The first step was to tie the drum pad together in an attempt to hold all the pieces together. I cut a circular base from plywood, then used a hacksaw to cut the foam to match. The foam looks terribly jagged - I think it needed some kind of hot-wire cutter for that. I got some 1/4" shock-cord (at REI, this is what they call this bungee cord) and cord-stoppers (clips used to close up back-packs, etc), then I cut holes in the drum head, foam, and base through which to thread the shock-cord. The hardest part was punching holes in the foam and threading the cord through it. The drill won't cut; the foam just got all wrapped around the bit! Forcing a 1/4" rod through the foam worked pretty well, although it was sill nearly impossible to thread the shock cord through the foam. Once the shock cord was threaded and cinched up, I glued some 2x2s to the base so it didn't rock back and forth on the shock-cord and cord-stoppers underneath. I used Gorilla Glue, which is really strange stuff, attach the 2x2s. As it hardens it foams up like polyurethane and can be a big mess if you use too much. The last step was to attach the mouse mats to the top surface of the masonite with silicone rubber glue.
After all that, it was time to hook it up. I connected a mini stereo plug to the drum and plugged that into the input jack of a laptop running XOscope. This is a pretty neat program. The controls are pretty intuitive, but I'm not going to be able to get any useful amplitude data until I figure out some kind of calibration work-flow. The other thing I didn't do which I should have done was to put some protection on the inputs - I could have damaged the PC's audio circuit - or worse. Note to self: next time be more careful!
Here's a trace of a hard DON strike. Right click and open the graph a new tab to enbiggen it sufficiently. The green trace it the drum head and the blue trace is the rim. You can see that the green trace is clipped, but what I was looking for was how long it resonated for. This way I could know when to start listening for the next strike. The graph shows that the signal took about 75 mS to settle down.
The other thing we can determine is that the resonant frequency is 4/0.02125, or 188 Hz
What I see there is that the drum head needs more damping. I want a single bounce - like when you push down on the fender of your car to see if the shocks are shot.
Below is a plot of a softer strike; one that 's not clipped. From it we can obtain the damping coefficient Tau, or the time it takes for the sine wave component to decay to 0.27 times its origin amplitude. It looks like Tau is about 0.017 Seconds.
The other thing I see in this plot is cross-talk. Striking the center of the drum results in a signal on the rim sensors. Cross talk's roughly (0.25/3.5)*100% or 7% and that's not too bad.
Tapping on the rim shows a quite different crosstalk story. Here the cross talk is (0.5/1.2)*100% or 42%. Something will need to be done to improve this. I think I many need to cut a slot in the foam to separate the rim from the drum pad center, but I'm concerned that the remaining foam will be too thin to support the rim. Maybe the foam will need to be replaced by a stiffer elastomer.
From this base-line data, I'll be able to see if changes I make to the construction of the drum pad improve or worsen its performance.
