Page 17 - Acoustics Today

BIOACOUSTICAL MONITORING IN
TERRESTRIAL ENVIRONMENTS
Kurt M. Fristrup and Dan Mennitt
Natural Sounds and Night Skies Division, National Park Service Fort Collins, Colorado 80525
“Autonomous acoustical monitoring is an emerging tool for terrestrial studies of ecology and animal behavior, and compelling results will be forthcoming as systems improve and researchers become familiar with their features and idiosyncrasies.”
Introduction
Many animal taxa—birds, frogs,
and some mammals—conspic-
uously advertise their pres-
ence, identity, and behavioral status
through vocalizations. In many envi-
ronments, these species are more read-
ily heard than seen. Accordingly, many
bird and frog surveys obtain most of
their data by listening rather than look-
ing. Dramatic improvements in audio
recorder technology have created com-
pelling opportunities to make long
duration environmental recordings
with compact packages. This technolo-
gy extends the spatial scope and tem-
poral extent of acoustical monitoring
and provides archival records of eco-
logical conditions. Birdsong research experienced dramatic growth as recording and spectrogram analysis technology became practical.1 Based on a sample of recent publications, environmental science may be on the cusp of similar
2-6
and frogs.
Effective and efficient implementa-
tion of an acoustical monitoring project requires careful selection of equipment and software, a process that is best informed by a sampling plan that addresses explicit measurement objec- tives. Acoustical monitoring has been successfully utilized to measure habitat occupancy, local density, breeding phe- nology and success, activity schedules, habitat use and patterns of movement, and aggregate acoustical indicators of habitat characteristics. The kind of data collected, and the equipment best suited to collect it, will depend upon the objec- tive. Nonetheless, some engineering considerations will likely be common to
all applications. This paper reviews some of these issues, and illustrates the potential scale of terrestrial acoustical moni- toring using examples from the authors’ experience.
The typical acoustical monitoring system is made up of multiple components: microphone(s), windscreen, digital recorder, data storage, equipment housing and support hard- ware, and a power source. Although each component in an acoustical monitoring system may have particular features that must be considered, three factors should be evaluated for all system components: cost, size, and power consumption. Cost will usually limit the number of systems, and in many sampling plans a large number of systems will be needed to obtain adequate spatial coverage. Often the systems will have to be placed at locations far from a road. A compact, light- weight system is needed when deployment scenarios call for multiple systems to be placed during a single hike. Power consumption is often a primary determinant for cost, size and weight, because system batteries and possibly a renew- able energy supply are usually the most bulky components and can be the most costly.
Microphones
The noise floor of these devices is critical for passive remote sensing. The noise floor limits the effective listening area of a system. As long as the instrument noise floor is higher than the background ambient sound level, every 3 dB reduction in noise floor doubles the signal capture area— assuming spherical spreading is the dominant source of attenuation. Dynamic range may be important because it will determine how close and loud an animal can be before the recording is distorted. Distorted recordings are undesirable, but loud, distorted sounds may still be identifiable.
growth in autonomous acoustical monitoring.
Strengths of acoustic sensors include long range, high fidelity, and passive operation. Compared with visual tech- nologies like camera traps or video monitoring systems, audio systems offer omnidirectional coverage that is not immediately limited by the presence of obstructions or vege- tation. Performance is not limited by available light. Audio data are more compact than video, and are comparatively easy to process automatically. There are numerous software packages that support detection and classification of animal signals. As the discussion of equipment will show, autonomous recording systems can be compact, presenting an unobtrusive visual footprint and negligible noise. Removal of the human from the system helps minimize dis- turbance to wildlife and other artifacts of direct observation. Acoustical monitoring confronts several challenges. Increasing susceptibility to attenuation with frequency, the potential for refraction to limit detection range, and the dis- torting effects of scattering and multipath propagation—sys- tems must be designed and placed to record signals of ade- quate strength and fidelity for analysis. While many animals have distinctive signatures, all biological signals are variable and waveform matched filtering will rarely be a successful approach. Classification methods must be able to distinguish within and between class variations. An additional complica- tion is sorting out overlapping voices in dense choruses. Many species will produce most of their calls in this con- text—dawn choruses of birds, nocturnal choruses of insects
16 Acoustics Today, July 2012