# Monster surround back speaker.



## Aaron Gilbert (Nov 12, 2008)

Hi all! I thought some of you might enjoy a look at my latest creation, a monster surround back speaker, combining left and right into one enclosure. I don't have an official name for this beast yet. Before I go into the gory pictures, I'll give a brief summation of the design choices made.

As usual, cost was a major concern, as was the desire to make use of numerous components already on hand. These included woofers, tweeters (same as in my other five speakers), and crossover components - essentially everything but the wood and miscellaneous hardware. So, any design had to make the most of these parts. Unfortunately, the woofers have a rather high Qts, requiring a rather large box, and I'm using two woofers per side to match the high efficiency tweeter, making the box even larger. These were surplus woofers that I got from MCM electronics about 11 years ago - the original drivers from my Emerald Towers. They're a very nice cast frame poly cone rubber surround, with a quite smooth response. The tweeters are the shielded soft dome 1" units from ApexJr, which are very nice, especially at only $15 a pair! Crossover components are a mish-mash of all high quality stuff collected from past projects. I chose a vented box despite the high Qts, because I had previously used these same woofers in a similar sized vented box with good results. And, I always like more power handling, lower distortion, and significantly higher maximum output. 

This speaker is probably serious overkill for surround back duty, but I didn't have any tiny woofers on hand to use. The box alone weighs 38 pounds, and when all the drivers and crossovers are added, it's pushing 60. :yikes: And yes, it _is_ being hung on the wall. It's a good thing my wife is strong. In hindsight, two separate boxes would have been a lot easier to maneuver, but oh well. :duh: I don't want to think about how heavy this would have been if I had used MDF instead of plywood! :hide:

Here's the basic shell including the back, top, bottom, and center divider. The bottom is the shortest side:









Test fitting of the baffle and sides just to make it feel more like a speaker:









Here are the end caps, made from pine stair treads 1" thick. On the back side, you can see the bolts which I've secured to wooden discs screwed and glued to the caps. I screwed up my first cut, resulting in one cap being about an inch too short. I had to cut a matching piece and glue it on later. :doh:

























Here are my two vent assemblies. Each side has two 1.5" vents. 2" didn't quite seem like enough, but 3" would have needed an elbow and seemed like serious overkill, given that my tower speakers use a single 3" with almost twice the total displacement, run full range, and don't have any vent noise. These will be filtered at 40Hz or 60Hz depending upon my findings. The tubes are epoxied to the solid oak trim rings:

















Winter came early to Washington, but that doesn't stop a true speaker builder! Here's the baffle showing placement of the vent assemblies:









Side panels screwed and glued into place, glue blocks added to front side, binding posts and wiring to crossovers installed. Note the holes in the side panels, which match up to the discs on the end caps:

















































Here are the crossovers. This is a third order electrical on the tweeter, first order electrical on the woofer. Approximate crossover point is 2800Hz, about as low as I comfortably felt going with this tweeter, and the same as my other five speakers:









































Here's the baffle attached. It's held by a full tube of construction adhesive on all the glue blocks and around the entire perimeter of the panel, as well as screws into all the glue blocks running from the bottom to top panels. It's solid! 









Here are the end caps with a whole bunch of wood filler to smooth them out and fill the gaps. Yes, the speaker isn't pretty underneath, but it doesn't matter, as the entire box will be covered with fabric and the end caps. It just has to be solid and leak free, which it is.

















Here's a shot of the crossovers peeking in through the holes in the side panels:

















Here's how the end caps are held on. Two 1/4" thick custom wood washers fit over the nut and bolt on the end cap, which are then secured with a fender washer and wing nut. This was done because I wanted no visible fasteners on the end caps, and I also wanted to be able to remove them easily for refinishing or replacement.











That's it for now! You'll have to wait until tomorrow to see pictures of everything fully assembled. It's in the car now so that I can take it to work for testing tomorrow, and I'm not hauling it out at 1:00 AM for pictures. Although I can lift 60 pounds, it's a very awkward load to carry very far.


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## Anthony (Oct 5, 2006)

Nice, keep the pics coming.

And you, sir, need a garage.  No need to be assembling in the snow!


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## avaserfi (Jul 5, 2007)

Interesting design concept. Any reason you chose one single cabinet over two separate ones?


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## tcarcio (Jun 27, 2007)

avaserfi said:


> Interesting design concept. Any reason you chose one single cabinet over two separate ones?


I thought that it is one speaker because it is a surround back as you would use in a 6.1 system. At least that's what I thought.


