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ACOUSTIC001

© Bat Conservation and Management, Inc., Janet Tyburec, and John Chenger, 1998-2014. Absolutely no rights of distribution by sale or other transfer of ownership or by rental, lease or lending, preparation of derivative works, or reproduction, in whole or in part, is granted. No text, graphics or photos may be downloaded and used on another Internet site, without express permission of BCM, Inc. For information on obtaining photo useage and rights, please see our contact page. BCM, Inc. reserves the rights to actively protect against infringement.

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Even though each detector maker also produces its own signal-analysis software, the SonoBat suite is one of the few programs that is not only designed by a software engineer, but by one who is also a biologist, specializing in bat physiology and who has an excellent concept of bat detector applications in the field.

The result is a perfect storm of user-friendly interface with relevant, easy to obtain, analysis options.

Furthermore, current upgrade packs allow for automated signal analysis at never-before-achieved accuracy in echolocation call classifications.

ACOUSTIC015DR detectors generally have no audio output.

The AR125 will produce an audible signal thru the S’pectr software.

DR detectors convert the incoming high-frequency call directly to a digital format and store thru USB port (AR125), onto CF card
(D500x) or SD-HC card (SM2).

Because each audio-file is a friendly WAV format, signal analysis is possible thru popular software programs: Bat Sound (Pettersson units), Spect’r (AR125s), SongScope (SM2), and
SonoBat which will display and analyze any bat calls recorded in WAV format.

ACOUSTIC014Like the TE detectors, DR units are capable of preserving all spectral components (amplitude, harmonics, signal strength, etc.) of the original, incoming bat call. The microphone is very sensitive, and provides a broad band conversion of the entire frequency time scale for bat echolocation calls.It does not need to be manually “tuned” to record calls. The sound file output is in WAV format and can be easily read by popular sound-analysis software packages.

Call rendering and analysis must be done passively, precluding observation of bats and their flight characteristics while viewing sound files. And like the TE units, DR detectors output large WAV files that are between .5 and 5 MB in size.

DR units are excellent for passive surveys, especially long-term surveys, because the microphone/receiver and recording media is usually fully integrated. Also, because they record in full-spectrum and are capable of recording hundreds, thousands, tens of thousands of call files, outputs are well suited for use with automated classification software packages. They are also easy to deploy during mobile, passive transects.

ACOUSTIC013Direct Recording (DR): DR detectors output the original, unaltered, high-frequency echolocation signal. Such an output is then recorded on a device capable of recording high frequencies (e.g., a computer with high-speed sound card or a high-speed tape/digital recorder or stored on a Flash memory card).

The three main types of DR detectors currently available are the BAT AR-125/FR-125, the Wildlife Acoustics SM2, and the Pettersson D500x.

SPECT’R software available from Binary Acoustic Technology (Tucson, AZ) allows realtime views of direct-recording bat calls collected with the AR-125, but because the calls come in so quickly, and the software does not contain a real-time “compression” feature, it is difficult for users to discern subtle call characteristics in a sequence as can be done with both the FD and TE analysis packages. The BAT FR-125 detectors will store incoming calls to FLASH storage media for later analysis.

The D500x and SM2 detectors are also DR detectors suitable for passive deployment. These contain four slots for compact flash (CF) cards or four SD cards up to 32G in size respectively, providing plenty of digital storage space for incoming full spectrum calls.

ACOUSTIC012Software packages designed to display and analyze bat echolocation sounds include typical frequency/time and amplitude/time outputs for a visual display of the call shape and size and how the frequency (x-axis) changes over time (y-axis) as well as an oscilloscope display of relative sound pressure level (SPL) at a given portion in the call to indicate amplitude/time values.

In this display, the frequency/time parameters are represented by the blue sonograph in the main black window, with a “color weather radar” representation of sound intensity along the frequency/time slopes of the individual call pulses. The amplitude/time parameters are represented by the bright green spectrograph at the bottom of the main black window. Zooming in on an individual call and its corresponding oscillogram will show the detail of the amplitude difference in the waveform oscillation. The
SonoBat program is able to quantify this during analysis.

ACOUSTIC011TE units, notably the Pettersson D240x, provide various outputs and opportunities for signal recording and analysis.

The real-time HET signal of a D240x can be played thru the on-board speaker, broadcast thru headphones, or transferred to any analog/digital recording devices (tape recorder, MP3 recorders, laptop computers, etc.)

