Please help design an MTM center

[Don Mascali]

I would like suggestions and opinions on the box size, driver spacing and Xover design of a new horizontal MTM center channel speaker for HT/Music use. The components on hand are two 2123J drivers that I got new on EBAY and for the HF I have a 2416H that will go on a 2342 horn. I also have a 2425J and 2344 horn I could use too.

My current fronts are actively crossed over and are as follows per side;
2404-5khz and up
2426H/2344-1200hz and up
2-E120s-150 to 1200hz
B380 clone 150hz and down
4645C 150hz and down

I really am not concerned with deep bass or extended VHF performance. We are after voice and fill for the center channel, midrange clarity is most important and voicing compatable with the existing fronts.

My first thoughts are to Xover at 1k to 2k and possibly a sealed box.

I am not at the skill level of many of our members and will be gratefull for all comments.

Thank you very much,
Don M

[Zilch]

"Classic" design calls for woofs centers to be each spaced one wavelength of crossover frequency out from center of tweeter.

At 1 kHz, that'd be 13.56". I'd turn the biradial horn sideways so the "cheeks" don't scoop sound from the woofs.

Drawing posted several times elsewhere on the forum shows patterns achieved with various crossover alignments, but that's for dome tweeters. Biradial's gonna be different, probably....

[Earl K]

Hmmmmm,


I would like suggestions and opinions on the box size, driver spacing and Xover design of a new horizontal MTM center channel speaker for HT/Music use.

It would be a lot better if you were working with a woofer having a larger Xmax value - like twice as much ( such as the 2012h, 2251j ) but ,,,,,



The components on hand are two 2123J drivers that I got new on EBAY and for the HF I have a 2416H that will go on a 2342 horn. I also have a 2425J and 2344 horn I could use too.

- Yes, I would "tune" this big box to recapture some of the midbass rolloff ( that starts occuring below 250-300 hz ) and create some useful response in the 125hz area.

- The 2123h ( IME ) is rugged enough to a point ) to survive a system tuning ( Qtc ) in the .425 to .411 range. Personally I'm always nervous going below .5 Q(tc) since Xmax is invariably compromised . .

- .411 Q(tc) in my software equates to @ .65 cu' for each 2123 . That's fairly oversized . For reference ,my preferred .5 Qtc gives an enclosure size of @ .35 cu' .

- a .65 cu' enclosure tuned to 110hz wants a "tuning" port/hole of 9 sq inches, when cut directly into a 5/8" thick baffle board ( or 9.6 sq inches in a 3/4" thick baffle ) . You'll need to limit input power to each 2123j to 175 watts max ( or the woofer cone will over-excursion ) . This "limit" should not be a problem in a home environment. If you tune to 130hz, you can just about use the full rated input power.

- I'd use the smaller of the horns you have ( the 2342 ) because it allows tighter packing .

- That critical distance between the woofer centers ( for a straight ahead MTM with zero lobing ) is unknown to me at this time . Give me some time to do some handson experimenting and I'll get back to you. ( I own 2123s in @ .65cu' enclosures )

- In the meantime how high off the floor will these boxes find themselves ?

- How soon do you want to get moving on this project ?
( The reason I ask is because within 2 months I'm going to changeout the stock surround on a 2123 for a foam type . The idea is to see if this will offerup a little more natural midbass from this driver - somewhat like a shorter coil 2122 )

:cheers:

PS; I'll post some SIMs ( different tunings ) a little later .

[Earl K]

Well quickly ,

The smoothest power response was obtained with 2 woofers side by each - with the baffle board split in two down the center and angled outwards at an offset angle of 20 to 30° . This is a fairly typical look for a mtm center fill

Even side by each woofers display some midrange nulling when used together on a straight unbroken baffle - though very good response is obtained up to maybe 40 - 45° off-axis / then the nulls become obvious. That gives a good 90° "projection window" .

Tomorrow; I'll jury-rig a 2344 in between my test boxes and see what gives .


:cheers:

[Mr. Widget]

I really am not concerned with deep bass or extended VHF performance. We are after voice and fill for the center channel, midrange clarity is most important and voicing compatable with the existing fronts.


So why go to all the trouble? Get an LE8T or H and plop it in a 1 cu ft sealed box and you will have a very nice center channel. The LE8T is even shielded.

Widget

[Mr. Widget]

Don't be so sensitive.:D

It will be on it's own amp. Unless he wants to play stupidly loud the 80Hz filter will allow plenty of midrange out of an LE8T.

Keep it simple or pedal to the metal. Why waste your time with all the half ass sh!t in the middle.

Widget

[boputnam]

Well , Okay , I'm laughing ! :D

but

I think its you who's being sensitive here . I think it's just that you now hate MTM and want a return to the simple tranquil life - of no hulking boxes filled with 1500AL(s) . You're just projecting those wishful thoughts onto poor Don and his project. :) ;) Weird...

