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**GranteedEV** · 05-31-2011, 12:24 PM

Re: 2 way or 2.5 way MMTMM?

I think the 2.5 way will be optimal. I guess it comes to "semi-line source" vs "point source" and I guess there may be indivual preferences for either.

**XtremeRevolution** · 05-31-2011, 12:37 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by GranteedEV View Post

I think the 2.5 way will be optimal. I guess it comes to "semi-line source" vs "point source" and I guess there may be indivual preferences for either.

I haven't tried to model this in PCD yet, but wouldn't going with a 2.5 way setup make the crossover more complex and expensive?

**thekorvers** · 05-31-2011, 12:55 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by XtremeRevolution View Post

Since my project thread gets virtually no exposure , I figured I'll ask the bigger questions in independent threads.

I'm planning a MMTMM with a Vifa tweeter and 4 of these:

https://www.madisound.com/store/prod...oducts_id=8781

I'm pretty far into the design process, but I wanted to know if I would have any issues with combing and lobing.

From my understanding, if the difference from the center of the tweeter to the center of the midrange is smaller than the wavelength of the crossover frequency, I should be ok, which in my particular case puts me at around 2200hz. Will this be an issue, or should I look into modeling a 2.5 way design where the outer drivers play the lower frequencies only and allow me more flexibility with the crossover point?

Lobing is less of an issue with an MTM design than it is with a TMM design. That said you still need to place the drivers apart equal or less than a wave length for an MTM design to work properly.

If you don't have Speaker Building 201 by Ray Alden, I suggest you get it. It is a very useful book. You will find this information on pages 51 - 52.

More relevant information concerning lobing and spacing of drivers etc. can be found in Jim Griffin's white paper analyzing Line Arrays.

Some time ago I calculated the frequencies related to the major distances (wavelengths) and put them in a table which I use for easy reference. TechTalk does not allow me to attach a file, but the table is short and I am reproducing it here. You can copy and paste this in a word document for use:

From Jim Griffin:

The c-t-c calculation relates the frequency (F) in Hz wherein the c-t-c spacing (D) in inches becomes a wavelength. The formula is

F = 13500 / D

where 13500 is the velocity of sound in inches per second. Thus if you have a 4 inches c-t-c spacing (4 inch diameter drivers with flanges touching) you get a frequency of 3375 Hz. As I explain in my white paper this is the recommended crossover frequency between the woofer and tweeter lines.

1 inch ~ 13500 Hz
1.5 inch ~ 9000 Hz
2 inch ~ 6750 Hz
2.5 inch ~ 5400 Hz
3 inch ~ 4500 Hz
3.5 inch ~ 3857 Hz
4 inch ~ 3375 Hz
4.5 inch ~ 3000 Hz
5 inch ~ 2700 Hz
5.25 inch ~ 2571 Hz
5.5 inch ~ 2455 Hz
6 inch ~ 2250 Hz
6.5 inch ~ 2077 Hz
7 inch ~ 1929 Hz
7.5 inch ~ 1800 Hz
8 inch ~ 1688 Hz
8.5 inch ~ 1588 Hz
9 inch ~ 1500 Hz
9.5 inch ~ 1421 Hz
10 inch ~ 1350 Hz
10.5 inch ~ 1286 Hz
11 inch ~ 1227 Hz
11.5 inch ~ 1174 Hz
12 inch ~ 1125 Hz

If we assume that the diameter of your tweeter is + 4" and the TB drivers you are using 3.25", the C-T-C distance is + 3.5" - 4".

From the table: The cutoff frequency is + 3500 Hz. So 2200 Hz presents no problem.

**Brewski** · 05-31-2011, 12:57 PM

Re: 2 way or 2.5 way MMTMM?

You'd need an extra inductor on each side for the .5 woofer. Catch 22 is if you build it and decide you wanted to switch from 2 way to 2.5 way you probably need to rework the woofers crossovers at that point so you'd want to figure that into the cost.

Another thing to consider is your driver wiring while modeling. You may want try various options like having the .5 woofers in parallel and the others in series to see how those options look in your sims. The sims in PCD might help sway you in one dirrection or another once you get going.

Take it easy
Jay

**Ludo** · 05-31-2011, 01:04 PM

Re: 2 way or 2.5 way MMTMM?

With a pair of those 3" drivers only measuring a little over 6 inches per MM, and your 2.2kHz xo frequency, your vertical dispersion should be reasonable doing a 2-way.

**Brewski** · 05-31-2011, 01:06 PM

Re: 2 way or 2.5 way MMTMM?

If we assume that the diameter of your tweeter is + 4" and the TB drivers you are using 3.25", the C-T-C distance is + 3.5" - 4".

