M-T-M centre speaker question - AVS Forum
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post #1 of 62 Old 08-14-2012, 06:58 AM - Thread Starter
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If M-T-M centre speakers often have mid range lobing issues, then why do people add the second woofer?

Wouldn't a speaker with a single woofer perform as well and without lobing risk?

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post #2 of 62 Old 08-14-2012, 08:40 AM
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Yeah it probably would. However they add the second woofer for a few reasons: some ppl like the more uniformed symmetrical look, and it also adds more bass extension.

There have been attempts to correct this as in a cascade tapered array in which one of the Ms of the MTM is a mid/woofer and the other M is a woofer only so its a 2.5 way speaker. I may be wrong but I think Klipsch and Polk both use that method.

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post #3 of 62 Old 08-14-2012, 08:56 AM
 
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Quote:
Originally Posted by flickhtguru View Post

There have been attempts to correct this as in a cascade tapered array in which one of the Ms of the MTM is a mid/woofer and the other M is a woofer only so its a 2.5 way speaker. I may be wrong but I think Klipsch and Polk both use that method.

In response to your above, Klipsch's RC-64 II has four 6.5" woofers coupled with one compression driver tweeter. The rest of their center channel speakers use two, same size IMG woofers. The G-28 flat panel speaker uses four additional passive IMG woofers. FWIW, the top three tier Klipsch center channel speakers all have similar bass extension of 56-59Hz.
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post #4 of 62 Old 08-14-2012, 09:06 AM
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Originally Posted by rick240 View Post

If M-T-M centre speakers often have mid range lobing issues, then why do people add the second woofer?
Wouldn't a speaker with a single woofer perform as well and without lobing risk?
The reason for a second woofer in an MTM is to be able to use two smaller woofers rather than one larger woofer, allowing a higher crossover point to the tweeter, eliminating the complexity of a 3 way system with the same dispersion. It also gives better vertical pattern control. The key word is 'vertical'; for an MTM to work per the design intent the speaker must be vertically aligned. When horizontally aligned as is done in most center speakers there are no advantages and many disadvantages, including lobed response and narrowed dispersion. So why do manufactures do it anyway? Because consumers aren't aware of these facts, and where there's a buyer willing to spend money on a product there will be at least two sellers ready to take his money.

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post #5 of 62 Old 08-14-2012, 09:19 AM
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Originally Posted by Bill Fitzmaurice View Post

So why do manufactures do it anyway?

Because not everyone can use a vertical speaker as their center channel. I have matching MTM LCRs across the front but I have no way of using a 26" high center channel. Much easier to fit a center thats only 5-8" high.

Manufactures don't create demand, the consumers demanded a product to fit their needs so, manufactures created the supply.
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post #6 of 62 Old 08-14-2012, 09:20 AM
 
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Originally Posted by Bill Fitzmaurice View Post

So why do manufactures do it anyway? Because consumers aren't aware of these facts, and where there's a buyer willing to spend money on a product there will be at least two sellers ready to take his money.

Oh sure, now it's PT Barnum's fault. biggrin.gif

"A fool and his money....."

I was going with the reason for horizontal placement being, that's how most entertainment credenzas are set up. Viewing gets more and more difficult, the higher a television screen is positioned and at base height, setting a bookshelf size speaker in front of a television's center-line creates it's own set of vertical viewing troubles; not to mention the WAFF; wife approval factor frown.

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post #7 of 62 Old 08-14-2012, 09:27 AM - Thread Starter
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Originally Posted by gtpsuper24 View Post

Because not everyone can use a vertical speaker as their center channel. I have matching MTM LCRs across the front but I have no way of using a 26" high center channel. Much easier to fit a center thats only 5-8" high.
Manufactures don't create demand, the consumers demanded a product to fit their needs so, manufactures created the supply.

Accepted, most people can not have a tall centre speaker. Therefore the centre requires some kind of compromise.

