Low-frequency spectral estimation (f<1 Hz) employing PC soundcards

We propose a simple but effective approach for the estimation of power spectra at very low frequencies (down to the tens of mHz range) by exploiting the characteristics of the sound system (sound card) and the computational power of modern personal computers. A simple mixer board has been designed that allows to transfer the low frequency region of the spectrum at a higher frequency in the passband of the personal computer sound card. Proper digital signal elaboration allows to recover the entire spectrum (from very low frequencies up to the sound board passband). Results obtained by the proposed approach on test spectra are compared to the results obtained by laboratory grade dynamic signal analyzer boards. The approach we propose is mostly useful for applications in the field of low frequency noise measurements, where the ability to explore frequencies well below 1 Hz is more important than obtaining large bandwidths.