After the previous round of mods, and sitting here in my (sometimes) quiet home office, I realized that I could hear a long fading-out "ping", or at least an "iiiiinnnng", that was clear after I finished typing. This is often completely drowned out by music when I'm working though, so it wasn't that loud.
But once you start paying attention, and start making things quieter, it's a slippery slope.
This evening I dug out the spare switches (Kaihua Speed Bronze) for my keyboard, held one up my ear, and tapped it. And heard the same thing ("tap-iiiiiing"). But not quite so loud as what I hear from the keyboard itself.
So I recorded it, and poked at some other switches that I'd lubricated last week, to see if they did the same. And they did not...
But first, here's the spectrogram (from Audacity) of the stock switch :
The span on this is, for reference, is about 320ms. So that long smear at 4800Hz (and the bands ~1400Hz above it are really interesting). It looks like the fundamental might be around 1200-1400Hz (and there's a few spots of energy there during the impulse), and these are harmonics that are ringing.
Also visible is a similar pattern for 2, 4, 6, and 8kHz. Probably lots of things going on.
Now, here's the spectrogram (with the same timespan) from similarly tapping the side of a switch where I've coated the ends of the main spring, the slider's sides, and the leaf-switch contacts, in Krytox 250g0 grease (but not the click bar):
The difference is pretty stark. Both in sound to the ear ("tap", not "tap-iiiiiiiinng"), and it's visibly different in the spectrogram.
But then I looked back at the frequency plots from the up and downstrokes that I made:
I think one of the reasons it's so audible when typing is that there are a lot of keyswitches which all experienced an impulse when any of the keys was pressed (especially since I tend to bottom out the keys).
That lack of ping is also apparent when you press and release the switch (held up to my ear).
Lubing the switches is on my mod-list. But this might move it up in overall importance. This is really quite the difference. But as it's hard to reverse, I'm not doing it yet (I have a few others changes I want to test first).
A note: These are Kaihua (aka Kaihl) "Speed Bronze", a switch with a very low actuation point, and is a clicky-type switch. But the click mechanism is very different from that of a Cherry MX Blue. Instead of an internal piece that moves/up down against the contacts, this has a "click-bar", which is separate from the tabs on the slider that closes the contacts.
So, 3 springs total. But in this case, the grease is only on the main spring and the contacts, not the click-bar. Greasing the click-bar makes the click much, much quieter, and slightly mushy feeling.