The detector also produces a time-expanded signal which is lower in frequency and slower in time than the original input. This allows any modern day digital recorder to sample the entire spectral components of the incoming sound and digitize the entire time/frequency - amplitude/frequency content of the sound. This includes the recording of harmonics.

The sampled sound is typically recorded in *.WAV format, allowing for a nearly unlimited playback and analysis of the bat echolocation call using standard sound manipulating software, from iTunes and Garage Band to sophisticated bio-acoustic programs designed specifically for the display, playback, and analysis of high-frequency bat echolocation calls (e.g.,
SonoBat, Avisoft, SongMeter, Spect’r, BatSound, etc.

ACOUSTIC010The advantage of time-expansion over both FD and HET is that TE detectors are capable of preserving all spectral components (amplitude, harmonics, signal strength, etc.) of the original, incoming bat call. The microphone is very sensitive, and provides a broad band conversion of the entire frequency time scale for bat echolocation calls. It does not need to be manually “tuned” to record calls. The sound file is saved in a small buffer built into the device and must be replayed into a recording device 10x slower than real time (hence, TE). This allows calls to be analyzed by sound analysis programs, including BatSound (Pettersson), SonoBat, AviSoft, etc.

The main drawbacks of TE detectors is their high cost and the loss of real-time sonograph rendering which precludes simultaneous observation of the bat and its echolocation call.

And, because the detector samples the incoming call and then plays it back at a rate of 10 times slower, the effective sampling time is reduced to 10% of real-time. (Leaving 90% of the night effectively un-sampled.) While the current call is being stored and/or played back, other incoming calls may be missed, thus under-representing bat diversity or abundance. This can become especially troublesome in noisy environments where random high-frequency sounds (i.e., insect noise) could trigger the detector often, rendering it unresponsive to actual bat calls

In field trials with these units in side-by-side tests with AnaBat and the direct recording detectors indicate that when approximately 100 calls are recorded over a 2 hour period in AZ and NM, between 2-5 calls would be missed on the TE detectors due to the 17-second lag between expanding and saving a call. That is, in most situations, bat calls do not come in in less than 17 second intervals very frequently, so functionally, very few bat calls are actually missed.

Finally, because a WAV file is generated for each recording and WAV files are rather large, on the order of 500kB to 5MB, digital storage space for TE recordings becomes an issue.

TE detectors can be used in a quasi-active state . . . rendering calls 17 to 34 seconds after the bat has been viewed, but their true application is for passive surveys. TE detectors can be hooked up to certain MP3 players (e.g., the iRiver or
Samson Zoom; the latter being the better choice for getting time-stamped files) for passive recording and later automated ID using the Sonobat 3.0 software. They have also been deployed for mobile transects. But, some acoustic researchers reject the TE detectors because of the time it takes for the units to download and save the calls will presumably interfere with the ability of the detector to detect new bats, therefore missing substantial numbers of potential call sequences. Finally, TE detectors are essential for collecting voucher calls for sophisticated full spectrum analysis, comparisons with unknown calls, and building an echolocation call library.

ACOUSTIC009Time expansion (TE) + full spectrum (FS): TE conversions record the original high-frequency, broadband call and can play it back at a slower speed. This allows digital outputs to capture the entire signal, retaining important information about original signal strength (amplitude) and spectral components (e.g., harmonics). Why this somewhat convoluted approach? When the concept was invented, field computers of the day could not handle the data processing requirements to instantly record the sound at such a high sample rate necessary to record bat calls. TE detectors collect the high quality ultrasonic recordings, retaining all call parameters commonly used in species identification. Users of a Pettersson D240x select how much of the original signal to record (1.5 or 3.4 seconds @ 307 kHz) and the detector expands the bat pass by a factor of 10, producing a detailed output 15 or 34 seconds long which can then be recorded at a relatively low processing rate of 44.1 kHz onto any computer/MP3 recorder. This "long" recording is digitally reassembled and analyzed by popular software programs, including SonoBat with very high precision. Despite being a "legacy" method, TE still holds it's own for active monitoring purposes, voucher calls, and even passive recording where the data can be retreived each morning.

ACOUSTIC008What does the typical AnaBat output look like? Even though the AnaBat will only respond to the strongest (loudest) bat call in the soundscape, it is still possible to record quiet bats and two different species in a single call sequence.This call sequence was recorded in Nevada. The faint call pulses, represented by the steep, nearly vertical, widely spaced dots, sweeping from upwards of 70kHz down to 20kHz represent MYOTHY which has a broad bandwidth, short duration call. The darker, more solid pulses, represented by the shallower sweeps from 55kHz to 45kHz likely represent PIPHES, with its narrow bandwidth, longer duration call.

