Starting Plans for New MTM Pair - Input?

[bmwman91]

I have been running my first pair of DIY 2-way loudspeakers for almost a year now, and the bug has bitten me again. While I was tempted for a while to build a pair of Zaph ZRTs, I really want to make something with parts of my choosing. This is not an insinuation on my part of possessing superior skills or having plans to outdo Zaph's great work (because I most likely cannot). In fact, many thinks go to him for his cataloging of distortion figures & testing work.

So, this time around I have a little more experience under my belt. I will be moving in the next few months, and I will have an entire living room at my disposal for listening purposes, rather than just my apartment bedroom as I have now. I presently have a tri-amp setup utilizing three Behringer A500 amplifiers & a DCX2496 programmable crossover. Regardless of opinions on the gear, I will be using it and will remain with the active crossover setup. My previous speaker projects include:
First 2-Way
First Sub - Overkill

Bass will be provided by the subwoofer that I finished a few months ago. I have been playing with plugging the port to improve transient response, but that is a different topic. The transient performance really only comes apart at frequencies that are not usually seen in music anyway. I will be setting the crossover for the sub/mid's at 250Hz, tentatively.

The drivers I will be using are SB Acoustics model SB15NRXC30-8. The two will be run in parallel in each loudspeaker, which will bring the mid sensitivity closer to that of the tweeter (88dB each nom, ~91dB nom effective combined). The tweeter will be a SB Acoustics model SB29RDC (93dB nom). Attenuation is dealt with on the DCX2496. I will likely be looking at a crossover frequency of 2500Hz for these. A big reason that I am moving to these is for power handling & distortion. The drivers & tweeters I am using now (Aurasound NS4-255 & NT1-208) are not rated to handle much power (25W & 16W nom, respectively), and they have rather low sensitivity (84dB & 91dB nom, respectively). I prefer not to melt them down since they are my first pair of speakers & I am quite fond of them.

Nailing down the form factor is really the big decision here since the crossover tuning does not really require any construction on my part at this point. Tuning is done with HOLMImpulse & a Behringer measurement mic feeding into an M-Audio Fast Track Pro. I have been reading what I can on baffle design & MTM setups. Here is the front baffle as I plan to have it right now. Notice that it is solid Cherry. This is wishful thinking...there seems to be some amount of debate about using solid hard wood for enclosures. Based upon my furniture building experiences, I will stick with MDF since its dimensional stability is something I greatly prefer. I will veneer the 5 other surfaces, and probably do a satin black enamel job on the front. The baffle will have 3/4" roundovers on the edges, and the edge of the drivers will be 1-3mm from the start of the roundovers (still deciding how brave I am feeling). Internal volume is being kept to a minimum (~13L including drivers & planned bracing), and the drivers will share the volume (no separator planned, can add one if it is recommended). I have a plethora of Acousti-Stuf laying around, as well as some butyl rubber + aluminum deadening for the enclosure walls. Open-celled egg crate foam will go on the inside as well. All walls are 3/4" MDF. If I use bracing, it will be 1/2" MDF. At the very least I will gusset the corners.

So, take a look & please let me know if you have any feedback. Constructive criticism is always welcome, and if you have done MTM's yourself I would be interested to hear about the experience. Baffle design is something I am very new to and pointers are appreciated. I typically like the KISS principle, and really a 2-way would be more along those lines, but I want to try something different this time around. Thanks!

[bmwman91]

The width & height of this design are fairly simple to nail down based upon driver dimensions. From what I ahve read, the baffle should either be as small as possible, or as large as possible to reduce diffraction effects. Since space is a factor (I need some room to move around, and there is the almighty SAF too), I have decided to stick to the smallest possible profile.

This leaves the depth as the enclosure's independent variable. Are there any solid guidelines out there relating depth to width? I would imagine that you do not want the depth/width or width/depth aspect ratios to be too extreme, but I really have no factual basis for this assumption. Can anyone point me to good information pertaining to enclosure dimensions? I have read Vance Dickason's LDC, but it didn't seem to address this in much detail.

The other independent variable are driver positioning in the baffle, and internal damping (foam, stuffing, acoustic -> thermal energy conversion materials). The LDC had some interesting studies on baffle arrangement, and I should probably review that. Can anyone recommend good books / links that discuss MTM configurations? As far as the damping goes, I know that I am on my own there. The enclosures will get the butyl damping material & acoustic foam for sure. I will just have to work out the stuffing with my measurement mic after building them. Obviously it won't be used for any sort of bass extension, but rather to reduce internal reflections.

[mcmahon48]

I would thing that these would work out even better Parts-Express.com:Tang Band W6-789E 6-1/2" Woofer | BLACK09 Tang Band W6-789E woofer 6-1/2 woofer poly cone woofer midbass tangband tb speakers and for the tweeter Vifa DQ25SC16-04, 1" Titanium Dome Tweeter from Madisound crossed over at 3.4 Khz resulting in a 93 to 94 sensitivity range and fairly flat response

[bmwman91]

Thanks for the suggestion on the tweeter. It is a fraction of the price, and the HD / transient response (per Zaph's site) are really good. I will give it some consideration. Thanks.

[planet10]

If the box is too narrow you will get early reflections off the side walls back thru the cones. We have found on a number of builds that we have needed to rebuild with a wider box.

dave

[McCormack]

Originally Posted by planet10

If the box is too narrow you will get early reflections off the side walls back thru the cones. We have found on a number of builds that we have needed to rebuild with a wider box.

That's been my experience as well.

[bmwman91]

Originally Posted by planet10

If the box is too narrow you will get early reflections off the side walls back thru the cones. We have found on a number of builds that we have needed to rebuild with a wider box.

dave

Originally Posted by McCormack

That's been my experience as well.

Interesting. Could you give a ballpark figure for the aspect ratio where the designs were showing this? (i.e. D/W > 2.0 or something)? The design concept is slightly rectangular right now, but it could just as easily be a square footprint.

[planet10]

It isn't about aspect ratio. It is about how close the sidewalls are to the driver inside the cabinet.

A wider cabinet also gives room for a more usefully larger champher/roundover.

dave

[bmwman91]

Well, in that case the design might need a bit more thought. I kept the width as small as possible to get the drivers as close to the edges as possible. Perhaps a more trapezoidal shaped enclosure would be beneficial? As the design stands now, the drivers probably only have 1" of distance from the side walls on the inside.

Most MTM setups I have seen look like they have drivers really close to the sidewalls. Perhaps it is a trade-off between early internal reflections & increased diffraction losses from a baffle that is not narrow enough (reading about baffle design more or less led to the conclusion that you want it as narrow as possible, or as wide as possible but not in the middle). I assume that acoustic stuffing could help to reduce internal reflections a bit, so I might stay with the narrow design & try that.