In the southeastern United States, the ring-tone chirping of crickets at night is accompanied by a percussive rattle that's common from mid-summer through fall. Where I live in northwestern Virginia, the dominant percussionist in our nighttime chorus is the Lesser Angle-wing Katydid. Similar to cicadas in timbre, but far less annoying, these bugs "shake their rattle" intermittently, calling once or twice and then going silent for five to fifteen seconds. Compared with a regularly chirping cricket, the delay adds complexity to an analog audio "Katydid circuit." You have to create the timbre and tempo of a rattle sound, and then you have to space out "packets" aka "envelopes" of this effect with silence in between --ideally randomized for natural effect.
In a first step, I built a machine gun circuit using a 555 timer. While the tempo is right, the timbre of a square wave is dull compared with a Katydid's sizzle and resonance. I reverted back to my cricket strategy and built two discrete transistor astable multivibrators, but that didn't work either because the simplicity of my cricket circuit depends heavily upon the acoustical properties of a 2.8 KHz piezo disk element. Katydids are resonant in a different frequency range than crickets. To make a long story short, I had to build a filter.
After some reflection on the sound quality of a Katydid, I decided that I needed a filter for "snare" or "high hat" as used in analog drum machines. Research led me to this very handy circuit by A. MAGIC PULSEWAVE:
With some minor adjustments (not needed for functionality--purely my preference), this circuit works perfectly to add sizzle, resonance, and noise to the output of an astable multivibrator. Note that I decided against a piezo disk element because --for a Katydid-- I prefer this little speaker (now obsolete but hopefully similar exist or I have more reverse engineering in my future). Here's a video clip to demonstrate the Katydid sound timbre. Notice the subtle complexity thanks to amplified noise from the abused bipolar transistor (gosh, horrible sentence out of context).
Since I'm on the topic of speakers and timbre, here's another video showing an overview of the circuit (including a second astable multivibrator for regular chirp) in which I demonstrate the Katydid's rattle through several different types of speakers. I end with a cool way to "play" a piezo disk in its housing. Electronic musicians: even if you find the earlier part of this video boring, check out the last 10 seconds for inspiration on how to physically manipulate the output of your synth to get a new electronic sounds.
Because I am piezo-obsessed, here's one more clip showing a bare piezo disk element as the output for this circuit (which provides sufficient amplification that my custom printed circuit board is not needed). I mess around with different connections to the feedback of the piezo (white wire), the last seeming to make a change -- a potential source of interesting effects that warrants further exploration. At the end of the clip, I show how the piezo sounds (differently) in different positions inside of a plastic housing.
In my next video, I show some of the cool effects that you can get by playing around with capacitors in an analog circuit. Be forewarned that you risk blowing one of your components (so careful if you don't have replacement parts). It amazes me how much one capacitor can change the behavior of a circuit -- and this fact extends to resistors and diodes as well. Especially note the weird chatter sound that I get about 1/2 way through this clip (start at 00:36).
So what does this say about Katydids --the ease with which capacitance alters "their" sound in an analog electronic circuit? Of course, my entire approach to hacking nature's musicians could be a logical fallacy. Maybe analog electrical engineering tells us little or nothing about the inner workings of nature and I just *feel* like there's a comparison to be made. Oh well. There's no accounting for the compulsions of an artist. But seriously, all you entomologists out there, what do you think?