How low can one hear Huh?? Say again

Fooling around with a test disc I found (in watching the speaker cone move)a large dropout in my own hearing at about 20-25hz. I know it's there, but at that point it is out of my ears and into my belly.

Upper end I'm getting worse, it goes OK to about 12k and that's it. My son can hear to 16k. I understand the sound needs to be there to be full- but the tone escapes me...

Get a signal generator and start out at 30 Hz and go lower. You will soon stop hearing the signal, and it will be replaced by feeling it, including in your ears as pressure fluctuations.

I've just done exactly that: Fired up my modular synthesizer, plugged my headphones (5Hz-35kHz) straight into the low frequency oscilator (0.001Hz-500Hz)set it to sine wave and turned the frequency down. The result is a hum that goes lower and lower, past the well known mains hum and the pressure inside the headphones gets stronger.
But then there seems to be a transition phase at about 15-20, I'd guess, where the pressure rapidly diminishes and the hum turns into separate click noises! It doesn't sound like a sinewave at all anymore.
Yet when I use this signal as a modulator it turns out to be still a sinewave. So my guess is that we can hear tones below 15Hz but our brain interprets these as short burst of a tone with a much higher pitch.
Any other ideas?

" Any other ideas?"

Headphones and in-room are not the same so kind-of apples and oranges. The ear does not respond in the same way to each and is the only reason why tiny headphones can do deep bass.

Dave

golix, stop playing this signal in your headphones. You are bottoming out the drivers. The signal, as you note, is coming through fine, but the headphones cannot reproduce it.

Try it on speakers that can go below 35 Hz or at least have some signal below that.