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## avaserfi (Jul 5, 2007)

tcarcio said:


> I thought that it is one speaker because it is a surround back as you would use in a 6.1 system. At least that's what I thought.


That's what I would have thought except for this statement:



Aaron Gilbert said:


> Hi all! I thought some of you might enjoy a look at my latest creation, a monster surround back speaker, combining left and right into one enclosure.


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## tcarcio (Jun 27, 2007)

Well that's what I get for just looking at the pictures....I am not sure if combining them would be the way to go, I thought they needed seperation in order to perform correctly?


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## avaserfi (Jul 5, 2007)

tcarcio said:


> Well that's what I get for just looking at the pictures....


I know where you are coming from. Hopefully Aaron clears it up for us soon .


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## atledreier (Mar 2, 2007)

Looking at the pictures I'd say there was plenty separation to get the proper effect.


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## Aaron Gilbert (Nov 12, 2008)

:blush:


Anthony said:


> Nice, keep the pics coming.
> 
> And you, sir, need a garage.  No need to be assembling in the snow!


Oh I have a garage, where most of the construction took place. It's just _way_ too crowded and messy for picture taking at the moment. :gah:


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## Ricci (May 23, 2007)

Aaron Gilbert said:


> :blush:
> 
> Oh I have a garage, where most of the construction took place. It's just _way_ too crowded and messy for picture taking at the moment. :gah:


And I was just about to comment on how dedicated and hardcore you are for constructing these out in the snow and 15deg weather. :teeth:. You should've played along with that assumption for a bit.


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## Aaron Gilbert (Nov 12, 2008)

avaserfi said:


> Interesting design concept. Any reason you chose one single cabinet over two separate ones?


It seemed like a good idea at the time - this was before I found out it's 60 pounds. :bigsmile: 

Seriously though, there were several reasons for the single cabinet. First off, I wanted the unit to fit between these two posters:










The right one actually needs to be moved further right slightly, and the THX banner needs to be lowered a bit.

Second, one cabinet is ever so slightly a cleaner install, as I have a single 4-conductor wire into the cabinet on the left side, vs running a second wire across the wall. Third, a single cabinet makes the most of the available volume for a given external size, since there's just the single divider, vs two exterior walls. Basically, since I wanted to use the entire width, I figured it's easier to make one cabinet than two! Fourth and most pertinent to your questions, THX recommends placing the surround back speakers of a 7.1 array right next to each other directly behind the listening position. As I've stuck to their placement and orientation recommendations for the rest of the system, why deviate now?:

http://www.thx.com/home/setup/speakers/71.html

The baffle is 48" wide, so there's actually a bit more physical separation than THX optimally recommends. It'll be a few days before it's fully mounted and proper listening tests can begin.


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## Aaron Gilbert (Nov 12, 2008)

Ricci said:


> And I was just about to comment on how dedicated and hardcore you are for constructing these out in the snow and 15deg weather. :teeth:. You should've played along with that assumption for a bit.



Sorry, I'll try to remember that for the future. :yes: The garage is unheated, and it got down into the 20s, not quite into the teens though.


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## avaserfi (Jul 5, 2007)

Aaron Gilbert said:


> Sorry, I'll try to remember that for the future. :yes: The garage is unheated, and it got down into the 20s, not quite into the teens though.


I know how that goes. The last project I did was this past weekend. The whole week it was sunny and in the 60s-70s, but come the weekend and it is raining and it dropped below 20 degrees! To finish the project I ended up having to do some work in the garage and some in the house if I wanted the paint/adhesives to cure properly. My parents were sure happy when I put a whole in their wall when I tripped up the stairs!


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## Aaron Gilbert (Nov 12, 2008)

Here are a few pictures of the mostly finished product. I'll probably apply some wax to the end caps at some point and might see if I can iron out any of the creases in the fabric. In initial testing, bass is considerably better than I expected, and tuning frequency seems a bit lower than expected, in the low 30's (was supposed to be upper 30's). Vent noise is not an issue at all, unless you sent a pure sine wave at ridiculous levels, and even then it's fairly benign. The upper end is hot, which is pretty much to be expected, since these same tweeters needed about 3dB of padding in my towers to sound right, with woofers of nearly identical sensitivity. I'm not too worried about it since they're behind us, but I have plenty of parts to whip up an L-attentuation circuit if needed. I do wonder if Audyssey corrects a hot upper end, beyond it's typical upper end rolloff? Anyhow, here you go:





























Here's the back where I hid the staples holding on the fabric (also hidden under the end caps). The metal pieces are eye-bolts which go through the back. This is how the speaker is hung on the wall - though after an experiment last night, we're going to put additional support on the bottom, if only to make it seem more solid, and a lot easier to put up! The speaker is going in the top corner against the ceiling.


