[VLM]

Around the turn of the century when PSK31 data modulation was new to ham radio operators, it was a very standard demo at club meetings or whatever to fire up two laptops using mic/speakers instead of radios and talk a couple feet which was considered impressive because most other modulation methods are wider band and will not work well.

The relevance to your question, is hams being hams, they gotta experiment, turn of the century laptop hardware often had codecs that operated up to 96 KHz or whatever, but speakers usually cut off somewhere around the high level of human hearing, so operating at 10 KHz worked and might be silent to old people with partial hearing loss, but you could forget about 25 KHz even if the signal looked great on an oscilloscope because the speakers and mics were not broadband enough. Very few customers care about audio quality in frequency bands they can't hear.

The same hardware hooked up to a simple I/Q software defined radio works fine up to the limits imposed by Nyquist and the claimed hardware sample rate, so its obviously limited by mic/speaker not the A/D converters.

This topic comes up repeatedly on HN. If HN had a wiki this meme would be a strange attractor for the HN community.

[m0ther]

I could see "there's a message being transferred, and it's just outside of our ability to sense it" being catnip to the kid in all of us. Like invisible ink handwritten notes for adults. I looked up specs, my mic doesn't go beyond 20KHz. Do clubs like this still exist? I get the sense that I wasn't alive when they were most popular.