> "a train of pulsed sounds is made at the low frequency of 250-300 Hertz
...

> We all know these sounds are inaudible to the human ear (reason I never
> heard the sounds).

250-300 Hz is well within the frequency range that most humans can hear.
http://www.allchurchsound.com/ACS/edart/fmelc.html
http://www.webervst.com/fm.htm

In fact 'Middle C" should be 256 Hz on a properly tuned instrument.
http://www.bcm.tmc.edu/oto/research/cochlea/Volta/03.html

Some people can recognize this frequency (and others) and detect slight
deviations from it without a reference tone.

allen