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Benefits of vertical MTM or MMTMM vs TM?

mikedn

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Stupid question: Is there any real benefit of adding woofers, beyond higher sensitivity / SPL?

E.g. 6.5” woofer, like the JBL Studio 6 architectural (in-wall) series, for use as identical LCR all vertically behind an AT screen. But question applies to monitors vs bookshelf speakers too.

One could do:
6.5” TM config i.e. single woofer like the 6IW
6.5” MTM config I.e. dual woofer like 66LCR
5.25” MMTMM quad woofer like the Theater model

You read about better soundstage for MTM compared to a single woofer, but some say worse imaging. Not much objective data that I can find, or proper theory on sonic differences along these configurations (when vertical - not talking well known horizontal MTM center issues)
 

staticV3

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More woofers can reduce distortion, increase max SPL, increase LFE, or any combination of these.

The tradeoff is that a 2-way or 2.5-way design with >1 woofer will inevitably result in beaming, which you can learn about here: https://youtu.be/GZrdsxrcpBw

To combine the wide and even directivity of well-designed TM speakers with the benefits of multiple woofers, the speaker needs to be converted to 3-way with one tweeter, one midrange, and x woofers.

This inevitably increases cost and complexity, which is why few brands do it.
 

terryforsythe

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Is there any real benefit of adding woofers, beyond higher sensitivity / SPL?
Lower harmonic distortion and less compression. E.g., with two midranges vs. one, for the same SPL, each midrange is receiving much less power (about one-quarter if I remember correctly), and is moving with half the excursion.

The issue with MTM and MMTMM configurations is that interference node patterns form vertically off-axis. This can negatively effect in-room response due to uneven vertical reflections. There also may be an issue when listening to the speakers above or below the tweeter axis.
 

voodooless

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The issue with MTM and MMTMM configurations is that interference node patterns form vertically off-axis. This can negatively effect in-room response due to uneven vertical reflections. There also may be an issue when listening to the speakers above or below the tweeter axis.
Yes! It makes horizontal directivity wider, but vertical directivity get narrower. Interference happens because of the center to center distance of the midranges. You’ll need a relatively low crossover point to mitigate this. Most designs do not adhere to this though.
 
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mikedn

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Wow. Thanks! And all these issues happen when using the MTM or MMTMM config vertically? Horizontally it’s even worse right?

If I have a small room and listen below reference levels, why then would I go with any other config than TM for my LCRs?

Does beaming not occur with TM?

So if I want to step up from the 6” TM it’s better to go to a 8” woofer instead of dual 6”? I also read about a suckout issue with bigger woofers in 2-way designs… hehe. Edit: maybe that’s the beaming issue? I thought Erin spoke about horizontally placed centers not vertical ones
 

kemmler3D

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Wow. Thanks! And all these issues happen when using the MTM or MMTMM config vertically? Horizontally it’s even worse right?

If I have a small room and listen below reference levels, why then would I go with any other config than TM for my LCRs?

Does beaming not occur with TM?

So if I want to step up from the 6” TM it’s better to go to a 8” woofer instead of dual 6”? I also read about a suckout issue with bigger woofers in 2-way designs… hehe. Edit: maybe that’s the beaming issue? I thought Erin spoke about horizontally placed centers not vertical ones
The issue with all 2-or-more-way speakers is that you get harmful comb filtering / interference between the tweeter and woofer when they are not placed close enough together and when the crossover is too high. Basically if they are both playing XYZ Hz at the same time, and the wavelength of that frequency is comparable to the distance between the drivers, you get harmful cancellations that mess up directivity and overall FR.

TM or TMW designs try to avoid these cancellations as much as possible. MTM or MMTMM designs try to gain a partial advantage from these cancellations by using them to control / limit vertical directivity in specific ways, by making the cancellations symmetrical. TMM would tend to be worse than the equivalent MTM design because the vertical directivity will be more asymmetrical.

