When you sound a tuning fork, you hear an pure tone/sine wave of usually 440Hz. Yesterday, I tried hitting a table knife made entirely from stainless steel against a grapefruit. When I held it up to my ear, I heard a frequency of roughly 698Hz, working nearly exactly like a regular tuning fork, but with a slightly less sustained tone. It got me wondering; what are the conditions for a pure tone like this to occur?
I'm going to assume that the tones from a tuning fork (and my table knife) aren't entirely pure. I know there are some upper frequencies that ring out aswell, and possibly some other stuff. But some objects make an entirely different sound all together, with much clearer overtones and a more specific timbre. In contrast to those, I think it's safe to say that tuning forks essentially sound a relatively pure tone.
But what are the conditions for a pure tone like this to occur? I know they can be picked out as overtones from a note with harmonics. But what makes a tuning fork not sound a standalone tone with no harmonics, or at least rather inaudible ones? What are the actual, physical conditions for something like this to occur? And how can one possibly recreate it?
Apologies if I've missed anything obvious or formatted anything badly, this is my first post on this site. Thank you in advance!