Introduction: Low-cost Spherical Speaker Array

Acoustic instruments radiate sound in a wonderfully complex, 360 degree fashion, while conventional loudspeakers radiate in a much more boring, spotlight of sound. You could spend a ton of money on fancy products:

Hemisphere from Electrotap
Experimental Meyer Array

or you could follow these instructions to build a cheap array out of IKEA salad bowls and surplus automotive speakers. It won't sound as "accurate" as those other speakers, but it's surprisingly good and looks pretty cool on stage.

Special thanks and apologies to Dan Truman and the researchers at CNMAT, who's scientific work directly inspired this project.

P.S. Here's another cool loudspeaker array.

P.P.S The Stanford Laptop Orchestra built some nice ones here.

P.P.P.S. This one is nice, too .

Step 1: Acquire the Materials

The goal of this project was to be as cheap as possible. You'll want eight (8) loudspeakers. I would have used these speakers:

$5.50 speaker

but All Electronics was out of them. I got these, instead:

$7.50 speaker

The important considerations were an impedance of 8 ohms (which is normal for home stereo speakers) and good frequency range. (in this case 70-10k Hertz). Sound localization is more acute at higher frequencies, so response >1k Hertz is especially important. It would be better to add a sub-woofer to make up for thin bass, than to have no high end.

While at All Electronics, get some Speaker Terminals.

I used four (4) "quad"  terminals, but the four-pair ones look good.

(NB: If all electronics doesn't have these anymore. Try Parts Express or Radio Shack)

The other major elements in this project are the bowls. They cost $5 at IKEA:

Reda Bowls

Insanely cheap. Get two (2) sets because you need two of the biggest bowl. They also come in white. I imagined using one of each color, so this project would look like a big fishing float. Ultimately, though, solid red seemed best.

UPDATE: These bowls are no longer sold by IKEA.  Sorry.  

Finally, the miscellaneous hardware. 8 speakers x 4 mounting holes = 32. My local Ace hardware provided 32 machine bolts, locking nuts, and washers. Also, pick up 8 small nuts and bolts for the terminals. Most of the speakers were fine with 1/2" bolts, but the top and bottom ones needed longer (1 1/2") ones, as you'll see later. This may vary, if your parts are different.

- weather stripping
- speaker wire ( I had some 18 gauge stuff lying around the house, the project only needs a few feet.)
- heat shrink tubing

Step 2: Assemble Your Tools

For this project, I used:

A "high-speed rotary tool" with the following bits:
- #409 cutoff wheel
- #561 Cutting Bit

Soldering Iron, Solder, and heat shrink tubing
Wire Cutters, strippers
Scissors and a Pencil
Ear plugs and Safety Glasses

I'll emphasize ear plugs here. It wouldn't do to deafen yourself in the process of building a snazzy speaker array, would it? I like to wear earplugs when I do any work with power tools. (Or when I work with stupid fools.)

And of course, working with many of these tools is inherently dangerous. Just because I did this project without injury doesn't mean you won't hurt yourself. Be careful; as the agreement says: By using Instructables, you may be exposed to Content that is offensive, indecent, objectionable or unsafe. Each user must evaluate, and bear all risks associated with, the use of any Content, including any reliance on the accuracy, completeness, or usefulness of such Content.

Step 3: Cut the Hole for the Top Speaker

I would like to say that I was smart enough to have found a speaker and bowl combination that worked out like this, but I wasn't -- I got lucky. The speaker fits exactly inside the rubber ring on the bottom of the bowl.

Use the rotary tool and the cutting bit to remove the circle of plastic inside this foot.

Step 4: Make a Template

Since the hole for the top speaker was the perfect size, I used it as the template for all the other holes. Trace either the hole or the part that was cut out onto a piece of paper. Use scissors to cut out the template.

Step 5: Mark the Locations of the Speaker Holes

The speaker holes should be cut evenly around the sides of the salad bowl.

To make sure they are exactly 120 degrees apart, used a piece of heat shrink tubing (or the like) to measure the circumference of bowl. Then divide the circumference by three, and used the same tubing to mark where the center of each speaker should be.

NB: For one hemisphere I centered a speaker on the handle and measured from there. The other hemisphere has a speaker centered on the spout. That way, the speakers are offset when you put the hemispheres together. Also, make measurements from both directions to make sure there aren't any errors.

I decided to put the speakers an inch away from the rim of the bowl. That left me clearance to put the lids back on, if I wanted to. Don't put them any further away, or they will wack into the top speaker.

Use your template to trace the speaker hole onto the bowl.

