So I take a walk outside to bring the cat in, like I do every night, and the little critter is hiding out by the fence. It’s just one of those chicken wire fences with the wooden slats. So I’m crouched down calling the cat and snapping my fingers.
Then I hear: Chirp. Chirp. Chirp.
It’s night time. There are no little birdies flying around. And every time I snap my fingers, I hear this chirp.
Then I clap my hands, and get the mother of all chirps: NEEYOWMP.
Finally I figured out what was going on. The regular spacing of the slats was diffracting the sound, splitting it into its component frequencies. Since low frequencies are diffracted more, they took longer to echo off the fence, and the result was a reverse crescendo.