Malcolm Goodson writes ...
The brain is a truly remarkable organ, It is not only with headphones; how about a normal-sized room. I have, in my youth, perceived 30Hz on a ‘test record’ or perhaps a multiple thereof, in a room scarcely big enough to allow the wave form to fully develop. But my cartridge does not have a quoted ‘frequency response’ down to that level; nor do my speakers.
I do not concur with Michael Chang’s interpretation because the tympanic membranne does not vibrate at 40Hz; most, if not all, headphones are incabable of generating that low a frequency except at a level far beyond the normal range of human hearing. But still the brain picks it up, however insignificant.
Perhaps, it is only another manifestation of: what we hear is what we want to hear.
The brain is more canny than you will ever know!