Quote:
Originally posted by speco2003
Please keep in mind Phase is not equal to polarity. polarity is when you have the wires reversed. Something not in phase is a time difference. The 2 are not =
This is all explained here

http://www.prosoundweb.com/studyhall...rity_and_phase
Electrically out of phase and acoustically out of phase are different things. In this discussion people are talking about speakers that are wired internally electrically out phase (in other words one driver pulls in at the same time the other driver pushes out), as opposed to acoustic wavefronts that arrive at differing times.
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post #8 of 23 Old 06-21-2004, 10:09 AM
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If 2 drivers are moving opposite each other that is polarity not phase. Read the article and understand. The terms are not freely defined per application.
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post #9 of 23 Old 06-21-2004, 11:16 AM
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Quote:
Originally posted by speco2003
If 2 drivers are moving opposite each other that is polarity not phase. Read the article and understand. The terms are not freely defined per application.
An AC signal has no intrinsic polarity. It has a phase reference to other signals. So in this case, the proper way to describe the signal would be that one speaker is wired 180 degrees out of phase with the other.

What the article you have linked is referring to is adding an additional phase shift to this by means of acoustic propagation. Wire two speakers out of phase 180 degrees, then move one speaker forward a few feet, and you've got an additional phase shift created by propagation of the wavefront. What they refer to as "out of polarity" is accurately (and more directly) described as 180 degrees out of phase.
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post #10 of 23 Old 06-21-2004, 11:50 AM
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Read the first line on page 6. If you wire one speaker in reverse that speaker is reversed polarity. If the 2 speakers reproducing the same signal are seperated one in front of the other that is phase shift and can be corrected by delay addition. Read page 10 after figure 18 and this backs me up. Phase and polarity are not the same.
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post #11 of 23 Old 06-21-2004, 12:12 PM
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Quote:
Originally posted by speco2003
Read the first line on page 6. If you wire one speaker in reverse that speaker is reversed polarity. If the 2 speakers reproducing the same signal are seperated one in front of the other that is phase shift and can be corrected by delay addition. Read page 10 after figure 18 and this backs me up. Phase and polarity are not the same.
What I keep saying is that common terms for situations with speakers do not override the correct technical terms to describe the situation. Polarity and phase, as described by that article, are identical. In this case, polarity simply refers to 180 degrees (either in phase, or directly opposite). Whether the phase shift (and any additional) is caused by electrical means or wave propagation does not negate that it's a phase shift of the signal.

BTW, if you could correct delay per speaker, you could easily correct for wiring one speaker opposite the other by applying 180 degrees of phase shift to the either channel.
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post #12 of 23 Old 06-21-2004, 12:32 PM
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Ok I am willing to be wrong but the article and my training tell me different. please so I can see what your saying point me to where the article says that they are the same. I cant seem to cut and paste but the top of page 6 of the article seems to back me up. If I am doing a measurement and I have 2 speakers playing the same source and wired the same but one is farther in front of the other they will be considered out of phase and can only be brought in phase by time correction. Now if one hi driver is wired correctly and the others hi driver is reversed they are not in polarity ie one pushing one pulling.
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post #13 of 23 Old 06-21-2004, 12:38 PM
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Ok here is another srticle by the biggest guru in our biz. Complete article here http://mixonline.com/ar/audio_understanding_phase/

PHASE AND POLARITY


Before we move to the next section, time for a pop quiz. How many milliseconds of delay results from 180° of phase shift?

Is that your final answer?

The correct answer is, “That's a trick question.†Why? Because a frequency must be specified. So how many milliseconds of delay are equivalent to 180° of phase shift at 250 Hz? The answer is 2 ms.

Now, how much time delay do we get when we put a phase reverser in-line? It reverses all frequencies by 180°, so it must delay each one by a slightly different amount. That is quite a fancy delay circuit there! And I thought it was just swapping two wires! In reality, there is no such thing as a “phase reverser.†It is a polarity reverser. Polarity reversers do not delay the signal. They invert the voltage or pressure component of the signal. While this does shift the phase, it does not change the phase delay time. That is not to say that a polarity reverse is unimportant. Anything that affects the phase will dramatically affect the way that different signals combine.

When two signals of the same frequency are combined, the summed response may be greater than or less than the original signals, depending on the phase. One plus one equals two, one or zero if summed at 0°, 90° and 180°, respectively. In a speaker array, the amount of addition will depend upon how close to 0° phase difference there is in arrival times. The tendency of speakers to add or subtract is shown in Fig. 2, which illustrates the hemispherical nature of combination. Signals that combine on the 90° to 0° to 270° hemisphere will achieve constructive addition. Signals on the 90° to 180° to 270° side will subtract. Constructive addition is easily done at low frequencies where it takes path length differences of several feet to move out the speaker of the addition zone. The highs, like football, are a game of inches.

