Calibration of the BATmode-System
Calibration of the BATmode-System is essential to make recorded bat activity data comparable. As we are continually asked calibration related questions, we would like to briefly explain and demonstrate in this article how you can ensure correct calibration of your system. To open this document as pdf, please click here.
The aim of the calibration of a microphone is to relate the electrical output of the microphone to the actual sound pressure level. This sound pressure level Lp (SPL) is generally expressed in the unit dBSPL and describes the logarithmic ratio of the squared effective value of sound pressure p̃ of a sound event to the square of the reference value p0. For airborne sound this reference value is set to p0 = 20 µPa.
The sound pressure level Lp can either be positive (sound pressure is greater than the reference value) or negative (sound pressure is smaller than the reference value). In short, the sound pressure level Lp in dBSPL indicates how much louder or quieter a sound event is in relation to a specified reference value. For example, a sound event with a sound pressure level of 6 dB SPL, is twice as loud as the related reference value and a sound event of -20 DBSPL corresponds to a 10 times lower sound pressure in relation to the reference value.
The unit of the electrical output value U of a microphone is given in dBFS (dB full-scale), which is, similar to the sound pressure level Lp, a logarithmized ratio of electrical output value U to the maximum output value (Full scale , FS) of the microphone. Consequently, the microphone output signal U is always smaller than the reference value and thus can only accept negative dBFS values.
As the essential factor in the determination of bat activity is not the loudness of calls per se but their number, it is especially important to define exactly for which real sound pressure level Lp a bat call can trigger a recording. This so-called "Trigger Level Threshold" AdBSPL thus represents a parameter for the sensitivity of the detection system and make results comparable. In general, it can be said that the quality of a bat activity monitoring can be increased by selecting lower trigger thresholds AdBSPL. This makes it possible to detect quieter calls and therefore calls of distant animals. In turn, this of course also increases the chance to obtain recordings of noise, which have to be laboriously removed from the resulting data set.
For the evaluation of the recorded data with the tool ProBat that results from the research project WindBat and thus is compatible with the BMU guidelines, a trigger threshold AdBSPL of 37 dBSPL is recommended when using the BATmode system. In the case of a wind turbine, which has a particularly high noise level, the trigger threshold AdBSPL can also be increased to 50 dBSPL to avoid the recording of noise. However, due to the lower sensitivity of the detection system in this case we can expect a significant reduction of quality of the monitoring. For data evaluation it is of outmost importance that all data was recorded with a uniform AdBSPL trigger threshold (that is, either 37 or 50 dBSPL). If one part of the data have been recorded with a lower trigger threshold AdBSPL, it is possible to reevaluate them for a higher trigger threshold AdBSPL using the RECORDER software. Hereby, quieter calls, which would not have been recorded using the higher trigger threshold AdBSPL are removed from the data set. Logically, this is not possible in the reverse direction and a data set, which was recorded with a trigger level of 50 dBSPL, cannot be converted to 37 dBSPL.
How does a calibration work in general?
To calibrate a measurement system, it is essential that a reference signal with a known value is given to which the measured values can be related. In case of an ultrasonic recording system, the reference would be an ultrasonic signal with a known sound pressure level Lp.
By exposing the microphone which have to be calibrated to this sound pressure level Lp, the electrical output signal U generated by the microphone can be related to this known sound pressure level Lp.
Calibration of the BATmode-System with microphone discs of bat bioacoustictechnology
The microphone discs of bat bioacoustictechnology are delivered with a calibration certificate, as exemplified in Figure 1.
On this certificate the so-called SPL of reference signal (LT) and the Full Scale Level of Reference Signal (UT) are given. The first one represents the sound pressure level in dBSPL of the in the microphone disc integrated test signal generator, which arrives at the microphone. The value Full Scale Level of Reference signal indicates the resulting amplitude of the microphone output signal in dBFS, if the test signal generator is activated.
For calibration enter the two values, which are also mentioned on the microphone disc, in the calibration dialog of BATcontrol (available on the page "Monitoring" by clicking on the button "Calibration", see Figure 2). Furthermore, insert the desired trigger threshold AdBSPL in the field "Level of Trigger Threshold” and click the button "Calibrate now!". By confirming the opening warning your system is calibrated. From BATcontrol version 1.1.10 on the calibration is also visually confirmed and the resulting trigger level threshold is displayed in % (A%). This value can be checked in the Configuration dialog of the RECORDER software (see Figure 3).
