If you live in an area that has fruit trees around, it is likely bats share your space. Bats are furry mammals that flit about at night, feasting on insects and fruits. Although they are not gifted with good eyesight, they locate prey and avoid obstacles using echolocation. They are expert fliers and it is difficult to observe them since they are so silent.
Although humans cannot hear bats, it does not mean these creatures make no noise. In fact, using the process of echolocation, bats produce a considerable amount of sound. However, humans cannot hear them because the sound bats produce has a frequency range beyond human hearing capabilities. Depending on age, humans can hear sounds produced in the frequency range between 20 Hz and 15-20 KHz. Bats can hear and produce sound up to about 110 KHz. That is why a Raspberry Pi or RBPi is necessary to collect process and graphically represent bat calls.
An analysis of bat calls shows the sounds they produce are quite loud and not limited to just one tone. Different breeds of bats produce a variety of sounds, differing just as bird chirping does. For example, their tone may sweep down from a high frequency to a low one, or move around a specific frequency.
Holger and Henrike Korber from Germany have used an RBPi to make a bat detection device. To collect the sound produced by bats, they use an inexpensive microphone of high sensitivity capable of responding to high frequencies. The algorithm they use allows not only a graphical representation of the calls, but also identification of the bat species as well. Additionally, the software allows manipulation of the calls to bring them into frequencies within the human hearing range and create histories of bat activity.
On their site, which translates to Bat Conservation in English, the Korbers offer a list of bat literature. If you can know the German language, you will find a treasure of information on echolocation and acoustic identification of bat species. To read in English, pass the page through Google Translate.
Details of their new WLAN-Raspi-Bat detector are available here. The detector, based on the RBPi Model B+, is wirelessly connected to an external notebook. That allows easy manipulation of the configuration and wireless recording of data. The RBPi bat project uses a UMTS stick for WLAN communication and a modified image of the RBPi OS.
The WLAN-Raspi-Bat detector sends SMS text messages automatically and at freely configurable times. For example, this could be just after the RBPi has booted or just before it shuts down. As the detector is portable, it is important to save on power consumption and data space on the SD Card. To keep the arrangement simple, the Korbers use a simple clock timer to start and shut down the RBPi. As bats venture out only at night, the RBPi can sleep during the day along with the bats.
As the detector communicates wirelessly, there are numerous applications. For example, it is able to operate at locations hard to access, such as in trees up to the canopy and in buildings with difficult access.