From the table: The cutoff frequency is + 3500 Hz. So 2200 Hz presents no problem.[/QUOTE]

Wouldn't the distance be calculated by the outside woofer in his MMTMM configuration. 1.75" + 3.5" + 1.5"?

Take it easy
Jay

**thekorvers** · 05-31-2011, 01:14 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by Brewski View Post

If we assume that the diameter of your tweeter is + 4" and the TB drivers you are using 3.25", the C-T-C distance is + 3.5" - 4".

From the table: The cutoff frequency is + 3500 Hz. So 2200 Hz presents no problem.

Wouldn't the distance be calculated by the outside woofer in his MMTMM configuration. 1.75" + 3.5" + 1.5"?

Take it easy
Jay

No. If that were the case, Line Arrays wouldn't work.

**XtremeRevolution** · 05-31-2011, 01:18 PM

Re: 2 way or 2.5 way MMTMM?

Thanks for all the wonderful help guys, this is all really good advice. This will definitely help me come to a design decision.

I do have speaker building 201, and have been reading it slowly. I will skip to the pages you recommended and look through that section.

The distance from the center of the tweeter to the center of the outer driver is 6.15". the drivers will be mounted right up against each other and right up against the edge of the tweeter.

I will be using this tweeter:

http://www.parts-express.com/pe/show...umber=264-1028

I'm leaning toward a 2-way configuration at this point. Am I correct in thinking then that a 2200hz crossover point will be sufficiently low to minimize any combing or lobing issues I might have?

**donradick** · 05-31-2011, 01:42 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by thekorvers View Post

From the table: The cutoff frequency is + 3500 Hz. So 2200 Hz presents no problem.

@Xtreme - that's what I was thinking. Plan on a crossover between 2700-3200 Hz. This will use the nice top end of that little mid-woofer, and relieve any stress on the tweeter.

Per your other comment about 2.5 crossovers. The simplest topology will only have 1 extra inductor used for the "bottom" woofers compared to a standard TM crossover.

See the Mini-Statements crossover for an example of this:
http://speakerdesignworks.com/MiniStatements.html

**greywarden** · 05-31-2011, 02:19 PM

Re: 2 way or 2.5 way MMTMM?

I've got a question I think is pretty on topic here, If you have a MMTMM where the woofers/mids are wired in series-parallel, how would you implement the crossover in that situation?

Would you cross the tweeter, then wire the 2 woofers closest to the tweeter in series and cross them over, then wire the furthest woofers in series and cross them over together, then the tweeter, closest woofers, and further woofers are all wired in parallel?

I think I just answered my own question, lol.

Hm... that would reduce the inductance needed on the woofers, too wouldn't it? Everybody Wins! Yay!

**XtremeRevolution** · 05-31-2011, 02:24 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by donradick View Post

@Xtreme - that's what I was thinking. Plan on a crossover between 2700-3200 Hz. This will use the nice top end of that little mid-woofer, and relieve any stress on the tweeter.

Per your other comment about 2.5 crossovers. The simplest topology will only have 1 extra inductor used for the "bottom" woofers compared to a standard TM crossover.

See the Mini-Statements crossover for an example of this:
http://speakerdesignworks.com/MiniStatements.html

I see what you mean about the Mini-Statements. Sounds like it may be fairly simple then. I'll try to model a 2.5 way design since it doesn't seem all that difficult, and two 3" mids should provide enough volume. I might be able to play around with impedance a bit as well.

**thekorvers** · 05-31-2011, 02:31 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by greywarden View Post

I've got a question I think is pretty on topic here, If you have a MMTMM where the woofers/mids are wired in series-parallel, how would you implement the crossover in that situation?

Would you cross the tweeter, then wire the 2 woofers closest to the tweeter in series and cross them over, then wire the furthest woofers in series and cross them over together, then the tweeter, closest woofers, and further woofers are all wired in parallel?

I think I just answered my own question, lol.

Hm... that would reduce the inductance needed on the woofers, too wouldn't it? Everybody Wins! Yay!

?????

**frugala** · 05-31-2011, 02:33 PM

Re: 2 way or 2.5 way MMTMM?

Slightly OT for this thread, but just wondering if you are going with the pseudo dipole (open back) that you were talking about previously?

**XtremeRevolution** · 05-31-2011, 02:36 PM

Re: 2 way or 2.5 way MMTMM?

Originally posted by frugala View Post

Slightly OT for this thread, but just wondering if you are going with the pseudo dipole (open back) that you were talking about previously?

No, I won't be doing that for a number of reasons. I want this to be wall-mountable, and I will need some way of controlling excursion.

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