The Pit Vipers
For the uninitiated, the term "Monkey Coffin" is used to refer to the large, heavy rectangular-prism-shaped speakers that adorned many homes during the 60s, 70s, and 80s. Their popularity dwindled in the 90s after to the emergence of Home Theater in a Box (HTiB) systems, which had a much higher "Spouse Approval Factor" due to the fact that they took up very little floor space, and mostly hid the speakers out of sight.
Yet as much as they may have fallen out of favor with the regular consumer, their iconic look is still part of our culture. Millions of people have fond memories of times listening to Monkey Coffin speakers at ear-splitting volumes at parties, at low-volume subdued listening sessions in sunken living rooms--and everything in-between.
As long as I've been in this DIY hobby, I've noticed that the desire for guys to build Monkey Coffins never went away. If anything, I think there's somewhat of a void in the hobby. What I mean is this: as much as guys claim to want to build a pair, I don't really see many unapologetic big, fun speaker designs out there--at least not when you compare it to something like, say, a bookshelf 2-way. So after all these years, I'm tossing my hat in the ring. Here we go!
Looking at my past through a different set of eyes
As a child of the 80s, these were the types of speakers I grew up with. Cerwin Vega, Pioneer, Sansui, Technics, JBL, Fisher, you name it. Yet as they fell out of fashion I was glad to see them go. A lot of them, for all their visually-imposing looks, didn't actually sound that good--at least, not compared to the kinds of things we can achieve now with modern driver technology and computer modeling. Honestly, after the first time I fired up a good DIY speaker with a well-designed crossover and bass tuning, I ditched all my Monkey Coffins. No joke.
But everything comes back around eventually, I guess. First I noticed that I was seeing articles about how Vinyl now outsells CD--and even watched non-audiophile friends of mine purchasing Vinyl. Then I noticed that JBL brought back the L100 (at a very hefty price!). I decided to read up on people's favorite "classic" Monkey Coffin speakers, such as those by JBL, Yamaha, Harbeth, AR, etc, and my feelings toward them warmed slightly. Maybe they weren't all so bad, after all!
So I started sketching up something similar to those perennial favorites. But there was to be a catch! If I was going to do a Monkey Coffin, it had to have a sound that I like, that brought out the best in my favorite recordings. I have no interest in recreating a "vintage" sound; and if you want that "vintage" sound, then go buy a pair of used, vintage speakers. On the other hand, if you've built or heard some of my other speakers and liked what you've heard, then read on.
The heart of a big ol' floorstanding speaker should be the woofer. And when we're talking about Monkey Coffins, that means something around 10" - 12". I looked all over the market for something that struck the right balance between low bass extension, high-ish sensitivity, and price. What ultimately drew me to the Dayton SD315 is that while it's technically sold as a "subwoofer," it has the higher sensitivity and midrange response of a big, paper woofer. And while I admit that it's not cheap, the sad truth is that not many 12" woofers are. At least not any that you'd actually want to use in a half-decent speaker. Consider this putting money where it matters most here.
Now, since I knew that my nice, big woofer was going to get us an efficiency around the 88-90 dB range, it actually posed a bit of a challenge of finding a suitable 5" mid. I was surprised to find out that most midwoofers currently available do not have the efficiency to keep up. The choice ended up being between the classic DC130b-4, or a dedicated Pro Audio mid. I chose the DC130 because of its stellar reputation in DIY designs over the last 15+ years, and also because it matched the same aesthetic as the Dayton woofer.
Finally, for the tweeter I knew I wanted something with a 1" dome and a 4" flange, like the Vifa, Peerless, and Seas tweeters we'd all come to love so much over the last 20-25 years. When I saw the Peerless D27, I just knew that was what I was after.
The Party Speaker
There are a few ways to build this cabinet. The first one I'm going to present is what I'd call "The Party Speaker." It's roughly 67 Liters internally (after you factor in space taken up by other stuff inside), and is actually relatively shallow, relative to its width. If you were ever in a dorm room or frat house during the 80s or 90s, the shape is instantly recognizable.
I used a pair of 2.5" inner-diameter ports, each about 5.5" long. This tunes the cabinet to around 35 Hz, and will get you an F3 in the mid 30 Hz range. It also uses a sheet of 4" thick roll-type insulation (eg: the pink stuff) along the back wall. It needs at least this much damping inside to tame the wide, gentle hump around 55 Hz. As it is, it's got an extra 2 dB or so in that hump, which actually works great for any modern recorded music like rock, pop, hip hop, dance, etc. If you want to flatten it out a bit more, you can either add another sheet of insulation, or some fistfuls of Polyfil / Dacron.
With the midrange chamber I played with a wide variety of ideas, and asked around on the Parts Express message board and got even more cool suggestions.
