Tradeoffs in sound quality and cost for passive acoustic devices

Abstract

Ecologists often collect data with automatic sensors such as passive acoustic devices. Traditional commercial passive acoustic devices that record ultrasonic sounds are usually > 700USD; less costly options now exist but the sound quality is unknown. We compared a low-cost passive acoustic device to a traditional device for quality of bat call data recorded. We simultaneously deployed two device types at seven sites to record free-flying bats over three nights. We paired call files from both devices when they occurred ≤ 3 s apart and contained ≥ 3 pulses; for each file pair and pulse, we measured indicators of recording quality, specifically duration, maximum frequency, and bandwidth. We compared the proportion of files classified as low frequency, mid frequency, or Myotis bats. The low-cost device recorded lower quality pulses (i.e., shorter duration, lower maximum frequency, and shorter bandwidth). The more expensive device recorded a higher proportion of Myotis calls and a lower proportion of low-frequency calls, but there was no difference across devices in the proportion of calls classified as mid-frequency bats. Less expensive alternatives for passive acoustic devices may be inappropriate for studies requiring high-quality data but may be critical when research objectives call for multiple devices and are constrained by budget.