David’s Bat Blog has a details of a bat detector (that’s “bats” as in “belfries”), partially inspired by sound mirror design.

The prototype is only 60cm by 20cm, so I wasn’t expecting miracles, but tests at varying ultrasound levels showed that, using the reflector a Duet bat detector could pick up the artificial bat between 30% and 70% further away than it could on its own. Positioning of the detector microphone is critical, as it needs to be at the precise focal point of the reflector for best results.

More at Bats, bombers and acoustic mirrors.