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.
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. Unfortunately, the housings and associated hardware of Y-tube and reflector arrangements still can generate reverberating echo distortion.
External microphones provide much better options to deploy and acquire high quality recordings with minimal distortion and echo noise. These arrangements enable placement of microphones out in the airspace away from echo-producing surfaces.
Pettersson, Binary Acoustic Technology, and Wildlife Acoustics all assert the weather resistance of their microphones, so long as not pointed vertical. Even the BAT AR125 element can resist weather if the electrical connections have protection from moisture.
Microphone under aluminum angle.
Simple external microphone mounting brackets can be fashioned using aluminum angle as shown above and below (right). The angle provides cover for the electrical connections and enables extending the microphone away from echo-producing surfaces. The brackets shown here were formed using 3/4″ x 1/16″ aluminum angle and spray painted. For best results, use a pair of flat crescent wrenches or another pair of gripping tools to flair out the angle where you intend to make the bend. Use larger angles as appropriate for other microphones or for longer extension.
In most situations, the higher off the ground, the better. Consider that the microphone picks up good recordings from smaller bats within about 20 ft (6 m). If so, then you can optimize the detection volume of the microphone by placing it at 20 ft above the ground to pick up bats 20 ft below the microphone and 20 ft above it. This provides much better sensitivity and recording quality than attempting to record from ground level with a reflector.
In some situations, incorporating local materials can provide convenient recording options, particularly for simple overnight recording. Unfoliated branches can avoid echoes and easily blend into background clutter and avoid preventing novel structures that bats may investigate.
* additional recording advice: 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. In general, the longer duration calls that many species produce in open air flight, i.e., away from clutter, provide more information content and greater species-discrimination confidence. Bats flying in confined spaces or near roosts will generally provide shorter, less discriminating and perhaps ambiguous call variants. If you need to identify bats in such situations, try to record them on approach to such a space or follow them out and away from a roost to acquire longer and more representative search phase calls.
To record search phase call sequences of bats along a flyway, place detectors out of the flyway as bats may investigate the novel object resulting in many recorded sequences of short “inspection calls.” Where possible, place detectors to blend in with vegetative clutter (but clear from it) and listen out into a 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, loose cables and other windblown components, or metal structures cooling in the evening. Detectors with microphones on cables separate from the detector electronics provide the best options for placement and best results.