Saved these two links:
40 Hz Pure Tone - Sine Wave | Bass Test Frequency Sound Therapy | 4 Hours BLACK SCREEN
https://www.youtube.com/watch?v=Ld0SoozrBBw
40 Hz Low-Frequency Sound for Subwoofer Testing. Tone Audio Signal. Sine Waveform - 3 min
https://www.youtube.com/watch?v=udhlm7ydlDk
I’ll have to try those on something other than my laptop. Apple does something both cool and non-authentic - when there’s a really low bass note, like 40 hz, its sound logic produces either double, triple, or 4X that frequency. So 40 hz might have a strong second and/or fourth harmonic. 80, 120, or 160. Possibly skips the third, I’d have to read up on it.
If the laptop were effectively reproducing 40 hz, I’d feel it. Now, on the desktop, I have a small subwoofer so it does produce some sound, though the low frequency rolloff starts well above 40 hz, rendering the low frequency note barely audible.