Parabolic Dish, Transducer
A parabolic dish will receive very weak signals and focus them to a microphone or transducer at the dish focal point. The dish will improve the sensitivity of the detector by about 30%. One drawback with using a dish is that it is directional. Its strongest signals will come from straight in front of the dish. Sounds that are off to the side are not received nearly as well. The dish works best when you can see the bats. This way you can point the dish at the flying bat and follow it, enjoying a stronger signal.
For my parabola, I purchased a 12" dish from Edmund Scientific. There is a 3/4" hole in the center. Through the hole I mounted 1/2" PVC male and female adaptors. The threads on the male adaptor must be ground down somewhat as this diameter is a little more than the hole size. The fittings will still go together tightly. On the female adaptor, on the inside of the dish, I epoxied three turkey lacers. These are small, steel, pointed shafts. These point out toward the focal point, 3" in front of the hole. A rubber garden hose washer is the mount for the transducer. I punched three holes with an ice pick, through the flat side, equidistant around the diameter. This I mounted over the pointed turkey lacers, sliding it down to about 3-1/4" from the dish. The transducer fits snugly into the hose washer - about 5/8" diameter. The wires (sheilded cable) go through the PVC fittings and handle. Keep the cable as short as possible, about 2 feet. Attach the appropriate plug for your detector ( your bat detector will need a jack wired to the transducer so the dish plug will disconnect the internal transducer and connect the dish transducer) - you're ready to hear bats at a greater distance.