Introduction: ResoDrums -- Electronic Hand Percussion
Step 1: Parts & Tools
This is the list of things you will need to make the ResoDrums. These instructions assume that you are making a two-drum setup (the best plan for a typical stereo input computer). The software is modular, however, so you can build as many ResoDrums as you have audio inputs.
A) two shallow wooden salad bowls -- available everywhere. I would recommend getting them secondhand at a Goodwill or Salvation Army. Try to get flat bottom bowls that are around 1/4" thick and carved from a single piece of wood. Avoid bowls with a ridge around the base.
B) two piezo discs -- these will cost about a buck apiece at any electronics shop. Get the largest ones you can find (the ones shown are about an inch and a half in diameter). If the piezos have any plastic housing or wiring, remove it. You will only need the discs themselves.
C) two electro-mechanical transducers -- these are basically the magnetic drivers of speakers without the diaphragm (cone). Whatever you attach them to becomes the diaphragm, in this case the bowls themselves. I like using either Vidsonix Phantoms (for larger bowls and lower frequencies) or Vidsonix Ghosts (for smaller bowls). They're less than $20 apiece from Vidsonix via their
D) an audio cable -- 1/8" stereo on one end and dual mono on the other (for example dual RCA as shown). This will connect the output of the ResoDrums to your computer (here assumed to have an 1/8" stereo jack). Alternately, you can make your own quite easily with just an 1/8" plug and some wire.
E) an audio cable -- to connect your computer's audio out to your amplifier's audio in (1/8" to 1/8" cable shown---your equipment may vary).
F) an amplifier -- a stereo audio amplifier that is 8 ohms and at least 15 watts (so just about any home stereo amp will do). Here I've used a class-D amp, which is small enough for easy transport.
G) 6 wood screws -- the screws that come with the transducer are too long (they will go all the way through the bowl). You will need shorter screws (about as long as the thickness of the bowl).
H) 6 washers -- these will distribute the pressure of the screws so you can tighten them without cracking the transducer's mounting plate. You'll want ones that are about the size of a quarter with a small hole in the middle.
I) spacers -- the cheap rubber bits with adhesive (available at any hardware or home furnishings store)
J) glue -- epoxy or jut plain superglue
K) a laptop -- mac or pc
L) a soldering iron & solder
M) pliers/wire cutters
N) a screwdriver
O) an awl
P) tape -- gaffer's tape is the best, but just about any tape will do the trick
Step 2: Attach the Transducer to the Bowl
Since the transducer is transferring energy to the bowl, we want to make sure this connection is solid. A bowl with a flat bottom will work best. Place one of the transducer's mounting plates in the center of one of the bowls (with the center bolt facing up). If the plate is wobbly, use additional smaller washers between the plate and the bowl. Make sure that the head of the bolt in the center of the plate rests securely against the bowl. If it does not, use additional spacers or other material to fill the gap completely.
Put a few drops of glue on the bottom of the bowl. Set the plate in the middle of the bowl and use the awl to punch shallow guide holes for the screws. Use the screws and washers to attach the mounting plate to the bowl. Tighten the screws as much as you can without cracking the plate. Once the screws are tight, drop a tiny bit of glue on each. This will help to keep them from vibrating loose later. Put a drop of glue on the bolt that comes out of the mounting plate and thread the transducer on tightly.
Step 3: Glue the Piezo to the Bowl
Glue the piezo to the underside of the bowl as close to the center as possible without touching the mounting plate. Make sure the piezo is firmly against the bowl, even if it curves. Hold or clamp the piezo tightly in place until the glue is completely dry.
Step 4: Connect Wires to the Piezo
Take the cable that is 1/8" stereo on one end (D from step 1) and cut off both of the connectors on the other end. Strip the ends and there should be two leads in each cable (see pic). Tin all of these leads generously.
The piezo has an outer (gold) ring and an inner (silver) disc. Take one of the cables and solder one of the leads to the gold ring and one to the silver disc (it doesn't matter which goes to which). Notice the shabby, shabby soldering job in the pic. It is more important to be quick than neat. Piezos do not tolerate heat well, so you don't want to linger with the soldering iron any longer than you have to. Just make sure that your solder does not connect the gold and silver rings to each other.
Tape the wires to the bowl. This will relieve strain on the soldered connections, protecting them from coming loose.
Step 5: Stick on Spacers
Put at least four spacers around the lip of the bowl. Invert the bowl and make sure that it rests firmly against the table. If it wobbles, adjust the spacers. Make sure that the wires can pass freely between the bowl and the table without being pinched.
Step 6: Building the Second Bowl
Repeat steps 2-5 to build the second bowl. The piezo in this bowl should be soldered to the other leads coming from the 1/8" cable.
Step 7: Download the Software
Step 8: Connect the Wires
Plug the 1/8" stereo cable coming from the two ResoDrums into your computer's audio in. Connect your computer's audio out (headphone jack) to the audio amplifier's input. Connect the amplifier's output to the wires coming out of the transducers.
The Theory (feel free to skip this part):
When you hit the bowl, energy is transferred to the attached piezo. The piezo converts this kinetic energy into electrical signal which is sent to the computer. The computer constantly measures the instantaneous amplitude of this signal. This measurement is used to run a series of ramp signals (basically averaging amplitude over time). These ramp signals control the volume, eq and pitch of sample loops (wavetable synths). The audio output for each bowl is sent to its transducer which in turn vibrates the bowl, producing the sounds. The latency (time lag) for the whole system is less than 50 milliseconds (1/20th of a second) and so below the threshold of human perception. Additionally, the acoustic sound of the bowl further masks the lag.
Step 9: Play the ResoDrums
Fire up the software! Select the appropriate audio I/O using the two drop down menus in the bottom right. There's a manual included, but for starters you can just hit the vertical row of buttons on the right to try the presets. If you get too much feedback, try turning down the outputs (or turn down your amp). If you hit one drum and it makes sound through the other, swap the wires coming out of your amp (the ones that go to the transducers).
For video examples, see http://www.vimeo.com/user930154/videos
Step 10: What Else?
I originally picked salad bowls as the main body of this instrument because they are roughly the size of hand drums and are thin enough to function as speaker diaphragms. But you could use a wide range of materials instead. Different cavity shapes and sizes will result in different timbres. Something with a larger cavity (such as a water cooler bottle) would create better low tones (and possibly some more interesting feedback).
The sound of the ResoDrums will vary considerably with the material they are set on (wood, carpet, concrete). It might be interesting to create a fully enclosed drum for a more consistent sound.
The "thump" ramp (controlling the volume of a trio of low oscillators) can be used to keep the ResoDrums at the edge of feeding back like crazy. Click and drag the frequencies (listed to the left of the ramp display) to change them. The differences between them will act something like a LFO (i.e. 134 and 140 Hz will simulate a 6 Hz LFO). See the video clip below for an illustration of this effect (especially noticeable around 0:05).