How to Record Audio Signals Below 5 Hz Using a Standard Sound Card?

Recording audio signals below 5 Hz using a standard sound card is challenging due to typical input coupling capacitors and resistors that create a high-pass filter with a cutoff around 10 Hz. To capture signals in the 0.5 to 4 Hz range, such as earthquake or wind noises, hardware modification like changing input coupling components may be necessary, though this can increase noise. Alternative approaches include frequency upconversion techniques: using a Phase Locked Loop (PLL) to multiply the low-frequency signal into the audible range (50 to 400 Hz), or analog chopping by modulating the signal with a higher frequency carrier (e.g., a 5 kHz square wave). This upconversion shifts the signal into the sound card’s passband, allowing standard audio hardware to record it. The chopping method is analogous to chopper-stabilized amplifiers and is historically documented in early patents. Both methods require additional circuitry and design effort but enable effective recording of sub-5 Hz signals with conventional sound cards.
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