This is the same reason you get a "suckout" from a large woofer + tweeter. The bigger the woofer, the further the tweeter is from the center of the woofer, and the lower you have to push the crossover to avoid the unwanted cancellations. However, tweeters can only go so low before distortion gets out of hand. This is why a good 2- or 3-way using a woofer larger than (IMO) 5.25" needs a large waveguide. The waveguide lets the tweeter play lower and in some sense makes it larger, and so easier to pair with the woofer.
 

Battlebeast

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Wow. Thanks! And all these issues happen when using the MTM or MMTMM config vertically? Horizontally it’s even worse right?

If I have a small room and listen below reference levels, why then would I go with any other config than TM for my LCRs?

Does beaming not occur with TM?

So if I want to step up from the 6” TM it’s better to go to a 8” woofer instead of dual 6”? I also read about a suckout issue with bigger woofers in 2-way designs… hehe. Edit: maybe that’s the beaming issue? I thought Erin spoke about horizontally placed centers not vertical ones
Kef or some other concentric type design.
 
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mikedn

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The issue with all 2-or-more-way speakers is that you get harmful comb filtering / interference between the tweeter and woofer when they are not placed close enough together and when the crossover is too high. Basically if they are both playing XYZ Hz at the same time, and the wavelength of that frequency is comparable to the distance between the drivers, you get harmful cancellations that mess up directivity and overall FR.

TM or TMW designs try to avoid these cancellations as much as possible. MTM or MMTMM designs try to gain a partial advantage from these cancellations by using them to control / limit vertical directivity in specific ways, by making the cancellations symmetrical. TMM would tend to be worse than the equivalent MTM design because the vertical directivity will be more asymmetrical.

This is the same reason you get a "suckout" from a large woofer + tweeter. The bigger the woofer, the further the tweeter is from the center of the woofer, and the lower you have to push the crossover to avoid the unwanted cancellations. However, tweeters can only go so low before distortion gets out of hand. This is why a good 2- or 3-way using a woofer larger than (IMO) 5.25" needs a large waveguide. The waveguide lets the tweeter play lower and in some sense makes it larger, and so easier to pair with the woofer.
Super interesting, this is what I was also thinking about. When you say large waveguide, do these JBLs qualify? https://www.jbl.com/studio6-architectural-series/#

I.e. can I expect the 8” woofers not to have worse suckout issues compared to the 6”? Any reason to choose 6” over 8” with these JBLs? Just trying to understand why one would pay more for the larger woofers or dual or quad woofers in a small room, unless you play reference level or above maybe?
 
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mikedn

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Kef or some other concentric type design.
I was advised that compression drivers like the JBLs have a better sound for home theater applications, at least there seem to be some strong opinions in that direction
 
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mikedn

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kemmler3D

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When you say large waveguide, do these JBLs qualify?
Yep, when I said "large" I mostly meant any waveguide that is comparable in width or overall size to the woofer.
I.e. can I expect the 8” woofers not to have worse suckout issues compared to the 6”?
Hard to say. Really depends on how well the horn / waveguide is designed, where they set the crossover, etc. When it comes to recent JBLs with horns or waveguides the crossover region tends to be decent-to-good.
Just trying to understand why one would pay more for the larger woofers or dual or quad woofers in a small room, unless you play reference level or above maybe?
In general more woofers = more SPL and less distortion, both good things. Check out Amir's recent review of the 2.5-way Polk floorstander. It has really amazingly low distortion in the bass region because several drivers are working in tandem, and excursion can stay low while still cranking out bass. So maybe you want to play loud, maybe you just really want clean bass at normal SPLs.
I was advised that compression drivers like the JBLs have a better sound for home theater applications, at least there seem to be some strong opinions in that direction
You can find strong opinions in every damn direction when it comes to speakers. If you want one more opinion, compression drivers / horns are good for dynamics and wider/smoother directivity, concentrics are good for smooth in-room response and directivity. You can find a lot of opinions on whether you should prefer one style over the other. If you can't decide, the best bet is to go listen to some. :)
 
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Interesting read, learned a lot, thanks for sharing.

The thing is that the JBLs are 2-way MTM / MMTMM so the 2.5-way explanations might not carry over?

That is true. I hadn't realized the Theater model was a straight two-way. My bad.

Jim
 
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