Digital Mist Netting
Anyone who has sat beside a mist net knows that most bats that approach the net avoid it, and many others just fly by out of its reach. What bats are those? Well, those bats may avoid the net, but they don't avoid emitting echolocation calls that you can automatically record with a bat detector and analyze using SonoBat.
Certainly there are reasons to capture bats, e.g., to attach radio tags for tracking. But for a goal of species presence, digital mist netting offers an easier and typically more effective solution.
Why use a recorder?
At first everyone using a a Pettersson 240x bat detector simply plugs the detector output into the microphone port of a computer. Incoming calls are recorded manually in SonoBat or using the SonoBat Autorecorder application; all is well.
One advantage of acoustic monitoring is that multiple detectors can be deployed in areas that are difficult or impossible to sample using traditional mist nets. The logistics of maintaining multiple detector/laptop setups in the field quickly spirals out of control, as there is often few good options to keep a typical laptop running for an all night unattended field session.
Historically, people have recorded calls onto voice activated tape recorders. As the recorder senses the beginning of the Pettersson output, the magnetic tape starts up and records the sequence. Besides concerns of tape stretch, hiss, and speed changes due to battery condition, most people have to recapture the sound in real-time into a computer before any analysis. This means a lot of time. A lot.
Saving calls on a digital medium has the promise of accelerating the time wading through any given night's collection of call sequences. There are pitfalls to avoid, as most digital recorders manufactured total fall short of being ideal for bat work. We have bought, experimented with, and subsequently thrown out a number of recorders in the last few years and have created a criteria by which to judge digital bat recorders.
44 kHz sample rate. This is a high quality recording rate suitable for bat recordings.
Zoom H2: The digital mist net
Laptop free recording for up to one night
Accessory for the Pettersson D240x
Samson Zoom H2 setup instructions:
Currently available and in stock from electronic retailers, Samson Zoom H2 digital recorders work well for an evening of auto recording. These units accept a line in connection from a Pettersson D240x or other detector and record directly to an onboard flash card in .wav format readable by SonoBat. Samson Zoom H2 units cost about 200 USD with a 4 GB card.
Zoom H2's do not automatically parse incoming signals into separate files. Instead, with this type of unit you record a single file, noting the start time, and then parse the bat pass events into single files with a date and time stamp in post-processing using the SonoBat AutoParser. For optimum convenience, leave the large recording file on the Zoom H2 and have the AutoParser read directly from it and save the parsed files to a directory on your computer or elsewhere.
Use these settings to record from a Pettersson D240x to a Samson Zoom H2:
When starting a recording session, push the record button twice to start. When recording, the Zoom H2 will display a constant red LED indicator and the LCD display will show the recording time counting. The Samson Zoom H2 only records in stereo. When AutoParsing the recording file select the right channel. The AutoParser has help messages to guide you.
H2 Zoom: $219.00 USD
In stock for immediate shipping.
Pettersson D240x sold separately.
Download a PDF brochure on the H2 Zoom.
Voice activated recording. Required so that the recorder and the auto-triggering Pettersson 240x can be left unattended. With the SonoBat AutoParser utility, we can now use any recorder up to the maximum length the recorder can record.
Digital Voice Recorders: These devices typically have low sample rates (16 kHz) and are fine for normal voice jobs like interviews. They often have the all important voice activated recording feature, and are inexpensive. Most will time stamp when the recording was made. High end models may support 44 kHz sample rates but may only crippled with low memory capacity (128 MB). In addition, most feature an automatic gain control, which floods the resulting files with noise between chirps, rendering them usable but extremely distracting.
Ipods. While we would love to use a hard drive based Ipod as our week long recorder, only one manufacturer currently makes a voice recorder accessory sampling at 44 kHz. It cannot be voice activated, nor can the Ipod be charged while in use, severely limiting the use.
MP3 Player/Recorders. Some of these devices have large memories, rigged for external power, and offer line level recording at 44 kHz. A precious few are also voice activated, while others are only manual recorders. Few, if any, time stamp when a recording is made. Unfortunately, newer MP3 players such as the current Iriver T30 series do not have the voice activated recording feature, severely limiting their use.
Current recorder options:
Longer term recording arrangements are possible with higher capacity units such as the iriver H320. These and other recording units can be configured using Rockbox open source firmware to record at specified times, auto-trigger, write files with a date and time stamp, and record using lossless data formats.
Although ideal for wildlife recording, iriver no longer produces the IFP series units. However, they can still be purchased through outlets such as ebay. Newer models limit recording capability, most likely over concerns regarding music copyright infringement. Other digital recording products to consider include the Samson Zoom H2 and the Edirol R-1.
Samson Zoom H2
Leading bat workers well versed in the complexities of bat acoustics have experimented with numerous models and found the Zoom H2 by Samson to be the ideal companion to the Pettersson D240x detector.The Samson Zoom H2 can be connected via a line-in audio cable from the Pettersson D240x (or any other detector). It records in .wav format, directly to an on board, 4G flash memory card. The Zoom does not automatically parse incoming signals into separate files, but instead stores them all into one continuous, large file. But by noting the exact start time of the recording, and running the AutoParser utility in SonoBat, this large file can be split into discrete call sequences, each with its own appropriate date and time stamp.