Comparing the Signal-to-Noise Ratio of the SM5BAT, SM4BAT FS, and Mini Bat 2
Depending on which external microphone is used, the signal-to-noise ratio measurements of these three recorders can be very similar.
About Signal-to-Noise Ratio
In the context of these specifications, signal-to-noise ratio (SNR) is a measurement that expresses the relationship between the sensitivity of a recording system to various frequencies and the electronic noise produced by the system at those frequencies. Increasing a microphone's sensitivity or lowering a recorder's electronic noise floor creates a recording system with a larger SNR. Decreasing a microphone's sensitivity or making the electronic noise louder produces a smaller SNR.
SNR will vary across the frequency spectrum because microphones are more or less sensitive to some frequencies than others, and electronics produce background noise with more amplitude at some frequencies more than others.
In ideal conditions, a recording system with a higher SNR at a given frequency can record quieter sounds with better quality than a system with a lower SNR at the same frequency. On the system with a higher SNR, an animal vocalization at the given frequency will be picked up by the microphone with greater amplitude, and/or the vocalization will be less obscured by the quieter noise floor. A triggered recording system with a higher SNR will tend to record more bat activity than a similar system with a lower SNR.
Comparing Signal-to-Noise Ratio
In this context, SNR is determined by both the microphone and the recording circuitry of the recorder. The combined system of the SM5BAT and its built-in front panel microphone has a different SNR than the SM5BAT with an SMM-U2 external microphone.
The SM5BAT and SM4BAT FS recorders have identical SNR measurements when they are tested with the same microphones. The SNR of either of these models paired with an SMM-U2 external microphone is generally within 5 dB/âHz of the SNR of the SM5BAT with its front panel ultrasonic microphone and the SNR of the Mini Bat 2 with its built-in ultrasonic microphone.
Comparing the Trigger Sensitivity of the SM5BAT, SM4BAT FS, and Mini Bat 2
Different recorders, microphones, and trigger modes change a recorder's ability to record quiet or distant bats during triggered ultrasonic recording.
Trigger sensitivity can be measured as the quietest sound at any given frequency that will cause a Song Meter to begin a triggered ultrasonic recording. Trigger sensitivity can be improved by increasing the sensitivity of a microphone, decreasing the electronic noise produced by the microphone and recorder, or using a more sophisticated trigger mechanism. Wildlife Acoustics tested multiple combinations of recorder, microphone, and trigger mode using an ultrasonic test speaker in a semianechoic chamber.
Single-Band Triggering Compared to Multi-Band Triggering
The Song Meter SM5BAT has two trigger modes that are well-suited to recording bats in most circumstances: single-band triggering and multi-band triggering. Each of these modes is also used by previous Song Meters.
Single-Band Triggering
Single-band triggering is an option on the SM5BAT and the only trigger mode used by the Mini Bat 1 and 2 recorders.
When active, the recorder monitors the level of incoming sound to gauge the average level of ambient background noise above its built-in high-pass filter. When it detects a bat call that peaks above the background noise by a certain amount, the recorder starts recording until the bat has flown away.
We call this single-band triggering because the recorder measures the level of all sound as a single, broad band. It doesn't matter whether the incoming sound is at 20 kHz or 80 kHz; it just has to be louder than the overall level of background noise.
Single-band triggering requires less processing power than multi-band triggering and therefore draws less battery power. On the SM5BAT, using single-band triggering instead of multi-band triggering extends the SM5BAT's battery life by approximately 20% when monitoring on a single channel or 50% when monitoring on two channels.
Multi-Band Triggering
Multi-band triggering is the default option on the SM5BAT and the only trigger mode used by the SM4BAT FS.
Rather than measuring all incoming sound as a single, broad band, the recorder splits up the incoming sound into multiple, narrower frequency bands. For a sound to trigger recording, it does not need to be louder than all of the ambient background noise; it just needs to be louder than ambient background noise in its specific frequency band.
For example, a bat call around 20 kHz only needs to be louder than the background noise around 20 kHz, and a bat call around 80 kHz only needs to be louder than the background noise around 80 kHz.
In quiet, laboratory conditions, we found that multi-band triggering is able to trigger on sounds that are 10â20 dB quieter than single-band triggering when using the same microphone.
Because multi-band triggering separately analyzes different frequency bands, multi-band triggering outperforms single-band triggering by a larger margin in very noisy environments.
Trigger Sensitivity Specifications
We measured the trigger sensitivity of multiple Song Meters with multiple microphones and trigger modes, where applicable.
The SM5BAT can optionally use either a single-band or multi-band trigger mechanism, resulting in different overall trigger sensitivities. We tested this recorder both trigger mechanisms with both the SM5BAT built-in microphone and the SMM-U2 external microphone.
Of the configurations tested, the most sensitive were the following:
- SM5BAT with its built-in microphone and multi-band triggering
- Both of the following, with identical performance:
- SM4BAT FS with an SMM-U2 microphone
- SM5BAT with an SMM-U2 microphone and multi-band triggering
The Song Meter Mini Bat 2 uses the same trigger mechanism as the SM5BAT's single-band mode, but the Mini Bat 2's built-in microphone is not exactly the same as the SM5BAT built-in microphone nor the SMM-U2 external microphone. Therefore, the Mini Bat 2 does not show exactly the same trigger sensitivity as any other configuration tested.