The question is, sonically, which is the best compromise (in this discussion let's assume that identical woofers and tweeters are used in all speakers):

1. W-T-W used vertically for L/R, W-T-W used horizontally for C (this is what many, many people do - but it has lobing potential)

2. W-T-W used vertically for L/R, T-W used horizontally for C

3. T-W for all three L/C/R, vertically for L/R and horizontally for C

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post #8 of 62 Old 08-14-2012, 09:35 AM
 
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Originally Posted by rick240 View Post

Accepted, most people can not have a tall centre speaker. Therefore the centre requires some kind of compromise.
The question is, sonically, which is the best compromise (in this discussion let's assume that identical woofers and tweeters are used in all speakers):
1. W-T-W used vertically for L/R, W-T-W used horizontally for C (this is what many, many people do - but it has lobing potential)
2. W-T-W used vertically for L/R, T-W used horizontally for C
3. T-W for all three L/C/R, vertically for L/R and horizontally for C

My understanding, in truth, the best measured numbers come out of a two speaker, vertically standing bookshelf with tweeter on top. In a real world environment, the, horizontal, b-stock, Klipsch RC-64 II is the best darn center channel I've heard.
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post #9 of 62 Old 08-14-2012, 10:08 AM
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Originally Posted by BeeMan458 View Post

Oh sure, now it's PT Barnum's fault. biggrin.gif
"A fool and his money....."
I was going with the reason for horizontal placement being, that's how most entertainment credenzas are set up. Viewing gets more and more difficult, the higher a television screen is positioned and at base height, setting a bookshelf size speaker in front of a television's center-line creates it's own set of vertical viewing troubles; not to mention the WAFF; wife approval factor frown.
rolleyes.gif
From an engineering standpoint there's no justification for horizontal placement, it's purely a visual thing, and as is often the case what looks pretty sounds bad. For instance, the Klipsch you referenced could only work reasonably well if the two outer woofers were low-passed at their 1 wavelength center to center distance, about 500Hz. Nothing in the Klipsch literature suggests that to be the case, making it an inherently flawed design. To put it simply, consumers assume that sound waves work the same way that light waves do, so a horizontal speaker will give wide horizontal dispersion. The exact opposite is true, but show me one manufacturer/seller of horizontal centers that points out this acoustical engineering fact to prospective buyers.
This way to the Great Egress. rolleyes.gif

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post #10 of 62 Old 08-14-2012, 10:35 AM
 
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Originally Posted by Bill Fitzmaurice View Post

From an engineering standpoint there's no justification for horizontal placement, it's purely a visual thing, and as is often the case what looks pretty sounds bad. For instance, the Klipsch you referenced could only work reasonably well if the two outer woofers were low-passed at their 1 wavelength center to center distance, about 500Hz. Nothing in the Klipsch literature suggests that to be the case, making it an inherently flawed design. To put it simply, consumers assume that sound waves work the same way that light waves do, so a horizontal speaker will give wide horizontal dispersion. The exact opposite is true, but show me one manufacturer/seller of horizontal centers that points out this acoustical engineering fact to prospective buyers.
This way to the Great Egress. rolleyes.gif

What's a poor married speaker buyer to do? We both know that in most homes, right or wrong, aesthetics wins out over practicality whether trimming unneeded landscaping or one's buying choice in a car purchase so yes, aesthetics are rationally in play. We also know that for the most part, societal conventions, coupled with traditional credenza styles available to audio-heads, convention wins out over objective reasoning. But I will admit, as to looking and sounding good, female Italian supermodels do both equally well. tongue.gif

FWIW, I LPF the RC-64 II at 60Hz. We're you the forum member who posted the graphs regarding the evils of horizontally placed M-T-M speakers vs vertically place two-way bookshelf speakers? Very eye opening article. As to the "Great Egress," I was hoping to make that the last attraction as I'm still enjoying all these other attractions. tongue.gif
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post #11 of 62 Old 08-14-2012, 11:42 AM
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Ok I looked it was Polk who uses this Cascade Tapered Array for solving lobing and comb filtering issues.

http://www.polkaudio.com/polk-university/technology/cascade-tapered-array-crossover

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post #12 of 62 Old 08-14-2012, 12:23 PM
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Quote:
Originally Posted by flickhtguru View Post

Ok I looked it was Polk who uses this Cascade Tapered Array for solving lobing and comb filtering issues.
http://www.polkaudio.com/polk-university/technology/cascade-tapered-array-crossover
Klipsch used to do it too. The RC-7 used a "tapered array" crossover:
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The RC-7 center channel features Klipsch's version of 2.5-way crossover technology, called tapered-array. With tapered-array, the two Cerametallic™ woofers work together to deliver high impact bass with one driver transitioning out at the mid-range frequencies. This provides more consistent coverage across the listening field, less tonal error and improved dialogue intelligibility.
CROSSOVER FREQUENCY: HF: 1950Hz, MF: 550Hz

http://www.klipsch.com/rc-7-center-speaker

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post #13 of 62 Old 08-14-2012, 12:52 PM
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Audioholics did some articles and measurements on this subject a little while ago:

http://www.audioholics.com/education/loudspeaker-basics/vertical-vs-horizontal-speaker-designs/250-mtm-horizontally-oriented-measurements.html

