Concrete Loudspeakers

Intro: Concrete Loudspeakers

Concrete is a wonderful material for speaker boxes: it's strong, stiff, dense, non-resonant, and (to me) beautiful Concrete's other ace is that it can be made into a variety of different shapes. For this project, I went with a simple rectangular sort of shape, because that's what my friend wanted :) However, using this method, you could make any kind of shape.

To make speakers, you'll need:

1. Speaker drivers (woofer, tweeter, full range, whatever) - I used the Continuum kit from Meniscus Audio
2. Crossover
3. wires
4. Binding posts
5. Screws

For the boxes you'll need:

1. Cement + sand + glass fibers OR premix. I used VOH from Sika and some additional fiber for the backing coat
2. Acrylic bonder/fortifier - like this from Home Depot
3. Some wood for front/rear baffles - 3/4 MDF from your local big box
4. Gasket material - like this from Parts-Express
5. Construction adhesive. I used this stuff which will glue anything to anything
6. (Optional) concrete stain
7. (Optional) Wood veneer
8. (Optional) concrete polishing pads and a sander (or strong elbows)

For the molds:

1. Melamine-covered particle board - from the local big box
2. 3/4" angle aluminum - ditto

Tools:

1. Router
2. Screwdriver
3. Saw
4. Caulk
5. (Optional) texture sprayer - like this one from HF

The main thing you'll want to do is ensure you have the best speaker components. If this is your first build, I _highly_ recommend getting a kit from Meniscus, Parts-Express, or Madisound.

For this project, I'll walk through the process of building the teal speakers I built for my friend, Noah. They are stained concrete, but (of course) you can keep them raw. You can see a similar project with the Linaeum tweeter that's in raw concrete.

After you've gotten your supplies. Plan the work carefully. Concrete isn't as forgiving as wood, so you'll need to know exactly what dimensions you need where.

STEP 1: Build Some Boxes!

Once you have acquired your material. You need to build the boxes.

The first step is to build a mold.

Cut your melamine board to the correct size and screw it together. Then cut the aluminum angle to size and screw it in. You can see from the photo how to align it. The angle will be the edges of the box and determine the thickness of the material. Next, run a thin beat of silicone caulk around all the seams to prevent leakage.

The next step is to mix up the face coat, load it into the hopper of the texture gun and spray it in. Follow the instructions of the cement you are using. There will not be any glass fiber in the face coat. The mix is thin, it's got to spray through the texture coat. This coat is thin - only about 1/16 of an inch. It will set up quickly, so next step is to mix the backing coat. Mix it quickly! You'll want to include some glass fiber and some acrylic fortifier to increase the strength. You'll be looking for "peanut butter" texture in your backing coat.

After your backing coat is mixed, pack it into the mold, into the face coat to ensure a good bond. Work quickly, packing the same side on each mold - i.e. do the bottom for both, the left side for both, etc, this will let the cement set up before it has to hang upside down. The backing coat will set up very quickly, but you don't want your walls to be hanging upside-down while they are too liquid.

Once the GFRC has set up, you can de-mold. After de-molding, do some polishing (if desired) and fill pinholes by mixing acrylic with the cement (no sand) and rubbing it into the holes; polish again. I took mine to 400grit. Then I stained the concrete with floor stain in teal.

Finally, I cut some braces to go around the edges out of some 1.5" scrap lumber I had laying around and glued that around the perimeter of the front and rear opening. I used polyurethane construction adhesive for maximum sticktuitness.

STEP 2: Woodwork (for Concrete?)

Since I'm using a wood (mdf) front and rear baffle to take my concrete box to a fully-enclosed speaker enclosure, I needed some front and rear baffles. To do this, I cut some 3/4" MDF to the size of my speaker boxes, then rounded over the edges with a router. I also used a 3d printed speaker template to match my (somewhat oddly shaped) speakers. This enables me to have the speakers sit flush to the baffle, reducing diffraction.

The next step (and maybe the hardest of the whole project) was to veneer the baffles. Veneering the corners was very tricky. I used paper backed teak veneer and stuck it to the baffles using wood glue that was allowed to dry, then heating the veneer with an iron to reactivate the glue.

After sticking the veneer on, I finished it with some danish oil and a spray of matte polyurethane. I probably should have done one or the other, btu it ended up looking pretty nice anyway.

I then added the speakers to the baffles and did some preliminary wiring.

Before the baffles were mounted, I used some spray adhesive to mount denim insulation to the interior of the box to absorb some of the rear wave.

To mount the baffle to the front, I drilled holes through the perimeter bracking and screwed screws into the front baffle. Be careful and don't overdo it! A little bead of construction adhesive acts as a seal and glues everything down. The screws are really just there to hold stuff in place while the glue sets.

Finally to mount the rear panel, I screwed in from the back, drilling and countersinking the screws. Before finally screwing it in place I added some speaker gasket to keep things airtight.

STEP 3: The Home Stretch: Crossovers and Final Assembly

To finish the speakers I had to assemble the crossovers in the tight space between the perimeter bracing. This was more work than I anticipated and I ended up having to use some acrylic (because that's the only thing I had). Eventually, I got the crossover wired up and mounted, then it came time to finish wiring, add the binding posts to the rear panel and seal it up for good.

A note on crossover assembly. It's not the easiest thing, you realy have to be careful where components go to minimize interference between the inductors. Here's a handy guide

Once everything was closed up, I could listen to them, finally. They sound great!

