AudioMoths are among the most popular - and in my opinion, serviceable - in a spate of low-cost programmable recorders now available. They are pretty cheap (+/- $60 USD a unit) and easy to deploy: basically, you load it up with three AAA batteries and either deposit the unit as is or drop it in a zip lock bag to protect from rain damage (with some signal attenuation, as a result). The 400-2000 Hz bandwidth detected by the AudioMoth sensor can be adjusted so that the units record everything from low-frequency gunshots to high-frequency bat vocalizations beyond the range of human hearing. 

Compared to other autonomous recording technologies - such as the Song Meters I cover elsewhere here - this device really stresses customizability. Like Song Meters, AudioMoths store uncompressed WAV files onto an SD card, which can result in several thousand recordings over a several month period before battery changes are needed. But AudioMoths support a range of additional user-led specifications too: such as the ability to make extensions to the board, e.g., external sensors or a wireless network unit. The acoustic sensor, moreover, can be programed with different kinds of classification algorithms that trigger different kinds of "event logging" (basically, the ability to be programmed to filter relevant sounds such that only those of interest are saved, thus reducing post-processing time, power usage and data storage requirements). As such, this tool is very much made for tinkering: and the free available hardware design and associated software will be a boon to those with the requisite computer programming skills (and a source of confusion and head-scratching for those who don't).

As the picture suggests, the idea is really to work with AudioMoths in the plural, and not the singular unit. They implied tradeoff here is one of quantity and extent: by reducing equipment costs and sacrificing expectation for high-definition recording quality, the AudioMoth promises to make it possible to cover large areas of inhospitable ecosystems with fields of sensors, much like a cabled hydrophone network underwater, and come up with different kinds of studies accordingly. I have yet to ascertain how durable the units are: and to be sure, Song Meters provide much more outer protection and can be used in very much the same way (at a much higher cost, of course). Nevertheless, AudioMoths have already been used to acoustically map large areas (such as the foraging habitat of the Cuban greater funnel-eared bat), and to search out difficult-to-find species (such as the New Forest cicada, an insect that’s probably extinct in Britain). But if the promise of the AudioMoth is to vastly improve the spatial and temporal coverage of certain surveys, it still provides surprisingly decent sound at close range!

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