How were the calibration values on the certificate determined?
- First, a sound event with a frequency of 40 kHz and a sound pressure level LRef of 96 dBSPL is produced in an anechoic acoustic chamber at room temperature using a measuring microphone from Brüel & Kjaer and an ultrasonic piezo.
- The microphone of the disc which has to be calibrated is exposed to this precisely defined sound signal and the resulting microphone output value URef is determined in dBFS (root mean square).
- Subsequently, the microphone output value for the built in test signal generator is determined in the same anechoic acoustic chamber. For this purpose, it is excited with a frequency sweep from 30 to 50 kHz in order to compensate for temperature- and type-induced variations of its resonance frequency. The resulting maximum microphone output UT corresponds to the Full Scale Level of Reference Signal in dBFS (root mean square) specified in the calibration certificate.
- Subsequently, the SPL of Reference signal value (LT) specified in the certificate is calculated to
Calibration of the BATmode-System with a microphone disc of Avisoft Bioacoustics
In the following, the calibration of the RECORDER software is explained in detail for the case that you are using a microphone or a microphone disc, which is not original from bat bioacoustictechnology. Since Avisoft Bioacoustics only indicates the sound pressure level of the test signal generator on its microphone disc, but not the resulting microphone output UT in dBFS, the output must be measured by utilizing the software RECORDER. (Please perform this measurement at room temperature and, if possible, in an anechoic chamber or hold the disc during the measurement away from walls, so that no possible sound reflective object is in close proximity in front of the microphone disc!):
- Close BATcontrol.
- Open RECORDER USGH of Avisoft Bioacoustics.
- Open the Trigger Level Calibration dialog of the software RECORDER (see figure 4) (Monitoring -> Trigger level calibration).
- Select "absolute dB SPL", "x10" and "take maximum signal level".
- Enter your desired trigger threshold AdBSPL (for example, 37 dB SPL) in the field trigger level (threshold).
- Insert the value LT of your Avisoft disc in the field reference signal level.
- Wait 60s and then write down the value of UT which can be found at reference signal level (root mean square) [dBFS].
- Close RECORDER.
- Open BATcontrol.
- Open the calibration dialog of BATcontrol (see figure 2).
- Enter the noted full-scale level of reference signal (UT) in the appropriate field.
- Enter the reference signal level LT in dBSPL of your Avisoft disc in the appropriate field.
- Enter your desired trigger level threshold AdBSPL in the field Level of Trigger Threshold.
- Click on "Calibrate now!" and confirm the opening warning.
- From BATcontrol version 1.1.10 on, the calibration is visually confirmed and the resulting trigger level threshold A% in % is indicated. This value can be checked in the Configuration dialog of the RECORDER software (see figure 3).
General information about the Trigger Level Calibration dialog of the software RECORDER
At this point, we want to refer to the official user manual of the software RECORDER of Avisoft Bioacoustics, in which the calibration of the software is described for a general case. You can download the manual from www.avisoft.com.
Below, the Trigger Level Calibration dialog of the software RECORDER is explained in detail to give you a broad background knowledge of the calibration functionality of RECORDER software.
Information which have to be provided by the user:
- Trigger level (threshold) AdBSPL [dBSPL]: Determines the trigger level threshold AdBSPL in dBSPL, for which a trigger event is triggered.
- Reference signal level LRef [dBSPL]: Indicates the reference sound level of a known sound source in dBSPL and serves as reference value for calibration.
- Reference signal level (root mean square) URef,dBFS and URef,%: Depicts a measured microphone output value in dBFS or %, respectively. If the field "take the maximum signal level" is activated, the maximum value over the entire period of time for which the dialog is open is displayed. Otherwise, the current microphone output value is displayed.
- Full-scale SPL: Specifies the maximum sound pressure level Lmax in dBSPL the microphone can record, if a calibration has been performed with the current reference signal level i>URef.
- Trigger level (threshold) AdBFS and A%: Specifies the trigger level (threshold) in dBFS or %, which represents the desired trigger level (threshold) AdBSPL after calibration. These values are updated by clicking on the button "calibrate".
Here, 3 dB must be subtracted to account for the reference signal level given as RMS (root mean square) value. The addition of 20 dB corrects the influence of the amplification x10 and has to be ignored for the case of a deactivated check box "x10". After calibration, the trigger level (threshold) A% [%] corresponds with the value of the Trigger Event Level in the Configuration dialog the software RECORDER (see figure 3).