Step 6: Cut and Adjust the Speaker Holes

Again with the dremel cutting bit, cut along the lines you drew in step 5. There is quite a bit of inaccuracy introduced by trying to draw a flat template onto a curved surface. The first hole you cut is going to be too small. Test fit the speaker, and cut out more material where it rubs against the speaker. Don't worry about ragged edges, they will be covered by the speaker flange.

NB: This might be a better time to do step 13.

Step 7: Take a Break

Actually, I made the hemispheres one at a time, over a number of days.

Step 8: Modify the Terminals

The speaker terminals have some extra plastic that prevents them from mounting flush to the bowl. Cut that off using the cutoff wheel.

Step 9: Prepare Your Speaker Wire

Cut a short (6-8 inches) length of speaker wire for each speaker. Strip the insulation about 1/2" back on each end of the wires. Tin all of the ends.

Step 10: Solder the Wires to the Speakers

It will be easier to solder the wires to the speakers before they are attached to the bowl.

Cut some short lengths (3/4") of heat shrink tubing and thread them on one end of a speaker wire. Solder the ends to the speaker terminals, slide the tubing over the solder joints, and use a heat gun to shrink the tubing.

RE: Polarity
My speaker cable has a reddish wire and a silver wire. I connect the red wire to the positive (+) terminal and the silver wire to the negative terminal. Some wire has colored insulation, or just a stripe on one wire. Conventionally, one connects a red or striped wire to the positive terminal. If you are inconsistent about this, you might accidentally reverse the phase of some of the speakers. While this will probably have no audible effect in this system, it would be a problem in a more scientifically calibrated setup.

For all the Rules of Hacking, see:
''Handmade Electronic Music: The Art of Hardware Hacking'' by Nicolas Collins, pp. 225-6.

Step 11: Prepare the Speakers With Weather Stripping

This step is optional, and cosmetic.

I ran a line of open foam weather stripping around the edge of all of the side speakers. (The flange of the top speaker sits flush against the "foot" of the bowl.) The purpose is to cover the gap between the speaker flange and the bowl, and hide any messy edges.

Step 12: Attach the Speakers to the Bowl

I started this process with the top speaker. Fit the speaker into the hole, and use a pencil to mark where the bolts should go. Remove the speaker and drill holes that are the same size as your bolts. If you get the right size, the bolts will actually cut some threads on the way through the plastic. This isn't very strong, of course, so on the back put a washer and a locking nut. I used locking nuts because there will be a lot of vibration in this system.

There is going to be a fair bit of muscle and bending in this step. As you can see, both the speakers and the bowl are a bit bent by the end. That's fine, it just makes the whole thing stronger. :-)

Repeat for the three side speakers.

BTW: It's possible that by this point you accidentally poked a speaker, and dented it.
Here is an instructable on "unpoping" dented speakers.

Step 13: Attach the Terminals to the Bowl

It would have been neater to cut the holes for the terminals back in step 6, but that's not how I did it. The terminals each need a hole big enough for the connections to fit through. Hold the terminal in approximate location, and mark the place to cut with a pencil. The terminal block is much bigger than the necessary hole, so a little imprecision is fine.

I was imagining this speaker hanging from the handles, so I kept the terminals as close to them as seemed reasonable. Check out the photograph to see how the positions are different on each hemisphere. There are two terminals on each hemisphere. On one, the terminals are close together near the handle. On the other, they are on either side of the speaker that is centered on the handle.

Since you've already got the weather stripping, stick some to the bottom of the terminals.

Finally, fit the terminal blocks, drill holes for the small bolts, and bolt the blocks in place.

Step 14: Solder the Wires to the Terminals

For each speaker wire, cut some short lengths of heat shrink tubing, and thread it on to the ends of the wire. Solder the wire to a pair of terminals, making sure to solder the positive wire (red, striped, etc.) to the positive (red) terminal. After the joints cool, slide the tubing over them and use your "heat gun" to shrink the tubing.

Step 15: Enjoy the Sweet, Sweet Music

This array can be used either as a hanging sphere(oid), or as two hemispheres. Either way, the radiation pattern can be quite interesting. I like to put the lids in when they're in hemisphere mode.

You're going to need a few things to use your speakers, now that you've got them.

-- Eight channels of amplification. With the money you saved on speakers, you could by a Stewart DA-70-4 for each hemisphere. Or, to keep the whole thing on the cheap, get four t-amps .  Currently, I'm running some class D amplifiers I found on ebay.

-- A source of interesting sound. I am a MaxMSP user, so I've used that software with the nbody~ from the PeRColate collection to create simulated body resonances of string instruments. You can also do this all for free with Pd .

-- I use it to make electronic instruments, or electronically processed acoustic instruments, sound more "real" and blend with other instruments.

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