Time alignment question - diyAudio
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Old 15th September 2001, 10:46 AM   #1
Simon is offline Simon  England
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Is there a rule of thumb for determining a drivers 'acoustic centre' for time alignment purposes.

For example for a straight sided cone is it:
- half way down the cone ?
- at the back of the cone (pole piece cover end) ?

What about domes ?

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Old 15th September 2001, 06:16 PM   #2
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Default I think it is more complex than that.

The xover is going to introduce phase shifts that you will have take in consideration

dee
;-D
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Old 15th September 2001, 09:23 PM   #3
Super is offline Super  United States
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I'm not positive, but I believe that in terms of depth, its the voice coils of the drivers that are supposed to be aligned.
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Old 15th September 2001, 10:09 PM   #4
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To a first approximation, align the voice coils in the vertical axis. This works well for first order crossovers. You may need to adjust the alignment if you're using a crossover that is less friendly, phase-wise.

Grey
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Old 15th September 2001, 10:36 PM   #5
Simon is offline Simon  England
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...Penguin:

Crossover will be 'phase correct' (Linkwitz-Riley) at xover point. But there's not much point in sorting the filters phase-wise if the drivers aren't in phase. Might just as well stick with Butterworth or Bessel for all the good it will do.

Drivers that are time aligned in the vertical plane must be phase aligned (though the reverse isn't necessarily true). This of course assumes the drivers themselves have a good 'phase behaviour' - if they don't then the whole thing gets mangled and becomes a waste of time anyway.

Intuition tells me phase correctness is a good thing but to get it right I need to know where the centres are. Xovers are going to be active so some tweaking is allowable - however would like to get it as right by theoretical means as possible as my test & measurement equipment (and environment) is limited.


I suppose all these variables and what if's are what makes good crossover design a black art and why diy speakers with identical drivers can sound so different. Wouldn't be fun otherwise would it ????
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Old 15th September 2001, 10:40 PM   #6
Simon is offline Simon  England
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.... Grey (& Super):

Thanks.

I was thinking of 2nd order Limkwitz Riley - this family appears to have superior transient response (to 4th) and this was one of my design goals.

Simon
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Old 15th September 2001, 10:44 PM   #7
Geoff is offline Geoff  United Kingdom
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Simon

For a lot of useful information on time alignment of drivers, have a look at the download zip file (sb80-3wy.zip)associated with publication No. 15 at the Linkwitz site:

http://www.linkwitzlab.com/publications.htm

Geoff
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Old 15th September 2001, 11:46 PM   #8
Simon is offline Simon  England
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Geoff

Thanks. Looks like I have some interesting bed-time reading to do.

Simon
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Old 16th September 2001, 02:28 AM   #9
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Default Phase correct xovers

In that case i would say as a first approximation line up the centers of the voice coils. You will not be able to perfectly time align the drivers without taking some transient measurements.

dee
;-D
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Old 16th September 2001, 11:30 AM   #10
Simon is offline Simon  England
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Penguin

I'm beginning to think you're right. Ultimately the only way to really know for sure is to measure it.

Does any one have any recommendations for cheap(ish) test & measurement packages for this kind of thing. I would assume these days it would all be PC based. Please excuse the ignorance but since my last foray into diy audio was in the days of the IBM XT (remember those things ?!!) I feel somewhat out of touch.

Simon
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