Bob Masta skrev:
I am fully aware of this, I just wanted to mention that these low frequencies actually can be heard.
The experiment you mention seems to be a (whole-)body vibration, not a sound exposure and that is a completely different matter. (By the way, have you seen a proper scientific reference?). Our sensitivity to whole-body vibrations is highest at a few hertz, which e.g. is reflected in the weighting curves in ISO 2631 "Mechanical vibration and shock - Evaluation of human exposure to whole-body vibration".
For vibrations there are certainly a variety of resonance frequencies in the body, and quite many are below 20 Hz. There are also body resonances for acoustic exposure, but they are a bit higher in frequency than for vibrations, since the body is a lot stiffer for acoustic stimulation than for vibrations.
The 7 Hz phenomenon that I referred to was probably a fairly large amplitude, but I can't relate it to "hearing threshold" at that frequency (!). As I recall, subjects were in a jet pilot's seat atop a shaker system. The 7 Hz sensitivity was assumed to be purely visceral, having nothing whatsoever to do with audition... probably just the effects of mechanical stretching and scrubbing together of internal organs due to large motions at the visceral resonance frequency. I have no doubt that there would have been some synchonization of brain waves, as a side effect of having one's guts tossed around! Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!