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The Rule Breaker
We have all heard that a horizontal MTM is the worst orientation for a center channel speaker. Yet, it is still the most widely used. Why? Well that is pretty easy, it works the best in most people homes. It is easy to fit under a television in a cabinet. That got me and @Elliott thinking. Can we make a horizontal MTM that not only is functional for your living room decor, but also has the dispersion characteristics that you really need to give everyone the best sound? Well that is what we are trying to accomplish with the Rule Breakers.
The Problem
If you aren’t familiar with the issue at hand, a horizontal MTM, has a good dispersion up and down, but side to side we get a lot of cancellation of waves, which make any seats not directly in the center of the speaker terrible. Basically anyone off to the side will be missing a lot of content, and when we are talking about voices, the main dialogue of the movie, that is a really big deal. It is not uncommon for people on the side to ask you to turn it up or even ask you what they said. They aren’t hard of hearing (probably) but they are missing some of the most important content.
So how are we going to fix it?
It must be said, that some companies have already tried to fix it, similar to the manner we are. The most common fix is to put two woofers on either side with a normal tweeter and midrange sitting vertically. Basically the theory is to cross the midranges over with the woofers low enough that we eliminate most of if not all of the wave cancellations. The issue with this, is the size of the center channel is typically fairly large, meaning you still have the same problem with fitting it in a normal room. So how will our project differ?
Simple, we are still going to use a normal MTM with no larger than 4″ woofers (although you could size this project up with larger woofers if you want). Now typically you would cross this over with a tweeter. The issue with the tweeter, is it has to crossover too high. And we really do not want to crossover any higher than 500hz. Enter the BMR. BMR stands for Balance Mode Radiator. And without getting too technical it can get very low (as low as 100hz) and still extend it’s high frequency up to 20Khz! The best part, a BMR has amazing off axis response. By using this, we should be able to cross over low enough, to make this no longer an issue.
Goals
- Make a small MTM that does not suffer from the horizontal off axis issues most center channels do
- Make it affordable – as close to $100 as possible
- Make it small enough to fit most spaces
- Make a bigger brother that can dig deeper
Driver Selection
Woofers: TCP115-8 4″ woofers
BMR: TEBM46C20N-4B
The BMR is by far the most expensive driver, but seeing as we are asking so much of it, it really should be. But after looking at all the BMR drivers, this seemed to be the best one we could get our hands on. And it should easily be capable of what we are asking of it.
Now we choose the TCP115-8 in order to get a lower impedance. Two of them wired in parallel will give you an 4 ohm load. Now I know some people are going to think this is going to be an issue with their receiver, as most are only rated at 8 ohm. But based on where this going to be crossed over, I think this will be a very easy load for the amplifier. Of crouse, this we won’t know until we test it. Thankfully I have 4 different receivers I can test this on to make sure it is not an issue.
Suitable Boxes
This is interesting the TCP115’s can either go well in a sealed or ported box. WIth that in mind, there will be two boxes that you can build. A larger ported box that @Elliott is building and a smaller sealed box that I am building. The advantage of the sealed box is it should fit in anyone’s living room and in most cabinets. The downside is you will want to cross it over between 80-120hz based on your room. The Ported will be able to go down closer to 50hz and is more for those who really want to use these for the whole surround. And honestly, these really could be used for all speakers. And should make for a heck of a surround sound when finished.
Results
Stay tuned and make sure to subscribe if you want to know the updates as they come out. This is going to be fun!
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Amazing write up Nick. I can’t wait for everyone to see and even try out what we’ve been working on here!
Also, to iterate what Nick said, keep an eye out on this thread (by subscribing at the top) for any updates Nick or I might have for you! You may be surprised by what’s yet to come 😉.
Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student -
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Nick’s might be any time from this week to 3 weeks. With my version probably taking about 4-5 weeks (but my one is a very loose estimation, there are a lot of new contributing factors so could take much less or much more time than that). My version is the one where you 3D print a reusable mould that you pour a lightweight concrete mix into. 🙂
Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student
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ah cool! Thanks for the update. I’m excited to see these! It’s funny you are using concrete. I’m making a concrete 5.1 set up. Here are some renders.
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Those look really awesome! I love how you did that concrete with the wood.
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Nice renderings!
I wish concrete looked that nice unfinished! 😂
For the enclosures I’m designing, they’ll be designed to be fully concrete as the purpose of these is meant to be that you’re able to complete this project with the only main tool you need being a 3D printer (Just like how Nick’s one is for laser cutting). Doing it this way also means once the mould has been made it should be plenty easy to make more enclosures, since all you have to do is mix up a new batch of concrete and do the pours again.
…if all goes to plan that is 🤞😅
Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student -
That’s a cool idea. Especially for someone selling custom speakers. Concrete can look this nice unfinished actually. It all depends on the aggregate, vibration and mold. But I’ve been having really amazing results. There’s 4 different approaches I’ve tried: mixing my own with white Portland cement and super fine sand, using artisan off the shelf stuff from Buddy Rhoades, using polymer modified gypsum plaster from smooth on (called forton mg) and just coating an mdf box with lime cement (limestrong plaster brand). All of these produce great but different results and different costs. I highly recommend anybody considering doing your method of 3D printing a reusable mold and wanting the smoothest and most finished look out of the mold check out a polymer modified gypsum plaster like forton mg or Duo matrix.
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I thought I would share some updated pictures. For this, I’m gonna go ahead and use a laser engraver to manufacture the box. You don’t actually need a laser engraver and it’s really easy to make a rectangular box for this, but I thought it would look really nice.
I decided to do a stacked layer design out of 1/4 inch material. The laser is doing a really great job especially with the addition of air assist. I think in general doing a cut out for the BMR would be hard to do without a laser. Not impossible, but hard for a lot of people.
with the laser and the BMR fit perfectly.
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This looks great! I really like how your design hides the burnt edges from the laser cut. I also really like that you chose a light colored wood! It’s got a great modern and calming feel. Especially compared to the overly masculine look of walnut or worse; black ash vinyl. How many watts is your laser?
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Looks awesome. What are the box dimensions going to be? Can these be used as a left and right channel also?
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The box is 13 1/2 x 5 3/4 x 4 1/2 deep. Pretty small and compact. It should work really well in most spaces.
And yes these can be used for Left and Right or center.
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For the Sealed version the anechoic F3 will be about 100hz. Which will allow it to crossover between 80 and 120hz. Fairly typical for a center channel. If you build the ported version that Elliott is working on, then it will dig even deeper (40-50hz range). The size itself will be much larger though.
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Getting Closer
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