* What is a bat detector?
A bat detector is a device to make unhearable frequencies hearable. Such frequencies are produced from different species like bats, grasshoppers, rats, mice, dolphins. On the market are different types of bat detectors available. The right choose of a bat detector depend on tasks which should be done, because there exist a wide range from low level up to high end for scientific use. A nature enthusiast donīt need the equipment what universities need.
The main question is, whatīs your goal. Do you want make it only hearable, then you can use detectors like Laar TD15 or Laar TDM7 C? Or do you want to determine some species. Then you need a bat detector with high frequency accuracy like the Laar PX1.
But if you want to do computer based analysis, then you need a detector with time expansion (Laar Explorer) or a realtime high speed harddisk recording system (DSP system) like the Laar/Avisoft Ultrasoundgate.
You see that the right previous thinking will open a way for saving money.
How does the different technologies work?
1. Heterodyne detectors
Heterodyne detectors are the earliest bat detectors with the greatest spreading. There exist two different types. Normaly the heterodyne systems are selective detectors. They transfer only a frequency window, mostly +/- 5 kHz, into the hearable range. The incoming frequencies, in middle Europe from 13 kHz up to 110 kHz are very often mirrored until they are per random in the hearable range. The basic frequency information is lost and only the amplitude is original from the real signal. This demonstrate also, why this detector signals cannot be used for computer based sound analysis. Further, if you use a selective heterodyne detector you have to tune and select a window of 10 kHz. If the detector is tuned on a frequency, you donīt know whether another species in an other frequency band simultaneous active. There is the possibility to loose this information in field.
Since some years exist a new kind of heterodyne devices. The circuit is called PANORAMA. It is a broadband heterodyne detector. This technology donīt support the frequency tuning.
So this device type is very well for quick Yes/No statement about activities. But you never miss a species reasoned by the wrong tuned frequency.
2. divider detectors
The second group of bat detectors are the divider or zero crossing counter. The incoming signals are devided by ten in Europe or 8, 16, 32 in North America and Australia. The deviding by 16 or 32 depend on the fact that bats in the named regions often work with frequencies up to 200 kHz. If you devide a signal of 170 kHz by ten, then you would get a transformed frequency of 17 kHz. This high frequency cannot heared from most of people.
Other side effects of this method are the lost of amplitude information (by the most of such devices), the lost of 90 % of the original sound information. In theory is the divider system the cheapest way for computer based sound analysis, if there wouldnīt be a further negative effect especially by moving objects. The system work very well in optimal constellations, but a flying object isnīt ideal. There are changings in amplitude. This cause mistakes of counting of the zero crossing and this produce wrong frequencies in spectrograms. But it is a unexpensive system in comparison with other systems.
3. Time expansion detectors
The time expansion system work with highest accuracy and is only toped by high speed recording systems. All informations of a ultrasound are available. The detected signals are sampled by an AD/DA converter into a memory. The content of the memory is played back with a time factor of ten. So the signals come into hearable range. This method have only two negative side effects. The first is the limited recording time, mostly between 2 to 15 seconds. The second effect is that during the expanded signals are transferred to a tape recorder or notebook computer, no activities could be detected. For a realtime record of 2 seconds, you need a transfer time of 20 seconds. Both side effects are not very important. Only in tasks for social calls research could be a longer recording time helpful. The time expansion method is a middle way for professional work.
4. Realtime harddisk recording (DSP system)
This method is a further development from highspeed tape recorder. The newest devices are adapted via USB to a notebook computer and make realtime recordings up to 250 kHz with a 16-Bit resolution possible. It is called Ultrasoundgate. The sampling rate is 500 kHz. With the special recording software the incoming sounds are also displayed in realtime and can be heared as broadband heterodyne signal. The software also allows an event controlled recording. Nothing of sounds is lost. This system has only one negative side effect and this is the needed space for storing the datas. If you use the Ultrasoundgate with maximum sampling rate of 500 kHz (other, lower sampling rates and resolution are also possible, adjustable with the recording software), you have a data transfer volume of 1 MByte per second, but you have all informations you wish!