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Multi-Way Conventional loudspeakers with crossovers

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Old 26th January 2005, 05:50 PM   #1
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Default Align for time, or for phase?

These questions are hypothetical and theoretical - I'm not actually building this system, just trying to understand it.

Suppose we have a two-way system with a second order crossover at, let's say, 3 kHz. If I understand correctly (and that's a big "if"), then the low-pass driver's response will lag 180 degrees behind the high pass driver's response.

Note that at 3 kHz one wavelength is about 4.5 inches long, so 1/2 wavelength would be 2.25 inches long.

Let's look at three different cases:

1) Tweeter in front: Let's say the drivers are flush-mounted on the vertical face of the enclosure, and that the woofer's voice coil just happens to be 2.25 inches behind the tweeter's. In this case, at the crossover frequency the woofer lags the tweeter by 180 degrees due to the crossover, and by another 180 degrees due to the driver offset (which is really a fixed time delay, but corresponds to 1/2 wavelength at the crossover frequency), which will put the woofer's output a full 360 degrees behind the tweeter's at the crossover frequency. Is this correct?

2) Drivers aligned: Next, let's precisely align the voice coils in the vertical plane. Now the woofer's output lags the tweeters' by 180 degrees, even though their outputs arrive at the same point in time. So in order to not have a deep null at the crossover frequency, we reverse the polarity of one of the drivers so that now they are operating in sync (I hesistate to say "in phase" but maybe that would be accurate) at the crossover frequency. Is this correct?

3) Woofer in front: Finally, suppose we physically move the tweeter back so that its voice coil is now 2.25" behind the woofer's voice coil. In this situation, at the crossover frequency the woofer is 180 degrees behind because of the crossover but also 180 degrees ahead because of the physical offset, so now we connect the drivers with the same polarity and their output is indeed in phase in the crossover region. Is this correct?

It might be said that configuration 1) is wrong in both time and phase; that 2) is right in time but wrong in phase; and 3) is wrong in time but right in phase, at least in the crossover region. Or, maybe I fail to understand the principles involved and how they interact.

Now I would think they'd all do a decent job of passing a 3 kHz sine wave. But which would do the best with a square wave, or a step response, or an impulse response?

Assuming no cabinet diffraction issues, which alignment would be the most likely to sound coherent and image well? (Yeah I know first order crossover would probably be better, but I'm trying to figure out what would be the optimum configuration for second order crossover).

My thanks to anyone who takes the time to comment.

Alidore
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Old 26th January 2005, 06:27 PM   #2
tiroth is offline tiroth  United States
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I am very interested in this thought experiment. I posted a similar question recently but was unable to get any constructive feedback.
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Old 26th January 2005, 06:40 PM   #3
wimms is offline wimms  Estonia
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You desperately need to consume something like this: http://www.linkwitzlab.com/frontiers.htm
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Old 26th January 2005, 06:47 PM   #4
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In the case of the second order crossover a 90 degree phase shift occurs in both the HP and LP sections, and that combines for a total of 180 degrees, creating a null at the crossover frequency. You can cure that by switching polarity of one of the drivers. But then time aligning is also required to maintain perfect phase relationships, and to do that you align the voice coils on the vertical plane.

You can accomplish the same thing as far as phase is concerned by leaving the drivers electrically out of phase and off-setting the voice coils by 1/2 wavelength, but that addresses only phase and not time. The crux of the question is whether you can hear the time lead/lag in that instance. Studies have shown that you can detect differentials as small as .003 seconds or so in the midrange, but the lead/lag with a 2.25 inch path differential would be around .00016 seconds, so it's not likely in this case.

The accepted procedure is to always phase align electrically, and to time align physically if one wants to go to the trouble involved. If you don't physically time align you also will have a phase zit but again in most cases it isn't audible anyway.
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Old 26th January 2005, 07:37 PM   #5
Bricolo is offline Bricolo  France
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Quote:
Originally posted by BillFitzmaurice


You can accomplish the same thing as far as phase is concerned by leaving the drivers electrically out of phase and off-setting the voice coils by 1/2 wavelength, but that addresses only phase and not time.

Are you sure about this?

