How low can one hear Huh?? Say again

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.

Granted there are major differences in music reproduction/perception between speakers and 'phones but I fail to see the relevance when it comes to a pure tone (sine wave) as long as it reaches the ear at a high enough level to be perceived.

On a different tack: We all know that bats use ultrasound to "see" and that we cannot hear this.
It was recently discovered that elephants use infrasound (ie below 20Hz) to communicate we just never knew because we don't hear it! Apparently you can stand in the open savannah halfway between two elephants, which can be miles apart, and they are "chatting" to each other whilst you hear nothing!
I think we can feel very low frequencies if we are in an enclosed space but we can't actually hear them and in open space where nothing resonates in sympathy we don't feel them either.