On-Wall MTM Design: Advice Greatly Appreciated

[Sebreaux]

Hi all.

I have been lurking around this community for years. I've built a couple proven designs, but have always wanted to try my hand at my own design. It's just that every time I got to PCD, I couldn't quite wrap my head around it. I am currently in pursuit of a living room HT system with a higher WAF, and came back to the same topic like I always have, except that this time I found some new tools that are easier to understand, namely VituixCAD. I have worked a few things up, but am now at the point where I need input from those who actually know what they're doing. Here is what I currently have for an MTM using 2 x RS150P-8A and a PT2C-8.

I am very much at the end of the spectrum where I don't know enough to even know what I don't know. But I am also at the point where I will only learn more by doing. What should my next steps be? Are there any glaring issues in what I have? I don't have measurement equipment, so these are made with Dayton's supplied data. There are no provisions for baffle step, diffraction, or driver offsets in the models either.

It seems that the response falls off a cliff after 10K. Will that be audible? Is there a change I can make to accommodate the tweeter rolloff?

Thanks in advance for any help.

[Regore]

The spec sheet says 26,000hz.for response on the tweeter. There are many articles you could read
to help gain some knowledge before diving in to a MTM. Good luck with your project !

[billfitzmaurice]

I see two potential issues. First is using a 2nd order/2nd order crossover. That invites polarity concerns, as the total phase shift is 180 degrees. Using 3rd order on the high pass removes the polarity issue, while increasing the level of protection and lowering the potential for distortion with the tweeter. The other concern is the Allison Effect cancellation notch in the midbass when wall mounted. It's why my L/R are flush mounted in the wall. This explains:
http://arqen.com/acoustics-101/speak...-interference/

[mobius]

Without chopping up your walls, for both aesthetic and sonic reasons I would want to go with the slimmest (ie. the least depth) cabinets I could get away with and so I would be looking at something like the Peerless GBS-135 mid/woofers instead which take up only just over an inch of internal cabinet depth. I did some rough sims of an MTM along these lines once upon a time and I found a few cabinet dimensions that worked well when you factor in baffle diffraction and the rear wall boundary gain using Jeff B's baffle diffraction program. One of the best was probably something like 8" x 30" x 4" (WxHxD) with 1/2" material giving 10L gross internal volume. Going sealed gives you an F3 of about 80Hz so that will work well with a sub.

The resultant FR though required a fairly low xo point of about 1700Hz so you would need to combine it with a tweeter that is comfortable with that and that preferably also had a small flange to allow closer center to center spacing for the woofers. I simmed something not sold by PE so I'll not mention it in this forum.

But you really have to include those pesky baffle diffraction effects and especially in this case, the rear wall boundary effects to have any hope of getting your sims to be half way accurate. Even then, measurements would be recommended in the end.

[Sebreaux]

Thanks. You posted this prior to my next response. I'll check those Peerless drivers out.

[Sebreaux]

Thank you. I am not dead set on TM vs MTM or on the tweeter choice. I would like to use the paper RS series, however. These speakers will be disguised as framed artwork, and will protrude 2-3 inches from the wall, not quite meeting Bill's flush-mount recommendation, but maybe better than a true on-wall design. I understand that I am entering into design situation fraught with compromise here, but again, I am viewing this as an opportunity to learn and have some fun. These would fill a roll as 50/50 music and HT in our family living room that is open on one side to the rest of the house (hence the desire by my wife to hide them). There is nothing about the situation that is ideal.

I guess that my main design goals would be that it have excellent off axis response, as my placement option is fairly set. I don't need tremendous bass extension, as I have a decent subwoofer that they can hand off to. But I would like them to be able handle HT duties fairly well at moderate levels. I will eventually build an identical vertical center channel into our HT cabinet (again hidden), so ideally the design would work as a complete LCR set.

[davidB]

If your frames use a moulding that angles back to the wall with an obtuse angle(a bevel as you see on some baffles), you will reduce the diffractive effect.

Convex, rounded and curvy is your friend here.

If your frames can have 14-16 liters volume, you can get into the 40s with the GBS135.
I thought they were weird, but I tried them and like them.

[billfitzmaurice]

These speakers will be disguised as framed artwork, and will protrude 2-3 inches from the wall.

That puts the Allison Effect dip in the vicinity of 1100Hz. If that's above the baffle step then no worries, which would require the baffle be a foot wide.

[Sebreaux]

Excellent. I was planning a 15x30" baffle. Is the 12" consideration only in the horizontal plane or both? I ask because if I were able to place the driver set lower on the baffle, the tweeter would be closer to ear height.

[rpb]

Excellent. I was planning a 15x30" baffle. Is the 12" consideration only in the horizontal plane or both? I ask because if I were able to place the driver set lower on the baffle, the tweeter would be closer to ear height.

Is part of the speaker box going to be behind the wall, or is the wall staying uncut?

[Sebreaux]

I intend to set them into the wall for a cabinet depth of 6-7".

[billfitzmaurice]

Excellent. I was planning a 15x30" baffle. Is the 12" consideration only in the horizontal plane or both? I ask because if I were able to place the driver set lower on the baffle, the tweeter would be closer to ear height.

It's a consideration in both planes, but less so on the vertical.

[Sebreaux]

Does anyone have a good reference explaining phase? Specifically with regards to what my goals should be in working on a design. I grasp some of the basics, but haven't found anything discussing what I should be aiming for with the phase graphs in the crossover design software.

[billfitzmaurice]

Some will go into far greater detail than actually matters, but the primary concern is that the midbasses and tweeter are not at or near 180 degrees of total phase shift at or near the crossover frequency. That's nearly always the case with 2nd order/2nd order crossovers, which is why they should be avoided.

[rpb]

Does anyone have a good reference explaining phase? Specifically with regards to what my goals should be in working on a design. I grasp some of the basics, but haven't found anything discussing what I should be aiming for with the phase graphs in the crossover design software.

Don't worry about phase! It pretty much does its own thing, and takes care of itself. Try to make the driver roll-offs smooth. Use the targets in the software for examples of the correct, or ideal shape. Nothing is perfect, and close is pretty good most times. Keep the drivers operating in the range they are capable of so that distortion, and breakup don't degrade the sound quality.

[rpb]

I had some ideas on the baffle, but it's hard to describe in a few sentences. Around the outer edges of your baffle, I'd have 2" depth of the firm type of fiberglass. Near the drivers, I'd have the baffle extend over a 1" depth of the fiberglass. In that portion, I'd drill some holes that allow sound to leak through the baffle, and into the 1" fiberglass. Hope that gives you the basic idea.