While playing songs on the floppy drives I couldn't help but notice that different notes played at different volumes, and the 2 drives I had had very different responses to the notes.
To see what was going on, I wrote a quick little program to go through a range of notes and measure the volume of each one, per floppy drive. These are the results:
The X axis is the MIDI note number. Look for a MIDI note chart to see what frequencies those are or what piano keys they would be.
I made measurements in 2 units, RMS and Peak, per drive. Peak is pretty much the highest sound in that 25th of a second or so, averaged over 5 seconds, while RMS is a "Decibel" of sorts. It's a log scale, but it's just an arbitrary unit because calibrating it to the Decibel scale would be way too much effort for this little program.
There's quite a bit of interesting information on the graph here. Let's start with drive 1 (green). It's pretty clear that it's best range is between 23 and 51. After that it goes surprisingly quietly. Until it reaches note 66, which I would guess is it's resonance frequency. It's important to note that this graph only shows volume when the note command is sent. It does not necessarily mean it was actually making that note, as is the case for values above around 72 or so, where the drive starts stuttering from struggling to keep up with the high frequency. At note 84, the motor just gave up and the drive stood completely still. At this point, I need to physically disconnect and reconnect the power to get the drive working again for lower notes.
Drive 2 (red) is pathetic. It is quite soft for most notes, and gets a little inconsistent in volume where drive 1 starts going softer, though it can actually play notes 65 and 66 properly. It also survives through the whole range of notes, even though it has a much smaller stepper motor.
I'll repeat this test when I get more drives. I'd like to incorporate this data into my program somehow, but I'm not too sure how I would do so. One thing's clear though: Never send note 65 or 66 to Drive 1, unless we're trying to make a very weird kind of dubstep. Also, avoid notes above 72 which temporarily disable Drive 1.