Horizontal MTM Center channel: avoiding off-axis nulls - Page 2 - diyAudio
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Old 27th July 2009, 12:29 AM   #11
tresch is offline tresch  United States
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In progress!

just a test-fit. Looking good! need to put holes in the bracing and build the boxes for the mid-tweets.

Center is very similar, need to make the miter cuts, yet, but it's about ready for a test fit, too
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Old 27th July 2009, 02:26 AM   #12
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Quote:
Originally posted by DcibeL
I am a little disappointed in the FF85K. The one I received had excessive black goop between the aluminum cap and the cone, and was not very evenly distributed.
The black stuff is the glue used to attach the dustcap... for the last year & a half the execution of this assembly step has seemed to be a bit random... i changed part of my EnABL recipe on the FE126/127 to accomodate the difference... i've not seen an FF85 with that much thou.

dave
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Old 27th July 2009, 02:52 AM   #13
Few is offline Few  United States
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Quote:
When an MTM is vertical, off-axis nulls will occur at at +45 degrees, and -45 degrees on the vertical plane, at the crossover frequency.
I don't think this is universally true. When D'Appolito first introduced his version of an MTM arrangement (AES paper) it included a specific crossover configuration that, together with the driver arrangement, yielded much more uniform polar response. I'm certainly not suggesting that the project that is the focus of this thread isn't worthwhile--I'm looking forward to reading about its successful development and completion. I'm just suggesting that a close look at D'Appolito's work on MTMs might yield some design ideas that will be helpful as you try to achieve your goal of more uniform polar response.

Few
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Old 27th July 2009, 03:46 AM   #14
tresch is offline tresch  United States
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There are other benefits of using a full-range as a tweeter, of course, as well. It's these other reasons that made me decide to go with this configuration, especially as I'm not an extremely experienced designer.

1) Woofer choice - You can choose woofers based solely on their performance in the bass range, without having to worry about cone breakup and the like. Finding a woofer that is great in both the low end and midrange generally results in an expensive driver. This configuration allowed me to choose a relatively inexpensive woofer that performs great down low, without having to worry about its inadiquacies

2) crossover design - since there is a HUGE overlap between the tweeters resonance (125hz for the A5) and the woofer's breakup, I have a huge range of crossover points available that require no compensation for driver behavior

3) It's different and it looks cool?
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Old 28th July 2009, 01:23 AM   #15
Few is offline Few  United States
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Hi Tresch,
I agree. There's more to the project than the radiation pattern, and it's worth pursuing for lots of reasons. My motivation for pointing out D'Appolito's work was that posts have a way of taking on lives of their own and I didn't want other readers to overlook some of the MTM subtleties. I certainly don't want to discourage anyone from pursuing projects! I share the interest in finding ways to lower the crossover point to minimize the discontinuity between the tweeter (or wide range tweeter-substitute) and the woofers and I'm looking forward to seeing how the project develops.

Few
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Old 28th July 2009, 01:51 AM   #16
KSTR is offline KSTR  Germany
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To put some details on the D'Appolito configuration:

Interdriver spacing (acoustic centers) of 2/3 of the wavelength at the crossover frequency.

Locked 90 deg phase difference between woofers and tweeter, which can be achieved (for example, but not restricted to) with odd-order Butterworth acoustic x-over characteristics. With 1st order, the off-axis errors are in a -3dB band, with 3rd order it's a bit more, peaking at -9dB at about 30 degrees. But no deep nulls like that we get with a Linkwitz-Riley (which is 0 deg locked phase). To the latter the OP was referring to, me thinks.

- Klaus
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Old 28th July 2009, 03:17 AM   #17
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There's a really simple solution to this problem. Here's what you do:
  • The directivity of your speakers will narrow to 83.6 degrees when the distance between the woofers is one half wavelength. Therefore, do not run your woofers above this frequency.
  • For example, if you are using four inch woofers and the center-to-center spacing is seven inches, the beamwidth will collapse to 83.6 degrees at 964hz. (speed of sound / 7 inches / 2) Therefore, don't run your woofers above this frequency.
  • Once you do the math, it becomes clear that horizontal arrays are practical, but you need an extremely small center-to-center spacing, or a very robust tweeter.

Does that make sense?

I personally am not a fan of using a full range in lieu of a tweeter, as it reduces your efficiency, your extension, and your power handling. I think the best solution is to use an extremely small center-to-center spacing, or use a tweeter with gobs of excursion.

For further reading, download "Application of Linear Phase Digital Crossover Filters to Pair Wise Symmetric Multi Way Loudspeakers part 2", by DB Keele.
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Old 28th July 2009, 01:27 PM   #18
KSTR is offline KSTR  Germany
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Yes, with convolution filters sort of "everything" is possible, since one can adjust all driver's magnitudes and phases independently within reasonable bounds... and yield a pretty much optimized system in various regard (especiallly tailoring off-axis response to the needs). I'm about to follow that route, but for many DIYers that is not an easy option.

- Klaus
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Old 30th July 2009, 03:04 PM   #19
DcibeL is offline DcibeL  Canada
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Well I am currently looking at this project being completed (apart from the cabinet finish) at the end of August. I may have to order another FF85K and hope for the best, as the extra goop on the one I have is sure to cause some trouble, or at least change the drivers' response from the manufacturer's datasheet. I expect that sensitivity and high frequency response will suffer. I'll try it out though, and hopefully won't have to go that route.
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Old 1st September 2009, 03:20 PM   #20
DcibeL is offline DcibeL  Canada
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My cabinet arrived, but the builder screwed up the baffle so terribly much it's unreal. I am very upset about it, the builder has come highly recommended from various other projects on the interweb.

The cabinet arrived with the baffle glued on, so I can't remove it easilty and put a different one on. I'm waiting to hear back from him, hopefully he will wish to rectify the issue.
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