The lowest frequency the Song Meter SM4BAT FS can record depends on two factors:

• Whether Settings > Audio > 16k High Filter is enabled
• The microphone you use (most commonly the SMM-U2 or SMM-U1)

The 16k High Filter setting enables a high-pass filter (HPF) with a cutoff frequency of 16 kHz. The SMM-U2 and SMM-U1 microphones also have permanent high-pass filters with cutoff frequencies lower than 16 kHz. For all practical purposes, only the active HPF with the highest cutoff frequency matters.

An HPF is a filter designed to remove incoming signals below its cutoff frequency. In the case of the Song Meter SM4BAT FS, the purpose of the HPF is to prevent low-frequency sounds like wind or non-bat animal vocalizations from distorting your recordings or excessively triggering the recorder.

If you enable the 16k High Filter, sound lower in frequency than 16 kHz will be significantly reduced in volume. If you disable the 16k High Filter setting, then the microphone's filter will be the deciding factor:

• The SMM-U2 will reduce the volume of sound lower than 1 kHz.
• The SMM-U1 will reduce the volume of sound lower than 8 kHz.

Most filters take effect gradually as frequency changes. In the case of the Song Meter SM4BAT FS and its microphones, for each factor of 2 below the cutoff frequency, sound at that frequency is reduced in volume by 12 dB compared to sound above the cutoff frequency. For example, when using the 16 kHz HPF, sound at 8 kHz is reduced by 12 dB, and sound at 4 kHz is reduced by 24 dB.

The filters built into each microphone is sufficient to remove wind noise in typical environments. However, if you know that you do not need to record sound lower in frequency than 16 kHz, enabling the 16k High Filter will help avoid recording higher-frequency sounds such as insect drones and rustling leaves.

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