Hmmm, I can definitely hear it in the video; maybe it's a difference in PC sound cards / speakers? It's a bit louder around 18:45 when I turn up the amplifier gain.
I checked your video again, and I definitely cannot hear a single tone at 1'10 or at 18'45
Then I went to this site:
http://www.noiseaddicts.com/2009/03/can-you-hear-this-hearing-test/From that site, I can hear everything up to 14 KHz. At 15 KHz and above that I cannot hear the tones anymore.
I am using headphones. I do not know if this is a limitation of my headphones or soundcard audio path, etc.
Also on that page they say it's not uncommon for people over the age of 25 have difficulty hearing above 15 KHz, and I am in my 50's, so now I'm not surprised if it's there and I can't hear it
I guess Widlar's device only kept the noobs out.
EDIT: yes, definitely the audio path to my ears is not capable of reproducing the high frequencies. I listened with my speakers, and at the site I mentioned above, I can only hear to 12 KHz with my speakers.
I then went to the same site with my android phone and ear buds, and I can hear all the way up to 18 KHz tone.
So my hearing is not that bad, and likely my sound system on the PC just can't go above 14 KHz or so.