How to measure at very low frequencies?

Using the sine sweep it is possible to get very good results for low frequencies. In the figure below we have measured the loop-back output to input of the USB Duo. It goes down to –85 dB at 0.02 Hz!

To get the result above we used the settings below

We tried to go as low as 50 Hz in the Range [Hz] parameter, but were not successful. We did these measurements with sampling frequency 44100 Hz just to show that it is possible to get very good results for low frequencies using high sampling rates (some sound cards do not have low sampling rates available). But for low frequency measurements we recommend that you use as low a sampling frequency as possible.