6.5" mtm?

[DrewsBrews]

I'm sure this has been covered quite a bit, but mtms tend to be difficult to search for.

Are there rules of thumb for MTM design?

For instance: I've got beastmode tweets and Visaton wg 148 r waveguides. If I shave down the sides of the waveguides a bit I can attain a center to center spacing for the midbass drivers of about 11.5". Which comes to a wavelength just under 1200hz. Around the wavelength I would probably be crossing over at.

Thinking this probably wouldn't be good? Better off looking into a 2.5way?

[billfitzmaurice]

Very close CTC spacing is more a matter of listening distance than anything else. Beyond ten feet or so the separate wave fronts integrate into a single wave front. It's only with close listening distances that CTC spacing is an issue. That's why recording studio nearfield monitors, intended for listening distances of three to six feet, are never MTM.

[norman bates]

If I remember, dunlavy sm1, a mtm with 6.5", he recommended 10' listening distance.

6db time/phase aligned.

[DrewsBrews]

Ok so if I understand correctly there will be lobing due to comb filtering. The center lobe, perpendicular to the baffle (vertically) around crossover frequency narrows making it more difficult to be in the sweet spot. The lobe broadens with distance, hence the listening distance concerns?.

I looked up the dunlavy sm1. That crosses over acoustically at about 3.5khz wow.

[billfitzmaurice]

You initially have two lobes, three counting the tweeter output. That looks really nasty on a plot that shows nearfield response, but those lobes integrate with distance from the baffle. Joe D'Appoloto's 'Thor' MTM uses seven inch midbasses with a 2.5kHz crossover. The midbass CTC is 12 inches. The drivers used allowed tighter spacing, but he didn't do so. I wouldn't argue with the inventor of the MTM as to how wide to make the CTC.

[djg]

A popular speaker design, crosses pretty low.

http://zaphaudio.com/BAMTM.html

[a4eaudio]

One thing I searched for and found conflicting responses...is it the center-to-center of the two midranges or CtC of the midranges to the tweeter that matters the most here?

[djg]

https://www.youtube.com/watch?v=GZrdsxrcpBw

[johnnyrichards]

The lobing was the point of the MTM as developed by Joe D.

[billfitzmaurice]

The lobing was the point of the MTM as developed by Joe D.

He did it as an alternative to the TMM, as placing the tweeter between the mids resulted in a single coherent wavefront at shorter distances. Lobing is involved only insofar as it was more serious with a TMM, so the distance for wavefront integration was longer. The reasons for both the MTM and TMM is to allow for smaller midbasses that have wider dispersion, allowing a higher crossover, without losing Vd compared to a single larger woofer.

is it the center-to-center of the two midranges or CtC of the midranges to the tweeter that matters the most here

The midbasses, but in practice it doesn't matter as much as once thought. By the time Joe came up with the 'Thor' design he no longer adhered to his own CTC rule.

[DrewsBrews]

Found the simulator that was being used in the "Erin's Audio Corner" video.

https://falstad.com/ripple/

Seems the double and triple slit examples can somewhat give a depiction of half space radiation pattern if both sources are perfectly time and phase aligned (big caveat). There is no 3 source example unfortunately, but you can try to use the 4 source and place 2 sources on top of each other. The two source example can easily show the tilting effect that occurs when the sources are moved one farther ahead of the other.

I tried the 3 slit example and watched the resulting pattern as I slowly bumped up the frequency. Looking closer to the sources/slits you can see the chaos that occurs until the distance where the final pattern is formed. Some frequencies even create a central null before the final pattern. I also noticed there were frequencies where the total radiation power was greatly diminished. This is probably why high order slopes are used in MTMs so the frequency range all 3 are playing at the same time is minimised as much as possible.

Very rudimentary but pretty neat to play around with. Really shows the complexities of what is being worked with here. Tells me that, without being able to model exact distances and frequencies, any real world build would be a shot in the dark if not able to do a basic setup to test before hand.

[Steve Lee]

Awesome link!
Thanks for posting it.

[djg]

Mr. D'Appolito designed a home theater system for Audax decades ago. MTM L/R, TM surrounds, and a WTMW center. He recognized the issues of a sideways MTM. I built that system, my first real HT. Moved on mostly because of my primitive cabinets. I've had 3 HTs with MTM centers since, in my listening situation they all sounded fine.

AUDAX HOME THEATER-Master Doc.pdf

[JRT]

Ok so if I understand correctly there will be lobing due to comb filtering. The center lobe, perpendicular to the baffle (vertically) around crossover frequency narrows making it more difficult to be in the sweet spot. The lobe broadens with distance, hence the listening distance concerns?.

The lobing and comb filtering don't really cause each other, rather are both effects caused by interference, varied phase of coherent (non-diffuse) sounds in the intereference sums. Each point in the 3D field has its interference sum, variations of constructive and destructive interference. Incoherent sources (either noise or diffuse sound with well distributed phase) do not exhibit phase related interfere in the sum, rather just increase magnitude of SPL.

It isn't just direct radiation from the drivers in the interference sums, rather there are also other sources of interference such as coherent reflections from room boundaries or other acoustically reflective objects, and from eigentones associated with room modes around and below the room's Schroeder frequency, and from diffraction at the edge of the baffle, and from diffraction of tweeter radiation across the cavity of a cone midrange, and diffraction at the edges of a tweeter flange if it is not flush with the baffle surface, and from resonances/reflections in a tweeter pole piece, and from port resonances in a bass reflex alignment, etc.