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MTM crossover questions
MTM crossover questions
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Old 5th November 2010, 06:34 AM   #1
SpinMonster is offline SpinMonster  United States
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Default MTM crossover questions

Im new at this and have a simple question.

Calculating the optimum center to center spacing of an MTM design goes by the crossover frequency. The distance as I read was found by dividing 13768 by the crossover frequency. If I chose a 2000hz crossover, it would then be about 6.9".

My question is: Is that the center to center distance of one woofer to the center of the tweeter or is that the distance to the center of the other woofer. It would seem you need a really small tweeter. Can a longer ribbon be used such as the Aurum Cantus G3 (5") in an MTM?

Listening distance would be about 6' to the seated listener and its primarily a home theater speaker. I do want it to be a decent stereo speaker with a fairly fixed listening height but over a wider area for multiple listeners.

I heard steeper crossovers and greater listening distances allow higher crossover frequencies.

I am interested in a high efficiency ribbon with 2 18Sound 6ND430-16

4th order at 2400hz

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Old 5th November 2010, 08:53 AM   #2
speaker dave is offline speaker dave  United States
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That formula appears to use the speed of sound in inches per second. It would calculate the spacing for one wavelength. I assume the intention was to define the woofer to woofer center spacing as one wavelength apart.

The issue is the lobing that is created from two spaced point sources. In practive you would need to get the woofers within 1/2 a wavelength apart to prevent nulls in the vertical response. A 3.45" spacing is impossible so there will always be vertical lobes.

If you mount your speakers at ear level (or aim them up or down as needed) then the lobes won't be too troublesome. Left to right response won't have the issue so multiple listeners will be okay!

David S.
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Old 5th November 2010, 09:20 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

The simple fact is whatever the c/o point, the size of the drivers and the tweeter,
the closer they are together the better, there is no optimum spacing, there may be
an effective maximum spacing inversely related to c/o frequency, but to get any
sensible agreement on what that might be is very difficult.

TMM 2.5 ways reduce the lobing issues as there are 2 instead of 3 sources.

Quite a bit of info on Zaphs site. (Also on MMTMM 2.5 ways)

rgds, sreten.

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Last edited by sreten; 5th November 2010 at 09:25 AM.
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Old 5th November 2010, 10:16 AM   #4
SpinMonster is offline SpinMonster  United States
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Using the G3 ribbon and the 18Sound 6ND430-16 in an MTM arrangement with a 2400 crossover point, what slopes should I use and is there any other input you can give me as to what else to add into the crossover?
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Old 5th November 2010, 11:15 AM   #5
Helmuth is offline Helmuth  Netherlands
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Quote:
TMM 2.5 ways reduce the lobing issues as there are 2 instead of 3 sources.

Quite a bit of info on Zaphs site. (Also on MMTMM 2.5 ways)

rgds, sreten.
Does the lobbing issue with 3 sources give a better result as for a two soource one?

Because the lobbing is more average. As for a two source one?

I say this because I have good experiences with Dáppolito configuration.
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Old 5th November 2010, 11:44 PM   #6
mayhem13 is offline mayhem13  United States
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Quote:
Originally Posted by SpinMonster View Post
Using the G3 ribbon and the 18Sound 6ND430-16 in an MTM arrangement with a 2400 crossover point, what slopes should I use and is there any other input you can give me as to what else to add into the crossover?
My first criticizm would be crossing the G3 that low.......not a good idea IMO. HD is going to be near horrendous by the time 2khz rolls in. Second would be the MTM config with an 12" C to C spacing.....too much even for 2.4khz.....can't imagine the lobing at 3.5 or higher.

A conventional 3way with the 8ohm version of the 18Sound filled below with a 10-12" woofer or pair would be a better approach IMO. If you must use the 18Sounds in an MTM, the SBAcoustics SB29 Neo offset and crossed 2khz solves most of the issues presented by the G3......for less money and IMO a MUCH better outcome.
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Old 6th November 2010, 01:48 AM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Helmuth View Post
Does the lobbing issue with 3 sources give a better result as for a two soource one?

Because the lobbing is more average. As for a two source one?

I say this because I have good experiences with Dáppolito configuration.
Hi,

No. D'Appolito guarantees the main lobe in on the tweeter axis
due to symmetry and thats it for MTM's, though strictly speaking
it is the case D'Appolito theory imples particular acoustic (not
electrical) rolloff functions to account for typical driver offsets.

There are more narrower lobes with 3 sources than 2. For typical
2 ways (or 2.5ways) an asssymetric version of linkwitz/riley is
typically used to account for driver offsets, the main lobe of
such is far wider for a TM than an MTM.

rgds, sreten.

