# MTM crossover questions

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#### SpinMonster

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

Comments....flame suit on.

#### speaker dave

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.

#### sreten

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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#### SpinMonster

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?

#### Helmuth

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.

#### mayhem13

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.

#### 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.

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.

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#### mayhem13

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?

#### SpinMonster

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.

#### SpinMonster

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.

#### theresa

I would agree with Zaph audio, thorough testing is always more accurate than just listening, whether you've taken the speaker apart or not. People have all sorts of preferences that are based on a purely subjective opinion and they are often wrong. If blind abx testing is done all of these preferences into nothing. I have both a science and an art background and have found myself preferring something when I first hear it but then its irregularities become tiresome. This is true for ribbon drivers which typically require a high crossover thereby demanding too much from the woofer and they have high high order THD. I think that ribbons appealed to me in the past because of their exotic nature (and price). Personally I'll go for a modern well designed motor over exotic diaphrams (sp?).

#### mayhem13

Using a 2" or larger dome in a three way with the ribbon is an excellent idea.....IF you like the sound of a dome mid. I've used the Dayton RS52 which measures very well, is quite sensitive and sounds great to my ears. The directivity match up at the higher XO point to the ribbon would be much improved over the 18Sound as well. A parallel pair of 7" Ushers with a .5 inductor on the bottom could be very nice down to 35hz or so. BUT this is quite a daunting task to design from the XO standpoint. It would certainly be a kearning experience to say the least. If you have the 18Sounds, save em for a system as i described earlier.

#### SpinMonster

I would agree with Zaph audio, thorough testing is always more accurate than just listening, whether you've taken the speaker apart or not. People have all sorts of preferences that are based on a purely subjective opinion and they are often wrong. If blind abx testing is done all of these preferences into nothing. I have both a science and an art background and have found myself preferring something when I first hear it but then its irregularities become tiresome. This is true for ribbon drivers which typically require a high crossover thereby demanding too much from the woofer and they have high high order THD. I think that ribbons appealed to me in the past because of their exotic nature (and price). Personally I'll go for a modern well designed motor over exotic diaphrams (sp?).

I agree with Zaph too and dont think that my post was meant to say I didnt. I think a speaker should measure well. My position however is that some things are traded off to get other things. Everyone has their favorite area of performance. Some like distortion plots and even if its not popular, some like false detail.

For me, a speaker that cant radiate the sound to the back the way my apogees or martin logans did, are far more annoying to me than the distortion plot is produces. Real sound radiates in all directions at all frequencies. Soundstage depth is my thing. To each his own. By the way, I always got my money back buying and selling used high end.

Anyway this isnt about creating the perfect speaker. I simply want to experience a properly set up 5" direct radiating ribbon.

I can pay someone to design the crossover. PAYPAL for payment?

#### tinitus

maybe consider crossing the mids and ribbon as a 2.5way

#### SpinMonster

maybe consider crossing the mids and ribbon as a 2.5way

I said I was open to that after a few posts. If the efficiency from having two mids isnt going to increase SPL in the midrange I dont see a point in running two mids. I have lots of subs and dont need bass so a single midbass drive unit is fine.

#### SpinMonster

Using a 2" or larger dome in a three way with the ribbon is an excellent idea.....IF you like the sound of a dome mid. I've used the Dayton RS52 which measures very well, is quite sensitive and sounds great to my ears. The directivity match up at the higher XO point to the ribbon would be much improved over the 18Sound as well. A parallel pair of 7" Ushers with a .5 inductor on the bottom could be very nice down to 35hz or so. BUT this is quite a daunting task to design from the XO standpoint. It would certainly be a kearning experience to say the least. If you have the 18Sounds, save em for a system as i described earlier.

Agree.

I see you said 'IF' you like the sound of a dome. This illustrates a point I made earlier. You can make a speaker with cone mids and dome mids, both can have the same flat FR and yet they sound different. Looking at distortion plots wont get you the experience to know what either sounds like, hence you have to actually listent to a bunch of driver types to get to know what charateristics each has. Dome tweeters arent going to sound like a Fountek ribbon over 10k where the distortion becomes much less apparent.

I can venture a guess that the dome mids weigh so much less that at the higher frequencies they start and stop on a dime. I think this is part why so many like ribbon drivers. When your breath can get one moving it goes to show how little inertia it has and thus is faster in some ways especially at the bottom of its passband. Some may like this, distortion notwithstanding.

#### mayhem13

The RS52 is cheap enough to try, and worth the investment. Something i've been toying with and trying to get a handle on polar radiation, is what a single 5" ribbon....flanked by two dome mids 'on either side'? might work? We've already understood that the ribbon is generally crap in the vertical, so the MTM being the worst possible VD, and single mid not much better....why not to the sides? We pick up the eff. of the ribbon, and solve some other problems as well IMO.

As to your point on the electrostats and rear radiation, what you're hearing as far as imaging is excellent power response, something that can be done with almost any design as long as it's designed properly.........You're on the right track here with the 2" dome crossed high and tight to the ribbon.

#### SpinMonster

The RS52 is cheap enough to try, and worth the investment. Something i've been toying with and trying to get a handle on polar radiation, is what a single 5" ribbon....flanked by two dome mids 'on either side'? might work? We've already understood that the ribbon is generally crap in the vertical, so the MTM being the worst possible VD, and single mid not much better....why not to the sides? We pick up the eff. of the ribbon, and solve some other problems as well IMO.

As to your point on the electrostats and rear radiation, what you're hearing as far as imaging is excellent power response, something that can be done with almost any design as long as it's designed properly.........You're on the right track here with the 2" dome crossed high and tight to the ribbon.

Apogee did the side mount on their hybrid speakers called the centaurs. It placed the woofer at the center of the ribbon to the outside so its acoustic center was at the same height as the center of the various ribbon lengths which were 26 to 48". They used a plastic film backing on their ribbons like Fountek does which reduces distortion. Their 26" ribbon crossed over at 350HZ or so. The distance wasnt as critical so the idea of mounting a dome mid to the side of a ribbon that is 5" long would actually be only about 1.5" from the edge of the side of the ribbon faceplate and then only about 2" or so from the center of the dome mid. The crossover allowed would be about 3200-3400HZ without big issues coming from the center to center distance but does the verticle lobing even matter if the listening position has to be in a +/- 10 degree height window? The more distance you have from the speaker the greater the verticle window is. My home theater is 21x21 feet with a 16 foot screen.

One other advantage to your design is that the alignment of the ribbon is about 3/4"-1" into the baffle and the domes are forward of the baffle reducing the time alignment issue somewhat. I know the Tang Band 75 is really sticking out some.

#### tinitus

I said I was open to that after a few posts. If the efficiency from having two mids isnt going to increase SPL in the midrange I dont see a point in running two mids. I have lots of subs and dont need bass so a single midbass drive unit is fine.

in its own way it is resulting in better sensitivity
its about BSC
with one mid you will have to attenuate the upper mids relatively to lower mids
with 2.5way you increase lower mids, and maintain upper mids level, hence the better sensitivity

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