[GordonW]

Well, my calculations, using a 1.2 cubic foot box (.6 cubic foot for each 2123) with fiberglass wall lining, using an 80 Hz port tuning, gives an f3 of 85 Hz and a max acoustic power rating of around 115 dB (@ the biggest excursion frequency, which will be about 100 Hz), before the drivers run out of X-max. Mind you, they will NOT bottom out or anything... not until MUCH higher excursions... so it should be able to reach PRODIGIOUS output levels without damage.

I'd think the 2123s should work fine in such a setup...

Regards,
Gordon.

[Mr. Widget]

Gordon, The LE8T will only require 1 cu ft. :D:D

...and Earl, while I do think an LE8T would make a great dialog speaker I was being tongue in cheek.


As for MTM wait for the comments after our listening session this Saturday! :applaud:

Widget

[Don Mascali]

Thank you to everyone for the quick replies.

This box will sit on a shelf 40" from the floor just below my screen. I use a projector so shielding isn't an issue. I have attached a pic of the setup.

GordonW:
The 1.2 cu ft box and 80hz tuning sounds perfect. I run the bass drivers to 150hz so 85hz f3 is as low as we need to go. The center can be set to "small" and crossed at 100hz.

EarlK:
I would like to stick with a flat baffle. My stuff images very well and for the dialoge I'm not worried about a full pattern. I sit 14' from the screen and the room is 20' wide so off axis response shouldn't really be an issue. This will be driven by my Yamaha A/V receiver w/100w and excursion limiting isn't a problem there. I'm sure my ears would give up long before. I don't know if I have the space to mount the horn above the drivers. I'll stick it in between, even if that is a compromise.

Zilch:
Agreed on the sideways horn. It may look strange, but it's needed.

Thanks again,
Don M

[Earl K]

Well, my calculations, using a 1.2 cubic foot box (.6 cubic foot for each 2123) with fiberglass wall lining, using an 80 Hz port tuning, gives an f3 of 85 Hz and a max acoustic power rating of around 115 dB (@ the biggest excursion frequency, which will be about 100 Hz),

Hmmmmm,

Gordon, my software ( MacSpeakerz ) doesn't agree with yours at all .

For Instance :

- My software shows a F3 plateau ( droop ) that begins around @ 150hz while endng at about 100hz before diving to a F6 point at about 77hz.

- I get maximum acoustic power of about 118db ( extrapolating backwards to a "J" instead of the "H" model seen here ).

- Biggest excursion occurs at around 125hz .

Can you post a jpg of a screen capture - that would show what you are looking at ?

I'll do the same - sometime today .

:cheers:

[Earl K]

I would like to stick with a flat baffle. My stuff images very well and for the dialoge I'm not worried about a full pattern.

Well ,, you've been warned .

- I just did a mockup with flat baffle vs outward angled baffles / using a 2425/2344 combo placed in between the boxes .

- Angled baffle wins on all counts . ( It's really best to not combine the uppers mids of the 2123j(s) - but do combine the lower midrange - this gives the flattest / smoothest passband ( 100hz - 1000hz ) over a broad 180° horizontal listening range )

- The flat/planar baffle has significant lobing issues that color the sound as one rotates ( walks ) in front. This is quite hearable and measurable using pink noise as a source .

- I strongly recommend you repeat my experimental mockups by first building (2) test boxes, each .6 cu' (net volume) . Then repeat this R&D before finalizing your design.

:cheers:

[Don Mascali]

How much of an angle would you recommend? would 15 deg on each side be enough?

I see by your graphs that a higher tuning makes sense.

Thanks for all of your input!

[Mr. Widget]

After seeing your setup, I would strongly suggest you consider building a version of your left and right speakers without the upper woofer. You have the mid horn and driver available. You would have to buy one woofer and 2404, but it would be worth it. Even though you don't think you need the "full" range here, there are some films that put much more than dialog on the center channel and if you watch a concert film you ideally want all five speakers to be identical. More importantly the sound quality as a soundtrack is paned across the screen works best if all of your front speakers are identical. You certainly have the room for a single woofer version of your L/R speakers.


Widget

[4313B]

I'd skip the Junk But Loud and toss a Tannoy dual-concentric in there.

:duck:

[boputnam]

Well ,, you've been warned . :yes:

Don - what is going on over in the corner, here...?

[Mr. Widget]

I'd skip the Junk But Loud and toss a Tannoy dual-concentric in there.

:duck:

Not a bad call, but I'd go with three. :D:D:D

Widget

[Earl K]

How much of an angle would you recommend?

25° ( per panel ) is what produced the smoothest midrange polar pattern ( least amount of midrange lobing ) .

NOTE :

- I'm using electronic 24db/octave LR crossover slopes on this setup - set to around 1250 hz .

