Step 1: Parts & Tools
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
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
Step 4: Connect Wires to the Piezo
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
Step 6: Building the Second Bowl
Step 7: Download the Software
Step 8: Connect the Wires
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
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).