Cost-effective monitoring: comparing the range of ultrasonic detection for two autonomous recording units

Effective wildlife monitoring generates valuable information, but it needs to be cost-effective because conservation funding is scarce. Passive acoustic monitoring facilitates monitoring and occupancy estimates of cryptic species, but is rarely used for terrestrial mammals vocalizing in ultrasound (> 20 kHz). Here, we compared the recording quality of two autonomous recording units: Open Acoustic’s AudioMoth and the Wildlife Acoustics Song Meter Mini Bat 2. In June 2024, we conducted an experiment in north-east Thailand, playing modulated ultrasonic sounds in three forest environments. We conducted 108 trials at varying distances during the day and night. We measured the average decibels for each trial and found that the AudioMoth consistently provided equivalent dB readings of ultrasound on spectrograms and detected ultrasonic tones from further away than the Song Meter Mini Bat 2. These data indicate that the AudioMoth could be a cost-effective option for detecting modulated ultrasound at distances of up to 32 m. Cost often limits deployment of monitors, and the ability to maximize efforts while minimizing cost is essential for improved species monitoring.