I’m sure I could just google, but I trust people on here more what determines the noise level of a switch? The housing, the spring, what?
Just my opinion, but I think by far it’s the stem hitting the top housing first, and then the stem hitting the bottom housing for people who bottom out when they type. Everything else, like the leaf and the spring and the stem rubbing the rails, contribute to the sound much less.
I can lube the top housing in my switches and nothing else and it deepens the sound signature the most making the switch sound quieter (although it might not be if actually measure, but just deeper)
Ha, in seriousness, though - basically every physical aspect of a switch contributes to its sound or lack thereof.
There are lots of ways to mitigate switch noise - sometimes one is enough, sometimes not. Here are some common ones:
Dampened stem or housing; usually the stem but housing dampeners are becoming more common in the MX-compatible space. TTC’s silents use a combination of stem and housing dampeners. Both can significantly reduce the volume and sharpness of a given switch’s sound, but anything close to “silent” is still pretty rare.
Spring weighting that discourages bottoming-out the stem. The best example I can think of is the Halo True spring; it actuates at some totally normal weight, but bottoms-out at 100g. “Fast curve” springs aren’t popular at the moment, but they’re not unheard-of.
Stem style is a factor; the loudest switches tend to be ones that bottom-out on the center pole, and the more quiet ones usually don’t. (TTC the odd ones out again; their silents bottom-out on stem poles, giving them a sort of “thud” character.)
Housing style & material apart from any dampeners play some role, if less than the others. If there are any impacts that you want to minimize, softer materials will generally help round those out more than hard ones. PA / Nylon, UHMWPE, Ink Stuff, Boba Stuff, etc. catch high-frequency vibrations better than Polycarbonate or PBT (not many switches use this but they do exist). On the other hand, the harder plastics tend to do better at mitigating scratch / texture, with some blends having mostly the best of both worlds but coming at an increased cost.
Tuning can play a big role; tightening-up the housing fit if need be with films or gaskets and lubricating the springs and plastic parts. In terms of sound, this can make a chattery switch clean, and a sharp clack a bit more rounded on the ear. Lubrication can and usually does lower the pitch overall.
Some examples with the above information in mind:
Bobagums are the most quiet stock switch I know, and only get more-so with some measured tuning. They have a fairly soft housing plastic, and have very soft silicone inserts in the stems, on which they bottom and top-out. This does make them feel a bit squishy, so the name fits beyond the color.
Moyu Blacks are one of the most harsh, clacky switches I can think of that don’t contain a click-bar. They have an extended center pole on the stem, and that slams down on an unusually hard PBT housing after cresting an unusually firm tactile bump. I think this is what some folks would call “extra”. No surprise, these feel crisp like a block of ice and are loud like a baseball bat doing its job.
Deep Seas are a recent switch using dampeners in the housing top and bottom, and compared with the extreme examples above, it’s a bit more balanced and practical. It isn’t silent - but it is dampened and anything but harsh. No high-frequency sounds; just gentle thuds.
Unholy Trues are a frankenswitch made of leftovers, from back when you had to make your own Holy Pandas. The housing and stem are generic enough not to matter, but the spring is the Halo True one I mentioned before - and using it with a linear stem that has a not-long pole will give you a switch that’s quiet by virtue of the spring absorbing more of the energy near the bottom of travel. It starts light, actuates at an average weight, but quickly gets heavy after that. If you’re practicing the art of not bottoming-out, these are really helpful. If you aren’t, they’ll still make a lot less noise than the same switch with normal springs.
In the end it’s down to how a given stem, spring, and housing work together in practice, but some of the general information might help narrow your search.
Switch stem striking the housing makes most of the sound IMO. Everything else make most of the noise.
Well… technically it is the air. Though impracticable, operating a keyboard in vacuum would make all the sounds disappear. Well… works in theory.