- Even with the use of these steep electronic slopes, the "acoustic slopes" when measured below the crossover regions, were both in the neighbourhood of :
Low Pass ; 13.5 db per octave
High Pass ; 12.5 db per octave

- What's that mean ? Both the dual 10 inchers and the horn have a ton of midrange "bloom" that needs attending to. So, designing your crossover to a usable 12 db per octave slope (the acoustic slope ) for these particular components will take a bunch of design tricks plus some trial & error on your part . The design for the high pass should start with the 4430 circuit while the lowpass will need to be a custom pursuit. At this time, I'm not the guy to do that for you . Sorry .
- Oh , you likely need to use the "H" type compression driver ( I'd use a 2425h ) to matchup with twin 2123j midbassers .

- Remember, build test boxes !

:cheers:

[Don Mascali]

So Giskard, you have a truck load of those things you're trying to dump off or something ? :D

Here is the description from the "Describe your system" thread:

http://audioheritage.csdco.com/vbulletin/showthread.php?t=147&page=10&pp=15


Amp rack gear:
Beheringer:
DEQ-2496
CX-3400
2xPEQ-220
EX-1200
Yamaha:
P-2500
P-5000
2xP-7000

[Earl K]

Hi Don

(i) Baffle : (again)
- If you just have to go with a straight baffle board then put the speakers right next to each other . Put the horn of your choice, centered over these 2 midbass drivers . This arrangement will give solid 80° to 90° performance .

(ii) Horns :

- So far, the best measuring horn over the twin 10(s) , is the 2344 closely followed by a 2307/2308 combo .

- I also tried my standard small CD horn. This is the RCF 9040 ( plastic horn ) but didn't like it as much as the 2307/08 combo .

- All three horns can use HF help in the form of CD horn boost .

(iii) Visual Aid :
- Here's JBLs' answer to the same dilemma. This centerfill from the "Synthesis" Series uses twin 10" (j types ). Personally I'm not keen on the look of that horn - but, that's beside the point since I'm not buying and ( apparently ) JBLs' not selling. One midbass 10" is crossed over lower - effectively boosting the LF performance ( as in the cabaret 4602 ? ) while breaking up midrange lobing. This is something you might want to explore a bit in a passive crossover design.

:cheers:

[Earl K]

Hi Don

Your third option might be to put a slender horn ( 5 to 7" wide ) between the angled 10" woofs. JBL will be releasing a twin woof centrefill from their new series - this spring.

The "look" is of a sideways biradial jammed between the 2 woofers ( they somewhat look like the older 2345 radials oriented sideways ) . This type of horn ( like the 2344 ) has the diffraction/spray slot oriented in the vertical axis.

Here's a horn of a similar type made just for this purpose ( this has been around for about a decade ).

Unfortunately , this horn from DDS ( Design Direct Sound ) (http://www.ddshorns.com/content.php?page=home) is not available in the 1" entry format. Only 1.4" and larger . A clever person would take the 2344 metal throat entry/adapter and add his own exponential curves ( bells ) - to get a hip new horn.
:cheers:

- Remember,,, always build test boxes !

[boputnam]

Amp rack gear:
Beheringer:
DEQ-2496
CX-3400
2xPEQ-220
EX-1200

Yamaha:
P-2500
P-5000
2xP-7000

Yamaha RX-V1400 receiver
Zenith DB-318 DVD player
Sony DVP-NS700P CD/DVD player
Sony SLV-777HS VCR
Sanyo Z2 LCD projector

That is what I thought, and why I asked.

Behringer:
DEQ-2496 - tried it, found it obviously digital. A/D conversion not acceptible quality.
CX-3400 - don't like the sound of the filters.
2xPEQ-220 - don't know it
EX-1200 - I have no idea what you need this for

Don, I'm very soft on the Behringer stuff. To know them, though, I have to admit I've at least tried them... :o: My advice is, with all the worry you're going to on the cabinet(s), you need some serious attention to the rack. You need to move-up the price-point into some better filters and benefit from all the good advice EarlK and the rest are offering. You need some good Rack Time!! :dancin:

[GordonW]

I'd skip the Junk But Loud and toss a Tannoy dual-concentric in there.

:duck:

Yeah, but I doubt the horn inside the dual-concentric drivers would keep up with the SPL this system can do. Great for "normal" home apps, but this would probably nuke it... :cooked:

Now, if you took an Eminence Beta 10CX 10" coax driver and dropped a JBL 2426 on the back of it for the high-frequenc... now, THAT might do something serious...

But, I agree. if you're already got 2123s and the big baby cheeks, no reason not to add the small baby cheeks 2404. That'd give pretty darn close voicing to the sides...

OTOH, you could just use a 4425/4435-type "HF-lift" circuit for your highpass crossover on the 2426 on the big horn, and let it roll... works well enough for those monitors!

Regards,
Gordon.