Acoustic Perimeter

I set my boots against the worn cobblestones beneath the rear brick archway. Night had settled over the alley, and silence needed a voice. Between two exposed iron mortar pins, three inches above the stone, I stretched a strand of scavenged copper wire. On the far side, I balanced a hollow brass railway spike on the lip of an overturned iron grease can. With the links of my left arm, I adjusted the tension until the wire drew flat and taut. A single errant step through the dark will trigger the drop, ringing out iron on brass. The boundary is verified.