Ok, just done it. Attached is the 30Hz tone* repro'ed through my home hi fi (kept level modest but still annoyed wife!)
The analyser plot is interesting? You can easily see the 30Hz picked up by the very small speaker, 4" woofer but the third harmonic at 90Hz dominates. Whether that is my speakers distorting or the wee one acting as a microphone I cannot say, bit O both I would guess.
Still, the principle works and you can use a relatively small speaker to record very low frequencies.
The speaker BTW was plugged into my NI KA6 and the mic gain whacked to max.
Hope this helps.
* I tried to record a 20-100 sweep sine but that did not turn out well.
Great recording, I’m going to try this and let you know how it goes. If it doesn’t work i might start crying