At the XO, there's a 180 phase difference, so connecting the tweeter backwards gives a correct phase @xo frequency. If the accoustical centers are aligned

You speak about a tweeter in reverse polarity, AND non aligned accoustic centers, this seems wrong to me
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Old 26th January 2005, 09:09 PM   #6
Pan is offline Pan  Sweden
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Alidore,

you are correct in all three examples you bring up.

The thing is that if you have a textbook 2nd order slope for both drivers at 3k and pull the tweet back to get them in phase at 3k, then you´ll have non flat summation above and below the crossing. And no example with true 2nd order slopes will have a good pasing of square waves/impulses though ther will be differences between the three examples.

Bill,

you are basically right but; "The crux of the question is whether you can hear the time lead/lag in that instance."

It´s not the time itself that is interesting here but the issue I bring up above. You will simply have ripples/non flat response.

Bricolo,

the drivers/legs will be both be 90 degrees away iow a 180 degree phase difference between the drivers, one connection summing flat and one creating a deep null.

/Peter
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Old 27th January 2005, 02:59 PM   #7
synergy is offline synergy  United Kingdom
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Quote:
Originally posted by BillFitzmaurice
time aligning is also required to maintain perfect phase relationships, and to do that you align the voice coils on the vertical plane.

i've always wondered about this method thinking "but the voice coils don't make any noise" they only give motion to the thing that does

fair play when it comes to aligning for a tweeter as there's next to nothing in it but say you're using one of the new breed of high Xmax drivers you're going to be way out on any cone you're using due to the extra length of coil involved

would you say the edge of the dust cap would be better or somewhere else entirely?
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Old 27th January 2005, 03:25 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by synergy
i've always wondered about this method thinking "but the voice coils don't make any noise" they only give motion to the thing that does...
But the voice coil does make a noise! Sound is vibration, and voice coils vibrate. This sound is conducted to the cone, where it is then coupled to the air. If air was incredibly dense, you wouldn't need a cone at all - the voice coil alone would couple well enough to the air.
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Old 27th January 2005, 03:32 PM   #9
pjpoes is offline pjpoes  United States
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My understanding here is limited, so I simply theorize here. I own a pair of time aligned speakers, and have also played around with time aligning the speakers by making test cabinets that could have the tweeters moved, and then measured with my RTA and computer. I dont have the ability to truely test what goes on, the resolving power of my equipment is terrible.

Anyway, I have often wondered if simply placing the tweeter's coil at the same plane as the midranges would actually time align the speaker. I had a few things I wondered about with this. First, I think high frequency waves can travel through the air faster than low frequency waves. One reason I think that is that bass waves disrupt the air more, and thus create more turbulance. Normally turbulance causes something to be slowed down, so I would guess bass runs slower. Im not sure of that, but looking at wave theory it just seems to make sense to me. Like I said, my understanding of this is all very limited.

If this is true though, then you would need to account for this with the tweeter placement. However, as was mentioned before, many of these differences are so small given the distances we are talking about that hearing it would be next to impossible. My guess is that, if what I said was true, it still isnt going to be audible.

another thing I have guessed, and clearly see in the design of both my JmLabs speakers and other aligned versions. Theil are infamous for the care in propper time aligning. I actually believe that I read mine are not truely phase aligned but only time aligned, or something along those lines. Anyway, if the speaker is tall, and there is a driver above the tweeter, and thus above ear level, then to properly time align, I would think you would need to angle them down. I may be confusing theories here, as it just crossed my mind that this might only have to with the directivity of the driver. Anyway, it seems that there is more to time and phase alignment than some simple wire swapping and distance changes. It would be my guess that is why these modern speaker DSP devices are able to so accuratly align every aspect of the speakers, so they measure so well.
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Old 27th January 2005, 03:45 PM   #10
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Since sound usually travels faster through most of the construction materials than through the air, the alignment of the voice coils isn't an exact one regarding time alignment.
I assume that the alignment of voice coils is just a coarse solution to avoid measuring of the actual delays involved. Since the plane of radiation is frequency-dependant one can only measure to get exact results. As an approximation the coil alignment might be better than nothing. One can assume that for a cone driver the acoustical center is in the middle of the cone at its upper working range, that's where the method might come from.

BTW: 2nd order filters with overlap and a recessed tweeter can give quite good performance in the frequency and time domain but for one axis only unfortunately.

Regards

Charles
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