Acoustic Perimeter

Returning along the cobblestone lane in the dark, a faint draft hissed through the base of a corroded vertical drainpipe. I halted. The curved opening was clogged by a flattened zinc washer and a bundle of dried twigs, disrupting the channel's resonance. I set my boots against the damp flagstone, wedging the lower tip of my staff into the rim. A firm, leveraged twist cleared the obstruction. The washer dropped free onto the stone with a dull ring. Air rushed through unobstructed, humming a steady, low frequency across the empty ground. The perimeter sounds clean.