At the beginning of the sequence, the MYOTHY was likely closer to the microphone, therefore its call pulses were picked up, then the PIPHES moved in and the detector triggered on its calls instead. Mid-way thru the call, the two bats were picked up alternately, as their relative positions and/or intensities triggered the detector. By the end of the sequence, the PIPHES had moved out of the cone of reception of the microphone, and once again the MYOTHY was the dominant species detected.

The AnaLook software allows the user to separate this single recording of two bats into two separate call files; one for each species. This will allow each species in the sequence to be analyzed and call parameters extracted separately.

ACOUSTIC007Frequency division (FD): FD detectors use a broadband conversion, transforming the entire bat call in real-time. The original high frequency call is converted to a "square" wave and divided by a user-selected ratio (usually 4, 8, 10, 16, or 32). The detector counts how many times the original high-frequency sound wave cycles from negative to positive, producing an output signal for every 4, 8, 10, 16, or 32 “zero-crossings.” This results in an output that is 1/4th, 1/8th, 1/10th, 1/16th, or 1/32nd of the input frequency.

The user-selected division ratio helps provide efficient processing. Low frequency calls are better represented at low division ratios, and high frequency calls should be recorded at higher division ratios. For most North American bat species a division ratio of 8 or 16 is used. For many of the low-frequency bats, especially big-free tailed bats, spotted bats, and lappet-browed bats out west, a division ration of 4 or 8 is better.

Though FD detectors are less sensitive than HET (or TE) units, they are capable of producing accurate representations of the bat call, including important information about the shape, slope, and characteristic frequency. This is helpful because these parameters are often used in species identification.

ACOUSTIC006Because the output of a FD detector is so much lower in frequency than the original signal, the sampling rate required to digitize the signal is low enough that a precise representation of the call’s duration and frequency-time course can be digitized and displayed in real-time. And, because the detector uses a “broadband” conversion process, the entire bat call is recorded, eliminating the need to “tune” the detector to a target frequency. This allows for real-time “active” monitoring where the bat’s morphology, flight-style, and behavior can be observed and associated with the incoming signal, which can aid greatly in species identification. Files generated by FD analysis are small in size, only 4-8 kB, therefore passive monitoring is extremely efficient and large numbers of files over a long duration can be recorded without taxing digital storage systems.

The drawbacks of FD bat detectors are that they are less sensitive. They cannot display a full spectrum of call parameters, such as: multiple harmonics (they will only play the signal with the most energy), and information about the amplitude or strength of the incoming signal cannot be displayed. Moreover, because the detector will only pick up the signal with the most energy (the loudest incoming signal) it will often over-represent bats with high-intensity echolocation calls when used in survey work. Or, preferentially switch to recording bats with intense calls that enter into the same airspace where quieter bats may be flying. Finally, tho small, the files generated with the AnaBat FD detector (unquestionably, the most popular FD detector in use) are proprietary and can only be analyzed using AnaLook software.

FD bat detectors are versatile enough that they can be used for either Active or Passive surveys. Active use requires an old AnaBat II detector, ZCAIM, and a DOS laptop OR one of the newer AnaBat SD1/SD2 detectors with the AnaPocket PDA. When attached to one of these devices, the AnaBat displays a sonograph in real time while the bat in flight can be observed. Without the “anapocket” the AnaBat SD1/SD2 is designed to be a passive detector. The detector can be programmed with a timer on/off to facilitate long-term recording to a CF card which can later be downloaded for viewing and analysis.

AnaBat detectors are useful for determining species ID in all but the most cryptic species. They are among the best detectors for mobile transects due to the realtime rendering of echolocation calls. And they can be used to record “voucher calls” for call libraries during hand-release efforts.

ACOUSTIC005Frequency division (FD): FD detectors use a broadband conversion, transforming the entire bat call in real-time. The original high frequency call is converted to a "square" wave and divided by a user-selected ratio (usually 4, 8, 10, 16, or 32). The detector counts how many times the original high-frequency sound wave cycles from negative to positive, producing an output signal for every 4, 8, 10, 16, or 32 “zero-crossings.” This results in an output that is 1/4th, 1/8th, 1/10th, 1/16th, or 1/32nd of the input frequency.

