I Could See Them Hunting but Heard Only One

Bats were the only thing I could see, and that was the cruelty of it.

Bats silhouetted over a river valley at dusk, a small homemade detector held in the foreground

Half past nine, the light gone that bruised violet it does over the river in late August, and there were four of them — five — wheeling low over the water where the mayflies were coming off. Unmistakable. Not the flap-glide of a nighthawk but the jerky, stalling, mid-air-correcting flight that only a thing chasing insects by ear can manage. Every few seconds one would snap sideways and drop ten centimetres, which is a bat closing on a bug. I was holding a detector built specifically to hear that. It heard nothing.

Let me back up to the bench, because the failure was already soldered in there. I just didn’t know it yet.

The circuit is honestly simple, and that was the trap — it looked too much like things I’ve built before. Ultrasonic capsule into a couple of op-amp gain stages, the amplified signal into a mixer, the mixer fed by a local oscillator I could sweep with a ten-turn pot. Beat the incoming ultrasound against the LO, keep the difference frequency, drop it into a small speaker. If a bat calls at 42 kHz and I’ve parked the oscillator at 40, I hear a 2 kHz tone — the offset, not the call itself. That is precisely the move from the theremin I built in the spring: a hand shifts one oscillator, it heterodynes against a fixed reference, and the difference becomes a pitch you can actually hear. The maths did not scare me. I’ve been beating oscillators against each other since I got my first receiver working.

(The man who built the first detector like this, George Washington Pierce, also has his name on the crystal-oscillator circuit inside half my radios. I learned that afterward and it stung a bit, like the universe underlining a point.)

So there I am on the riverbank, sweeping the dial, and — nothing. Hiss. Once, thrillingly, a stutter of dry clicks that ramped up into a wet raspberry and vanished in half a second: a feeding buzz, the sound of a bat’s call rate spiking past a hundred pulses a second as it makes the grab. One pass. Then forty minutes of me working that pot back and forth under bats I could plainly see, convinced my solder had gone cold or the LO had quit, while the valley fed above my head in total, insulting silence.

The capsule was the problem, and it’s a good enough failure that I’ll spare you the same night. I’d used a 40 kHz ultrasonic transducer — the cheap silver cans sold in pairs for distance-sensing robots. They are wonderful, and they are a resonant piezo element, which means they are ferociously tuned to a single frequency. My “tunable” detector could sweep its oscillator wherever it liked, but the ear on the front only heard within maybe two or three kilohertz of 40. The oscillator was never the band-select. The transducer was. And 40 kHz happens to sit near a little brown myotis and almost nothing else around here. The big brown bats calling down at 27, the hoary bats booming at 18 — deaf to every one of them, by construction. I’d built a receiver with the front-end filter epoxied shut and then cursed the tuning knob.

That single feeding buzz? A little brown that drifted close enough for its 40 kHz call to fall into my one open window. Everything else over the river was a species my hardware simply refused to acknowledge.

The fix isn’t a bend of wire. It’s a broadband capsule — a MEMS microphone that hears roughly flat from twenty kilohertz up past a hundred — so the oscillator finally gets to do the tuning I’d assumed it was already doing. Different part, same board. I ordered one before bed.

What I can’t shake is that I’ve made this exact mistake in other costumes: trusting the loud, obvious component and never once interrogating the quiet one that was actually deciding what I got to perceive. The moths, at least, came to the light on their own terms. The bats hunted them all night, ten metres up, and let me listen to precisely one of them.