Last edited by sreten; 6th November 2010 at 01:51 AM.
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Old 6th November 2010, 03:13 AM   #8
mayhem13 is offline mayhem13  United States
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What Sreten said.....or in other words the lobing is between the two midwoofers, not the individual mids to the HF device.

I've tried to work around what maybe you're attempting myself which might be the use of two midwoofers to match the eff of the ribbon....not wanting to sacrifice it?
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Old 6th November 2010, 09:57 AM   #9
SpinMonster is offline SpinMonster  United States
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Quote:
Originally Posted by mayhem13 View Post
My first criticizm would be crossing the G3 that low.......not a good idea IMO. HD is going to be near horrendous by the time 2khz rolls in. Second would be the MTM config with an 12" C to C spacing.....too much even for 2.4khz.....can't imagine the lobing at 3.5 or higher.

A conventional 3way with the 8ohm version of the 18Sound filled below with a 10-12" woofer or pair would be a better approach IMO. If you must use the 18Sounds in an MTM, the SBAcoustics SB29 Neo offset and crossed 2khz solves most of the issues presented by the G3......for less money and IMO a MUCH better outcome.
I appreciate your input and credit it as accurate. I post on other forums such as the Corvetteforum under the same name and have become something of an authority in building high HP cars. Why that is, isnt because Im a genious. I get a part and install it and can then say, first hand, that part works in that configuration. This is exactly what is going on here. I want to know what a ribbon and its distortion sounds like; nothing more, nothing less. Im one of those crazies who owns Krell equipment because I heard the difference in the real world and yes, the result is different with my Krell than with my Bryston 7B amps. You cant measure it. I wont ditch the use of ribbons becasue of distortion graphs. I would miss out on something.

I have (already own it) the G3 (not G3si) and for the education I want to build a system with them. Why? While I agree that the ribbons are eactly as Zaph said they are, I want to know first hand what this will sound like so I can say I heard it and base my future comments on my use of them in a real speaker, optimumly built, and not based on a distortion graph with a single cap in the line.

After this, I will ditch the front baflfles and start over with the same two 18 sounds and use a scan speak 7000 ring radiator but I will sit there knowing what the ribbon sounded like and why companies that make them stay in business. I dont care about value. Im aware i can move around the room while listening, I dont do that anyway. I sit in the optimum spot and stay there. Its not a party speaker Im building. I have those already in other rooms.

If you have a suggestion as to how I would go about finishing this project crossover wise, please contribute because Im trying to learn. Im not looking for cost savings so please feel free to use expensive parts recomendations and I want them to be as good as they can be so the result is a no-excuse finality.

I dont need anything more for low end. I have 500 pounds worth of subs in my listening room and love the low end. I need the over 80HZ sound only.
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Old 6th November 2010, 12:25 PM   #10
SpinMonster is offline SpinMonster  United States
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Quote:
Originally Posted by mayhem13 View Post
My first criticizm would be crossing the G3 that low.......not a good idea IMO. HD is going to be near horrendous by the time 2khz rolls in. Second would be the MTM config with an 12" C to C spacing.....too much even for 2.4khz.....can't imagine the lobing at 3.5 or higher.

A conventional 3way with the 8ohm version of the 18Sound filled below with a 10-12" woofer or pair would be a better approach IMO. If you must use the 18Sounds in an MTM, the SBAcoustics SB29 Neo offset and crossed 2khz solves most of the issues presented by the G3......for less money and IMO a MUCH better outcome.

Actually there may be a way to get around prdjudicial issues and take into account your concerns about the ribbon crossing over too low. I didnt want to use the 18sound mid in a conventional 3 way but I would be inclined to go with a dome that can cross over higher and be closer so the system will blend better in a 6' distance. What do you think about the use of a dome like the Tangband 75-1558SE out to 3500 or 4k to the ribbon and an Usher 8945P (or something more efficient) for some low distortion lows under 500hz? They can be mounted a bit closer and still sensitive enough over 500HZ. Any recomendation for a good bass driver (.75 cu feet)? It has to be 7". Im looking for a lean/detailed mid bass. I would like it to be tuned on the side of accuracy not peaky one note bass.

That mid should be extremely clean and its lower mass in the 2k to 4k range should rival the speed a ribbon (why I wanted to use it lower) without its distortion level. It looks to be even cleaner than the 18sound with a smaller footprint on the baffle.

Keep in mind the system doesnt need to make anything down low because of all my subs.
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