Daqarta
Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows
The following is from the Daqarta Help system:

# Sound Card Low-Frequency Response Via Decimation

You can use Decimation with the White noise method to measure response at very low frequencies. The Generator output produces continuous White noise at the full sample rate (48000 Hz default), while the input is effectively sampled at a much lower sample rate that allows high resolution at low frequencies.

For example, with Decimate Factor set to 100, the effective input sample rate is reduced from the default of 48000 down to 480 Hz. The Daqarta Spectrum response is 512 spectral lines (from 1024 raw samples) spread over a frequency range equal to half the sample rate (the Nyquist frequency). The default spectrum thus has a resolution of 46.875 Hz, running from 0 to 24000 Hz. This is not adequate to measure the frequency response of a sound card's input and output AC coupling, which is typically 20 Hz or less. But with 100x decimation the resolution is 0.46875 Hz, from 0 to 240 Hz, which is more than adequate for most sound cards.

You can set the Decimate Factor even higher for finer frequency resolution, but note that this increases the time for the Spectrum Average to complete. At 100x, each frame takes 100 * 1024 samples, or about 2.133 seconds. Thus a typical 32-frame average takes well over a minute.

Even if you don't mind the wait, remember that as the effective sample rate goes down, so does the highest frequency shown in the spectrum. You typically want to see some of the flat part of the response above the low-frequency cutoff, to be sure you are getting the whole low-end response. If you set the effective rate too low, you might miss too much of that upper end.

Note that this method alone cannot separate the respective contributions of the sound card input and output AC coupling from the measured response. A DC measurement system can eliminate the input AC coupling, so the measured response only reflects the output coupling. A full Sound Card DC Input / Output Modification eliminates both, and the need to measure low-frequency response.