The conclusion I drew from their charts is that when choosing a center channel, one must take their room and seating positions into consideration. My room is long and narrow, and the front left and right listening positions are about 15 degrees off axis. By looking at the charts they did, it looks like about 20-25 degrees off axis is where things really start to deteriorate. So as long as you don't have listening positions in this area, you should be fine. Alternatively, if you don't care about the "cheap seats", again, you'll be fine. Just be sure to always sit in "your" spot.

Of course, the ideal situation is an acoustically transparant screen with three identical speakers behind it. But that isn't feasible for a lot of people. Also, you'll get arguments from people who say the screen isn't truly acoustically transparant. So instead of having the "vertical center" people yelling at you, you'll have the "grill off" people yelling at you.

Bottom line is do what works best for your situation in your room. No matter what you do, there will be some audiophile or videophile trying to crucify you for not doing it the "right" way (their way).
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post #14 of 62 Old 08-14-2012, 12:52 PM
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I would have a question regarding changing MKT capacitors for MKP once in an old loudspeaker crossover.The box is 20 years old and I red that MKP capacitors better open midrange and highs.So I took out old MKT capacitors from midrange and tweeter part of the crossover and put new ones from ClarityCap and Mundorf.Now the sound is much more open and clear but the box is more loud then before and some part of midrange is emphasised so it is somewhat unpleasant for the ears...The question is - Can I change only capacitors as I did or I should corect other parts of crossover?
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post #15 of 62 Old 08-14-2012, 07:05 PM
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Can I change only capacitors as I did or I should corect other parts of crossover?
Tweaking a crossover involves a lot more than swapping out caps. The best crossovers take literally weeks, if not months, to get right, and it requires good measurement gear.

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post #16 of 62 Old 08-15-2012, 07:31 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

Tweaking a crossover involves a lot more than swapping out caps. The best crossovers take literally weeks, if not months, to get right, and it requires good measurement gear.

What You mean by " they take weeks" - you mean they take time to become smooth (capacitors)...now it sounds a little bit hard
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Originally Posted by FineArts View Post

What You mean by " they take weeks" - you mean they take time to become smooth (capacitors)...now it sounds a little bit hard

My take, it takes time to bake a cake and it takes time to make changes, test and then make more changes until you get the crossover just right.

There's ready made cakes (crossovers in a speaker) and then there's do-it-yourself cakes. Not being baker nor EE, I buy both, ready made. cool.gif

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post #18 of 62 Old 08-15-2012, 08:25 AM
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Originally Posted by rick240 View Post

If M-T-M centre speakers often have mid range lobing issues, then why do people add the second woofer?
Wouldn't a speaker with a single woofer perform as well and without lobing risk?

To get more power from the tiny woofers.  


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post #19 of 62 Old 08-15-2012, 08:34 AM
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The wavelengths covered by the duplicate speakers relative to their physical displacements are the important factors.  With the long wavelengths from subs, this problem is unusual in most domestic rooms.
The best reason for multiple, as opposed to larger subs, is that the bass distribution in the room is less subject to the variations imposed by the room's dimensions.

I may have missed something here, but are the quoted posts relating to multiple subs or why a typical center channel is an MTM rather than a horizontal design with a single "woofer" (really a mid in most cases)?
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post #20 of 62 Old 08-15-2012, 08:42 AM
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Originally Posted by BGLeduc View Post


I may have missed something here, but are the quoted posts relating to multiple subs or why a typical center channel is an MTM rather than a horizontal design with a single "woofer" (really a mid in most cases)?

Yup.  Will correct post.


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post #21 of 62 Old 08-15-2012, 08:50 AM
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What if the T in the middle is able to rotate?? then wouldn't it be the same if u were able to place it vertical like these that i might be buying?? http://www.adam-audio.com/en/installation/products/gtc77/series..
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post #22 of 62 Old 08-15-2012, 09:07 AM - Thread Starter
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Quote:
Originally Posted by BGLeduc View Post

I may have missed something here, but are the quoted posts relating to multiple subs or why a typical center channel is an MTM rather than a horizontal design with a single "woofer" (really a mid in most cases)?

