The SMART System has a built-in filter to avoid triggering on signals within a set of defined parameters. This filter can help avoid false triggers on signals such as electrical interference or mechanical squeaks.
The constant-frequency filter accepts a maximum frequency, minimum frequency, and maximum bandwidth. The SMART System will prevent a signal from triggering recording if the signal meets all the following conditions:
- The signal's minimum frequency is greater than or equal to the CF Filter Min Frequency.
- The signal's maximum frequency is less than or equal to the CF Filter Max Frequency.
- The difference between the signal's maximum frequency and minimum frequency is less than or equal to the CF Filter Max Bandwidth.
If you use the CF Filter, you should make sure that none of the echolocation calls produced by nearby bat species meet all three of the above parameters.
Using the CF Filter on a Wind Turbine
A common reason to use the CF filter is when a particular wind turbine produces high-frequency squeaks as it rotates. If these squeaks are similar in duration and frequency to typical bat calls, you might get many recordings of these squeaks, and they may trigger your bat activity alarms.
Say that a turbine produces squeaks that fall between 20 kHz and 30 kHz. Each individual squeak is narrow-band and flat in frequency: a single squeak spans less than 1 kHz between its highest and lowest frequency.
To prevent these squeaks from triggering the SMART, we could use the following parameters:
| CF Filter Min Frequency | 20 kHz |
| CF Filter Max Frequency | 30 kHz |
| CF Filter Max Bandwidth | 1 kHz |
Before implementing these settings, we should make sure that the bat echolocation calls we want to record do not meet all of these parameters:
Say there is a nearby species of bat that produces very flat calls around 80 kHz. Even if the SMART detects one of those calls with a bandwidth less than 1 kHz, it will still trigger since the call is higher in frequency than the CF Filter Max Frequency.
There may be many other species of bats whose calls fall, in part, between 20 kHz and 30 kHz. As long as each of those calls has a greater bandwidth than 1 kHz, the filter will not interfere with those calls. However, if a call is faint enough that the SMART only registers a narrow portion of it, that call may be removed by the constant-frequency filter.