But they weren't for me, so I had to pack them up and send them to San Jose for my friend Noah to enjoy. He's still listening to them now!

Hopefully that gives you some idea of how to build some concrete speakers if you are so inclined. Let me know if you have any questions!

Comments

[Lennerton]

Brute force effort but nice result. I'd recommend making an internal box/form with removable front and back rails and then troweling your fast drying cement onto one side, letting it firm up, then rotating 90 degrees and repeating. This would reduce some of the challenges faced and you'd already have the interior wood installed.

[jack ruby]

IDK that I'd call it "brute force"... maybe the cinder block would be brute force :D
I like the idea of doing the rails in the cement. That would definitely save some time trying to cut all the bracing to the precise size/contour of the concrete.

What you describe with troweling, letting it set up, then rotating is basically what I did, except I spray a skin coat, then hand-packed. Basically doing a concrete countertop all the way around the mold.

The VOH cement sets up _very_ quickly, so by the time the bottom was done on both speakers, I could move to the left side, then the top, then the right side, so by the time the cement had to be upside down, it was already firmed up. The challenge comes because it sets so quickly, I had to work really fast to keep the stuff in the bucket from setting into a brick!

[jimvandamme]

Sometimes you can go to the local construction concrete dealer and find the hollow core brick or flue or drainage tile that's about the right size, for a couple bucks. That can save a lot of time. Unless you love playing with concrete, or want an odd shape.

I'd use the premade speaker jacks that are recessed into the back panel, so they can't get busted off.

[jack ruby]

That would be too easy :P
You can also use cinder blocks, but then you are limited in size.https://www.whathifi.com/us/news/cinder-speakers-i... Flue tile works well, but it's hard to cut to size and rings a bit (can be damped pretty easily though) check this out http://techtalk.parts-express.com//forum/tech-talk...

I don't like the plastic insert jacks because I've spent a bunch of time making the box thick, stiff, and non-resonant, only to add a thin bit of plastic over a big hole in the back. They are bit less "sticky outy" though :)

[grapenut]

Great job, I learned alot by watching you iron out the veneer, and I thought the Teal Patina stain looked very nice.
Q1: I'm not an audiophile, so I was wondering if you could expand a little on the use of concrete for the box. Is it simply personal preference, aesthetics, etc.. or is concrete in-fact the defacto standard for premium quality speaker builds?
Perhaps there is a link to something that explains how the use of concrete yields a superior build?
I did hear you reference a couple of times that the concrete was neutral or more neutral, so if neutral is good then what is considered the most neutral material?
Q2: Several times I heard the term 'monitors' used to reference the speakers. Could you please explain (or post a link explaining) the use of the term monitors?
Thank you so much for sharing such a great build.

[jack ruby]

Q1- the "standard" speaker box material is MDF (medium density fiberboard) which is not terribly stiff, but it's non-resonant. Some people use plywood, which is stiffer and stronger, but might resonant. Some companies use more esoteric materials like aluminum extrusions or carbon fiber.

The goal is to stop the cabinet from releasing sound energy, which can introduce "colorations" or "smearing" of the sound. If you think of a sound wave being generated from a speaker, the cone has to move in and out to create the wave. That means there's an equal amount of energy on the back side of the speaker that must be dealt with (absorbed, usually). The box contains that energy, and if it doesn't completely contain it, the box will release that energy out of sync with the wave from the speaker which will impact the tone of the sound or the ability to localize the sounds.

So concrete is ideal (in my mind) because it is strong, stiff, and non-resonant (and looks cool). It _is_ heavy though :) This company makes commercial speakers out of concrete https://www.concrete-audio.com/en/

Q2: monitors just means reference speakers used by audio engineers to "monitor" the mix. Usually, it will refer to smaller "bookshelf" speakers when used in the hifi world. This design is based on a true monitor - the BBC Ls3/5a which is considered one of the most neutral designs ever made. Jeff Bagby used that design as the basis of his monitor design. Since this is for my friend who is a composer, it's a true monitor, being used for composition and audio engineering.

Just a note on "neutrality" what we mean is a loudspeaker that reproduces the input signal as accurately as possible. Tonal neutrality means that the frequency response is as flat and accurate as possible.

I hope that helps clear up some of that terminology. Here's a couple of audio glossaries if you want to learn more! http://www.enjoythemusic.com/audioterminology.htmhttps://www.moon-audio.com/audiophile-terms-guide

[_Vyper]

Looks nice, But what non-audiophile is spending that much on a pair of speakers? my whole sound system cost less than that for a 7.1 surround receiver and all the speakers... granted they're plastic boxes with metal grills but its still the best sounding setup I've ever owned.

[jack ruby]

Indeed, these are audiophile pieces. If you think the price of that kit is high ... well don't go into a local hifi shop 👀
That said, you could build something much less expensive. Parts-express has a ton of kits: https://www.parts-express.com/cat/speaker-subwoofe...
some of which are $100 or less, and I guarantee that you don't have to be a "golden ear" to hear the difference from some home-theater-in-a-box speakers. Give it a try! Then once you have caught the audio bug, you can upgrade from MDF cabinets to some concrete boxes and see what additional improvements you get in sound :D

[seamster]

Very nice. I like the concrete/wood combo. Always a great look in my opinion! : )

[jack ruby]

Thank you :) I like the combo as well. I'd probably build some more stuff using that, but there's a limit to how many concrete things I'm allowed to build in the house :D