Thanks smile.gif

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Originally Posted by Kal Rubinson View Post

To get more power from the tiny woofers.  

OK - so multiple drivers is to get adequate SPL.

But if L/R mains are T-M vertically aligned, then a centre with the size M should be a good match (instead of two smaller Ms)...

...what are the implications of horizontal alignment of the T-M vs the vertical of the mains?

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post #23 of 62 Old 08-15-2012, 10:54 AM
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What You mean by " they take weeks" - you mean they take time to become smooth (capacitors)...now it sounds a little bit hard
It takes weeks to tweak , measure, re-tweak, ad infinitum until right. But as for caps breaking in, they don't, and most of the differences 'heard' are placebo effect. After all, if they're more expensive they must sound better, right? DBT has found no more differences between caps than between cables, ie., slight, and not cost related.
Quote:
...what are the implications of horizontal alignment of the T-M vs the vertical of the mains?
Lobed response on the horizontal plane if the drivers are too far apart.

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post #24 of 62 Old 08-15-2012, 11:05 AM
 
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But as for caps breaking in, they don't, and most of the differences 'heard' are placebo effect.

You do know that with your above, you're dangerously close to messing with the whole space-time continuum thingy and that can't be good.

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post #25 of 62 Old 08-15-2012, 11:05 AM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post

Lobed response on the horizontal plane if the drivers are too far apart.

Makes sense, but just in the crossover area - whereas an M-T-M has full range of the M plus crossover region.

How does one determine "too far apart"?

Bottom line I guess though is - vertical is always better.

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post #26 of 62 Old 08-15-2012, 01:55 PM
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Originally Posted by rick240 View Post

How does one determine "too far apart"?
1 wavelength on-center. If the crossover is at 2.5kHz that's 5 1/2 inches, so you can't have no lobing with a 61/2 inch woofer and a tweeter with a 3 inch frame..
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Bottom line I guess though is - vertical is always better.
True, because then the lobing occurs on the vertical plane, so no matter where you are on the horizontal plane it's not a factor. The same applies to the MTM. Originally Joe D'Appolito, who invented it, specified a 1 wavelength center to center of the midbasses at the crossover to the tweeter, but even he no longer recommends that.

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post #27 of 62 Old 08-15-2012, 03:01 PM
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Originally Posted by rick240 View Post
OK - so multiple drivers is to get adequate SPL.
But if L/R mains are T-M vertically aligned, then a centre with the size M should be a good match (instead of two smaller Ms)...
...what are the implications of horizontal alignment of the T-M vs the vertical of the mains?

The same as for the MTM since the M and T will be producing the same signals in the crossover region.  That is why you rarely ever see a modern full-range system with drivers not vertically arrayed.


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post #28 of 62 Old 08-15-2012, 03:04 PM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

1 wavelength on-center. If the crossover is at 2.5kHz that's 5 1/2 inches, so you can't have no lobing with a 61/2 inch woofer and a tweeter with a 3 inch frame..
True, because then the lobing occurs on the vertical plane, so no matter where you are on the horizontal plane it's not a factor. The same applies to the MTM. Originally Joe D'Appolito, who invented it, specified a 1 wavelength center to center of the midbasses at the crossover to the tweeter, but even he no longer recommends that.

I had always thought that the "safe" separation is no more than 1/2 crossover frequency wavelength since both drivers are usually no more than 6-12dB down an octave on either side.  


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post #29 of 62 Old 08-15-2012, 04:31 PM
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Originally Posted by Kal Rubinson View Post

I had always thought that the "safe" separation is no more than 1/2 crossover frequency wavelength since both drivers are usually no more than 6-12dB down an octave on either side.  
1/2 wavelength is unachievable with most driver/crossover scenarios. For instance, In an MTM loaded with 6.5s and a 2.5kHz crossover that would be 2.7 inches. Even 1 wavelength, 5.4 inches, can't be done, even if there's no tweeter between the midbasses.

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post #30 of 62 Old 08-15-2012, 07:57 PM - Thread Starter
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Thanks for all of the wisdom gents smile.gif

I think in my next system I'm going to try to source speakers that will do it for me with T-M vertical for my 5.0.

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