An unobstructed microphone element provides the highest sensitivity and quality recordings. However unattended and long duration recording requires some means of enclosing or protecting detectors and recording gear from the elements. A previous recommendation here suggested a scheme for weatherproof recording by listening through an elbow fitting. Subsequent field testing has shown that this method can result in a disappointing number of distorted calls. |
| Acoustic horns, as from a tweeter, improve upon elbows and provide shelter from the elements while conducting sound to a microphone. These can render good calls, but overall produce mixed results with some distorted sequences, probably from bats off axis to the horn. |  |
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Implementation and photos by Greg Green. |
| Y-tubes provide protection and an open path for sound to microphones. They can effectively sample the airspace below and outward from their placement, so if used alone to sample bats above ground, place them well above the ground*. A Y-tube or other cover can be implemented with a reflector to sample above the enclosure placement. However, this will attenuate the reception sensitivity. A lower angle of incidence, e.g., 30° rather than 45° will improve performance. The reflector should be sufficiently large to support the angular coverage of the microphone. Place the microphone element 15 cm or more above the reflector to avoid rain splatter, and slope the reflector off horizontal to allow drainage to further reduce splatter. | |||
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* recording notes: Avoid recording with a detector placed directly on the ground. Simply elevating a detector one or two meters above ground level can dramatically improve recording quality by reducing surface echoes, avoiding thermal layering, or near-ground air convection currents, all of which can distort ultrasound signals. Place detectors out of a flyway to best record routine search phase call samples. Where possible, place detectors to blend in with vegetative clutter and listen out into a flyway. The center photo above shows a good detector placement while the photo on the right shows poor placement directly in the flyway. That will interfere with routine foraging activity at that site and such a placement will often result in many recorded sequences of short "inspection calls" from bats investigating the novel object placed in their flyway. Avoid placing detectors near large echo-producing surfaces: asphalt, building facades, bridge structural surfaces, flat water, etc. When you must record near such surfaces, attempt to position the detector to listen away from these surfaces rather than toward them. When possible, use a handheld detector to acoustically sample the potential detector placement site to reveal sources of ultrasonic noise before a recording session. Many things that seem quiet to our human ears can emit overwhelming ultrasonic noise, e.g., dried leaves or other vegetation rustling in a breeze, insects, or metal structures cooling in the evening. |
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