In the end, I decided to keep things very simple, and used a 6" PVC end cap. These are fairly easy to procure at your local plumbing supply store (or Amazon, where I got mine). Gluing the caps to the back of the baffle was actually a bit of a challenge, though. The first thing I tried was Construction Adhesive, but this didn't stick, and the caps just fell off. Next, I used E6000 glue, which seems to be holding great, and created a nice, airtight seal.
Inside the cap I did a heavy stuffing of Polyfil, and drilled a hole in the back to pass the speaker wire through. That's it. Easy peasy.
The hardwood strip around the inner perimeter of the cabinet is what I screw the front baffle to, just FYI. I default to removable baffles whenever possible, because I'm always swapping stuff around.
This is a big box. It needs bracing. Without an effort to tie the walls together and/or dampen the panels, it resonates like a bass fiddle. You do not want this.
So what are our options? The sky's the limit, really. In fact, I encourage people to come up with good methods of bracing or damping this cabinet. What I ended up doing was using some 3" wide scraps jammed and glued in place between the walls, and fastened and glued to one another. This is the bare minimum; the more effort you put into making this cabinet inert, the better. For example, a lot of guys like to do a sort of "grid" or "honeycomb" with their inner bracing, and that would be ideal, but it would also be quite a bit of work.
The bracing should also tie to the front baffle, if possible. Or--even better--make the baffle double-thick.
I guess the only real downside I can see about lots of bracing here is that it's already a pretty heavy speaker as it is, and adding more mass is only going to increase its weight--so I guess that might be something to keep in mind as you add bracing and damping.
The crossover here is relatively simple.
The woofer uses a 2nd order electrical filter with C4 acting as a notch (or "tank" filter).
The mids are all 2nd order electrical, with padding resistors (R2 and R3) on either side.
Finally, the tweeter is gracefully simple, at only 3 parts. It's a 2nd order electrical filter, with a single padding resistor.
Of course, getting to this point wasn't necessarily so simple. For one, when a woofer is placed closer to the floor, you automatically start picking up reinforcement from floor bounce--so this gave me back an extra few dB, which I had to account for in the bass and low mids.
The other thing I felt was vitally important with this speaker is that it had to sound good on non-audiophile recordings. So voicing was done with that in mind. Also, it was a given that this speaker was going to be played loudly, and I didn't want it to become obnoxious or fatiguing even at these higher SPLs. So the voicing process happened at a wide variety of volumes (my 8 year-old son liked manning the SPL meter), and also with a wide variety of recordings.
I'm including the FR graph, even though it's not probably as accurate as it could be. Since I use MLS gated measurements, whatever you see there below 200 Hz doesn't count. But I figured it at least gave you a rough idea of how and where the drivers cross over one another.
Some may worry that since this speaker is based off a 4 Ohm woofer (technically, it's two 8 Ohm voice coils, wired in parallel), that it is going to be a tough load for an amp. Now sure it's not necessarily the easiest load in the world, but I've tested it with a variety of amps, and have not had any problems.
It should go without saying, however, that you ought to drive these with an amp with a healthy amount of power, regardless. With 100 Watts, you should be able to get around 108 dB. Technically, the woofer can probably take more; according to the simulations, it should be able to get to 111 dB (@200 Watts) before xmax becomes a problem. You may want to use beefier resistors in the crossover than the standard 10 Watt-rated ones we'd normally use if you plan on driving them this hard for extended periods of time.
This is a FUN speaker to listen to--which OK, was one of my initial goals. But it's so much more than just a party speaker. Obviously, with a pair of 12" woofers, you're going to get gobs of bass for most any average-sized room. And with an F3 in the mid 30 Hz range, it will handle pretty much all musical content you can throw at it.
As I mentioned with the Tarkus, the real bonus of using large woofers is the sense of separation you get with instruments in the bass range. The bass guitar (or bass synth) is clearly separate from the bass drum--which itself you'll notice has a complex timbre.
The midrange is ideal for rock and pop and country, and never becomes fatiguing. Voices sound natural and lifelike--which honestly is amazing, given the relatively cheap price of the Dayton DC130.
The tweeter exceeded my expectations here. When I first measured it, I was slightly concerned because its response didn't look all that flat. But sometimes you luck out with certain tweeters. That is, their unique dips and bumps can really lend themselves well to recorded music. (I've mentioned this many times before about the Vifa/Peerless DX25) The Peerless D27 tweeters here allow for fantastic imaging, and never become shrill or harsh, even when you're shaking the walls over 100 dB.
So, in spite of the fact that I've purposely avoided Monkey Coffins ever since I started in this hobby, I'm so glad I took the plunge to make this design. Despite the fact that I still sometimes find the design a bit goofy and over the top, I would be more than happy to keep a pair of these in my listening room for years of enjoyment!
by Paul Carmody | last updated July 30, 2022