BCM's Top Recommendation
for Passive Recording:

Pettersson D500x

D5002014FD1 Bat detectors that are appropriate for passive monitoring have features such as:

The Pettersson D500x was influenced by BCM and other bat workers specifically to obtain the highest quality sound recordings and be user-friendly to arm, troubleshoot, and retreive data. It has seen only minor updates since release in 2008, a testament to Pettersson's knowledge of bat acoustic technology and attention to detail and persistance to "get it right the first time." Users have benefited from high-quality, consistant data, year after year without being subjected to major hardware changes that can compromise data, torpedo projects, and end up costing more in the end with system upgrades - and most costly of all: excessive time expenditure for the user. The Pettersson (and Binary Acoustics Technology) microphones must pass a rigorous quality control/quality assurance before they leave the shop, providing high quality and consistency between units that has not been found in other manufacturer's offerings. In addition, Pettersson and BAT mics have been field-tested in 7 locations across the country to be equally sensitive on-board and on external cables up to 50-feet with no-signal loss, no other current manufacturer can claim this type of quality and reliability.

Extensively deployed at BCM workshops along with other similar detectors from other manufacturers, the D500x is the "gold standard" we compare others to for recording-quality and ease-of-use. This device has proven to be the easiest "all-in-one" recording solution for newcomers to most quickly grasp, program, and successfully deploy and therefore wins BCM's recommendation for best recommendation for passive recording. The "Grab and Go" kit includes everything needed for a short term passive monitoring survey (USFWS suggested protocol) in a weatherproof deployment/storage box, while the FD Special Edition includes a live audio output useful for active monitoring.

For information about some of our hardware tests, see the Chenger/Tyburec poster Comparing Bat Detector Deployments at Different Heights, In Different Orientations, and Using Different Microphone Types.


One way to rate a recording system, is to look at how consistently it picks up high frequencies. The higher frequencies attenuate in air very rapidly, so this is a good test of the quality of the detector's microphone. Below we have the same exact bat pass recorded by 4 different microphones. The white ruler on each image is set to the highest rendered frequency. Starting with the D500x, we see it recorded frequencies up to 90 kHz. The AR125 recorded up to 85 khz, the MiniMic reached 77 khz. The Wildlife Acoustics SM2 lagged behind, topping out at 67 kHz. Notice it has a lot of oversaturated pulses, suggesting it’s deficiency is not due to the range of the bat, but its probably because the microphone itself simply is fundamentally not as sensitive to higher frequencies as the others. Also all SM2/SM3 devices should be set to record at 384 kHz sampling rate; if the lower 192 kHz rate is used, these devices will not record frequencies above 96 kHz leaving certain species such as Northern long-eared (Myotis septentrionalis) and Fringed Myotis (Myotis thysanodes) underreported or misclassified.



Below we are looking at the same exact pulse recorded with 4 different microphones. A high quality recording features a crisp pulse, with good signal-to-noise ratio (SNR). Here we also have a harmonic indicating that what we are looking at is likely the entire fundamental signal, and we have not lost high-frequency data because of range or attenuation. While the SM2 seems to have good SNR, but because it’s not recording high frequencies, it is recording shorter overall pulses. This will have consequences when it comes to automated call classifying. Click here to view more microphone side-by-side tests.




BCM's Passive Recording Comparison Chart
a.k.a Passive Detectors for USFWS "Indiana Bat Acoustic Protocol Guidelines",
NABat Project, or any other similar professional level project where results matter.




See also: BCM's top pick for:

Passive Monitoring

Active Monitoring

Mobile Transects

Automated + Manual Bat Call ID Software