Bat detector
The bat detector is a device that transforms the echolocation calls of bats in human- audible sounds. Bats send for locating prey, for orientation or for communication with conspecifics sounds, which are located mainly in the ultrasonic range. For the detection of bats and better species identification of the bat detector is employed. The application range of the bat detector is not limited to the study of bats. Any other natural or artificial source of ultrasound in the frequency range of interest can be detected and analyzed, if necessary. The frequency range of a bat detector is different. The lower limit is - depending on the individual hearing - to the audible frequencies down. The lower limit of a bat detector is about 10 to 20 kHz. The upper limit of a bat detector is 100 kHz to about 250 kHz.
The observations of bat detectors goes from simple DIY projects kits and simple finishing devices to professional high- tech devices that cost several thousand euros.
- 2.1 frequency divider
- 2.2 frequency mixer
- 2.3 heterodyne receiver
- 2.4 Time Dehner
- 2.5 DSP detector
Functional units of the bat detector
Microphone
For use Commercially available microphones or special ultrasonic microphones. Also, ultrasound receiver, as used for example in ultrasonic distance measurement systems, are conditionally suitable.
Preamplifier
The preamplifier amplifies the weak electrical signal of the microphone. Ideally, the preamplifier is dimensioned such that it compensates for the frequency response of the microphone used.
Signal conversion into audible sounds
Here lies the central functional unit of the bat detector. Various methods are used to implement the inaudible frequencies for the human to lower frequencies.
Basic methods for converting the signals
Frequency divider
The signal is divided by a digital frequency divider ( synchronous counter, ripple ) by an integer factor. A common division factor is 1:10. A Fledermausruf at 40 kHz is reproduced after the division by 10 as a lute with 4 kHz. A disadvantage of the simple frequency division is that the information about the amplitude (volume) of the sound is lost. The amplitude of the digital output signal is independent of the incoming amplitude. By amplitude modulation of the digital signal to the envelope of the input signal can recover the information. One advantage of the frequency divider is that a wide range is monitored simultaneously.
Other common division factors are 1:8, 1:16 and 1:32. The powers of two are common in digital technology. The division factors 1:16 and 1:32 seem more appropriate here, since they better share the entire frequency range of interest in the audible range than 1:10 or 1:8.
Frequency mixer
The frequency mixer, the incoming signal is mixed with a fixed frequency. The resulting signal is a plurality of sum and difference frequencies, of which only the respectively audible played. Frequency mixers are relatively simple to implement in construction. Again, a large bandwidth is monitored simultaneously. Playback is patchy. If the resulting output signal at very low frequencies, so you hear nothing. For example, a Fledermausruf of 41 kHz can be heard by the mix with 40 kHz as 1 kHz. A Fledermausruf of 40.001 kHz is theoretically represented by the mixture as a one Hz signal, is thus inaudible. Since most bat calls are not on a fixed frequency, but in the pitch are variable, you will miss this with a simple mixer.
Heterodyne receiver
In the heterodyne receiver ( heterodyne receiver or superhet ) the incoming signal is mixed with a variable frequency and then filtered. In " Double superhet " are two mixing stages. The first with a variable frequency and the second at a fixed frequency (intermediate frequency). The heterodyne bat detector works basically like a radio receiver. It represents a detector to a specific frequency and hear only calls that are received in a narrow band around this set frequency. In the frequency of different calls are suppressed by filtering. Only in the frequency close to each other at the same time calls are heard. This is similar to two overlapping radio stations that can be heard in a broadcast receiver at the same time. The heterodyne bat detector is suitable to determine the exact frequency of the calls and thus to specify a species identification. For a general location of this bat is less suitable, since most frequencies are filtered out. This is (eg crickets ) also an advantage in disturbing noises.
The process can be simplified greatly if the mixture is made according to the IQ method and mixing products are processed by the sound card of a PC on. The display can also be immediately displayed as a spectrum. It is advantageous that the signals can be stored permanently in digital form.
Time stretcher
The previously described methods convert the ultrasonic sounds to delay. Not so with the time stretching procedure. The sounds are recorded permanently or in a continuous loop of several seconds and played back just to listen slowed. The method is technically very complicated, but it enables the most precise evaluation of the recorded calls. The chance to miss with this method is something quite large. Therefore, offers a combination of a frequency divider or blender to when needed - at your fingertips - to save an interesting sequence can. This is a later stage evaluable. Since suitable storage media are becoming cheaper, it is foreseeable that with a handy time expansion bat detector records longer be possible.
DSP - detector
DSP (Digital Signal Processing, German: Digital Signal Processing ) is a special case dar. In DSP, the analog ring signal is digitized and mathematical algorithms presented in a way that the result signal is in the audible range. Theoretically, all reaction processes described previously can be realized by digital signal processing.
Distortion of the sounds by the methods
A distortion of the sounds on the implementation in the audible range is mandatory, because unadulterated only the original signal that is inaudible to humans. Each implementation method distorts the calls of bats differently. In the heterodyne detector, the pitch of the playback of the selected frequency is also dependent. A species identification based on the reproduced sounds requires experience. The delay-free method ( frequency divider, mixer and heterodyne ) give at least the rhythm of the sounds unadulterated again. The output signal of a frequency divider without amplitude recovery is sometimes perceived as unpleasant.
Recording and analysis of sounds
Most detectors have an output for recording the signals. In particular, the time-expansion of this makes perfect sense, since the sound signals can be analyzed by appropriate software. This gives you additional possibilities Artansprache as well as information about the behavior.
A replay of the terrain is usually on memory cards. It is important for compression- free recording. The sound signals are then transferred later to a computer with a sound card, with wav files are generated. Recently, also real-time recording systems are used for the documentation of bats, the high sample rate sounds directly to memory cards (CF, SDHC) or disk record ( connected via USB to a computer). These are preferably used in the context of environmental assessment in automatic mode.
The analysis of sounds by means of suitable programs to call here is primarily BatSound Pro. Commonly used is also BatScan and the programs from the Avisoft group. Some universities are also developing systems for the automatic determination of bat calls. Pioneers are Dr. Parsons (New Zealand; previously Bristol, UK) and Martin Obrist ( Switzerland ). The first such software available - bcDiscriminator - there were 2007 Central European species.
Meanwhile, researchers from Switzerland have a new bat detector developed ( BATLOGGER ). This facilitates the observation and species identification of bats significantly. In addition to the recording and storage of calls each call information such as location or temperature can be stored. A complementary Compendium ( BatEcho ) allows account to reference images and access to further information. For the consistent storage of data recorded specifically, a software was developed ( BatScope ).