The relationship between phase and polarity is illustrated in the series of simulations in Fig. 3. In this series, two speakers are splayed 60° apart, and the response is viewed at 250 Hz. In 3a, the response of a single unit is shown as a directional pattern reference. In 3b, both speakers are enabled, and the response shows a beam at the center of the array where the speakers combined at 0° relative phase, yielding 6 dB of addition over the response of a single unit at that location. The sides of the beam are formed in the area where the speakers are 90° of relative phase apart, creating minimal addition. The nulls are caused as we move into the cancellation hemisphere, with the deepest spot being 180°. Side lobes appear where the signal is a full cycle (360°) out of time, allowing addition to occur again. At different frequencies, the position of the nulls and side lobes will change. The time between the cabinets stays the same, but the change in frequency causes the relative phase to change.

In Fig. 3c, there has been a polarity reversal in one of the speakers. The amount of energy generated by the speakers is the same as before, yet where it goes has reversed completely. Now, the side areas contain the bulk of the energy, while the on-axis area is in a null.

Finally, in 3d, we have a delay of 2 ms on the lower speaker. This is half a wavelength at 250 Hz. The 2ms delay steers the sound downward toward the delayed speaker in the direction of where the 0° addition area is centered. (This technique can be used to optimize arrays, the subject of Part 2.) Compare this to the polarity reversal above where the signal is flipped 180° (half a wavelength), but the result is quite different.
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post #14 of 23 Old 06-21-2004, 12:46 PM
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It seems like that article is using the term "phase delay time" for what is generally termed group delay. It does represent a phase shift, but in a different manner, and that was a point I thought I made earlier but may not have.

*EDIT*

On second thought, since music does not generally consist of periodic signals, I suppose that for the most part electrical phase shift would look very similar to wave propagation delay, but inversion of polarity would be unachievable (the problem being I've been sitting here thinking of sine waves and not a complex aperiodic waveform).
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post #15 of 23 Old 06-21-2004, 12:54 PM
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I think I see where you are coming from, but do we agree on polarity being in its simplest terms a reversal of leads? Phase also being a time difference, and that can be introduced by reversed polarity?
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post #16 of 23 Old 06-21-2004, 01:02 PM
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I completely agree with that. I was simply using the assumption of a sine wave to say that swapped leads was equivalent to 180 degree phase shift, which it generally would not be when playing a real signal.
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post #17 of 23 Old 06-21-2004, 01:03 PM - Thread Starter
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I threw this together to answer my original question, with lots of help from sdurani:

http://www.bobgolds.com/DipoleSurrounds/home.htm

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post #18 of 23 Old 06-21-2004, 01:14 PM
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Basement Bob I still have a problem with the use of phase here, it seems the correct term would be polarity, thats what the 2 of us have been bantering about before you rudely interupted us with this fine example to help the original post.:p
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post #19 of 23 Old 06-21-2004, 02:17 PM - Thread Starter
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speco2003

Please continue the phase vs polarity chat. I'm curious.

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post #20 of 23 Old 06-21-2004, 03:06 PM
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I think I am done, we seemed to have reached agreement etc...
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post #21 of 23 Old 06-22-2004, 08:46 AM
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Ferris,

> An AC signal has no intrinsic polarity <

Sorry, but that's not true. Suppose you put a microphone in front of a bass drum, then hit the drum from the other side. The head in front of the microphone will push outward, and in turn push the mike's diaphragm inward. If the mike is wired correctly that creates a positive output voltage on the "plus" pin of the output connector. If it's not wired correctly it will create a negative output voltage. With a single microphone recording a single instrument, absolute polarity is not a factor. But when you put seven mikes on an entire drum set, or three mikes above a large violin section, they better all be wired correctly or you'll have big problems!

> the proper way to describe the signal would be that one speaker is wired 180 degrees out of phase with the other. <

Phase implies a time component. Polarity reversal does not. It's as simple as that.

--Ethan
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post #22 of 23 Old 06-22-2004, 09:09 AM
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Ethan,

Re-read the discussion above. I've already conceded that I was referring to periodic signals in my discussion, and not thinking of aperiodic signals such as music.
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post #23 of 23 Old 06-22-2004, 09:47 AM
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Ferris thats why when mixing monitors for a live band we sometimes throw the kick drum mic to reverse polarity, because of that push pull. Its amazing how it will kill feedback issues and also make a nice boom chicka boom sound.:D
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