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## Aaron Gilbert (Nov 12, 2008)

avaserfi said:


> I know how that goes. The last project I did was this past weekend. The whole week it was sunny and in the 60s-70s, but come the weekend and it is raining and it dropped below 20 degrees! To finish the project I ended up having to do some work in the garage and some in the house if I wanted the paint/adhesives to cure properly. My parents were sure happy when I put a whole in their wall when I tripped up the stairs!


Sunny and 70 sounds really nice about now. At least we're out of the freezer, but still plenty of rain and wind. I brought my speaker into the house for much of the time, because the caulk just was not curing at all sitting in the garage. Bummer about the hole in the wall.


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## avaserfi (Jul 5, 2007)

Interesting finish on the speaker. It will look pretty good when you iron out the wrinkles



Aaron Gilbert said:


> Sunny and 70 sounds really nice about now. At least we're out of the freezer, but still plenty of rain and wind. I brought my speaker into the house for much of the time, because the caulk just was not curing at all sitting in the garage. Bummer about the hole in the wall.


The day after I finished the sub it was sunny and 70s of course. The hole was promptly fixed by me (as if I had a choice).


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## Aaron Gilbert (Nov 12, 2008)

I did quite a bit of listening yesterday, and the elevated high end was just not tolerable. I added an L-attentuation network to the tweeter circuit today via a couple resistors, for about 3dB reduction. The sound is much more balanced and listenable at ridiculous volumes without sounding too bright. Here are a few more pictures:


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## jeremy7 (Feb 7, 2008)

nice soldering joints.


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## Aaron Gilbert (Nov 12, 2008)

Well, she's mounted, connected to my Onkyo 805, and sounding great! Now all I need to do is rerun Audyssey. My secondary mounting plan worked out fine, but required a little modification to not freak out the wife. I admit this does look a bit better than the initial plan, and is a whole lot easier to mount/dismount. 

Here's the new rear wall, speaker conveniently located directly behind and centered on the couch and screen. Please ignore the hideous smoke alarm and cheesy thermometer - these will be replaced asap. And yes, the door does clear the speaker when it's opened, by about 1". 

















In this view you can see the brass rod which keeps the speaker from falling off the shelf. My initial plan with this mounting arrangement was to have the speaker hang by this rod completely - no shelf underneath. I do have a thicker steel rod for that purpose. However, mounting and removing was problematic due to the weight, and would have required blocks behind the speaker to keep it parallel to the wall anyhow. Plus, it just freaked my wife out, that huge heavy thing hanging off the wall with no visible means of support. So, up went the shelf, with the rod as a safety.










Here's the back showing how the eye bolts, eye screws, and rod work together. I can think of at least a couple ways that I could do away with all these and get the speaker right up against the wall, now that I have installed the shelf below, but oh well. It works for now, and it's high enough that nobody is going to bump their head on it unless they walk with their shoulder on the wall and are taller than 6'2".


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## Anthony (Oct 5, 2006)

the only problem I can think of there is potential for rattle. Look at Home Depot or McMaster-carr (www.mcmaster.com) and you can find some bushings or grommets that might damp out the vibration.

Some pipe insulation or door gasket tape right at the contact points might work as well. Just a though.


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## chadci (Feb 13, 2008)

Interesting mounting solution. The speaker looks great!


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## Aaron Gilbert (Nov 12, 2008)

Anthony said:


> the only problem I can think of there is potential for rattle. Look at Home Depot or McMaster-carr (www.mcmaster.com) and you can find some bushings or grommets that might damp out the vibration.
> 
> Some pipe insulation or door gasket tape right at the contact points might work as well. Just a though.


Anthony,

Right you are! In very high level testing yesterday, using sine waves from 5-50Hz, I was able to produce some rattle due to the cabinet moving a bit on the shelf. Any of your suggestions would probably fix that. In use with actual music or movies, even at extreme levels, I can't detect any rattle, so it's probably a non-issue. The only way I produced rattle with the sine waves was in the 'all channel stereo' mode, where equal signal levels are sent to all channels. Typically, there's a lot less bass sent to the surround back, even run as full-range. But hey, I've got some foam tape, so no problem applying it!


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