The user-selected division ratio helps provide efficient processing. Low frequency calls are better represented at low division ratios, and high frequency calls should be recorded at higher division ratios. For most North American bat species a division ratio of 8 or 16 is used. For many of the low-frequency bats, especially big-free tailed bats, spotted bats, and lappet-browed bats out west, a division ration of 4 or 8 is better.

Though FD detectors are less sensitive than HET (or TE) units, they are capable of producing accurate representations of the bat call, including important information about the shape, slope, and characteristic frequency. This is helpful because these parameters are often used in species identification.

ACOUSTIC004The HET bat detector is the lowest-tech bat detector option, and as such its outputs are very basic.

It produces an audible signal. This signal can be broadcast thru a speaker, fed thru headphones to allow individual listening and thus improved perception of sound, or played into an analog/digital tape recorder for later listening and “analysis.”

Because only a small portion of the original high-frequency sound of the bat is transformed, signal-processing to determine key identifying call parameters is not possible. There is no way to get at information about how the signal frequency, or amplitude, changes over time, or anything else about the call structure (e.g., maxF, minF, slope, harmonics, etc.)

ACOUSTIC003Among the advantages of heterodyne detectors are that they are inexpensive, rugged, and provide “good” signal-to-noise ratio which makes them highly sensitive to incoming signals. HET detectors are perfect for using in “real time” when the bat can be observed in flight while the signal is being transformed, giving investigators simultaneous visual and auditory clues to bat behavior and ID. They can also be hooked up to a recording device and used passively, allowing multiple units to be deployed concurrently in different habitats, to measure bat occupancy or behavior across spatiotemporal scales.

The main disadvantage to heterodyning is that the detector only selects a very narrow bandwidth of the original call to play back. It does not preserve any information about the original call’s “structure” e.g., duration, absolute frequencies, or the frequency-time course. It must be manually dialed to respond to the frequency of interest. This narrow sampling window may lead to under-representations of certain species in survey work because bats calling at frequencies outside the dialed frequency will be missed.

Best uses for heterodyne detectors include deployment for “active” surveys when the bat that is detected can be spot-lighted at the same time for cues to identification based on the audible rendition of the call and the morphology of the bat in flight. There are limited opportunities to identify bats based on the heterodyne output alone . . . except in areas of depauperate bat faunas where the different bat species produce calls that exhibit significant difference in duration and/or minimum frequency. That being said, heterodyne bat detectors have been used to great effect in the U.K. for identifying different morphologically indistinct pipistrelle bats which have very different echolocation call types.

ACOUSTIC002Heterodyning (HET): The most common method, HET is a very sensitive, narrow band technique whereby the user selects a short range of frequencies to transform using a manual tuning control.

Setting a HET
detector to 40kHz makes echolocation calls between about 35-45kHz audible, in real time.

Most bat calls are broad-band, sweeping thru many frequencies over time, so only a portion of the call transformed by the HET system is re-played.

Depending upon the dialed frequency of the detector and the characteristics of the bat call (i.e., the duration of the call that is within the dialed frequency range), the tonal output of the detector will vary, sometimes producing a "chirping" sound (indicating a narrow-band, longer call) or a "ticking" sound (for a broad-band, shorter call).

With practice, it is possible to identify some species based upon the dialed frequency and the detector output, especially in areas with limited species diversity.

Since many bats produce very intense (loud) echolocation calls, we have invested a great deal of effort into recording these calls in hopes to identify bats on the basis of their calls much like birders can identify birds on the basis of their songs. But, bat echolocation is also very high in frequency, above the typical range of human hearing (which is 1-15 or 20 kHz). Most bat calls are well above 20kHz, some reaching frequencies of 100kHz or more.

So, we’ve developed special microphones, sensitive to these high frequencies and sophisticated electronics in “bat detectors” which will translate those high-frequency sounds into humanly audible outputs that allow us to eavesdrop on bats and bat activity without physically capturing individuals. Some of these outputs can be digitized, allowing computer software programs to display the actual wave-form (oscillogram) of the bat call and how the frequencies of the bat call change throughout its duration (known as a spectrogram or spectrograph).

As such, bat detectors have become essential survey tools to complement mist nets and other physical capture devices for the inventory and monitoring bats.

But, these days, the startling array of bat detectors can be truly daunting for the first-time user, and for experienced devotees of a particular platform alike. So let’s take a few moments to discuss the myriad detectors, pros, cons, and applications.

Bat Detector Types

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