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It must be considered that electrolytic capacitors are inherently noisier than other types, and can suffer from microphony. In many instances, "designers" just throw in an electrolytic capacitor without considering its actual reactance and the practical frequency response of the circuit they're putting together. I've often seen guitar effects boxes with responses down to the milliHertz region! Good designers know that there's not much point in having a response much below the frequency of the lowest string on a guitar (or a bass), and that a response down to very low frequencies is often detrimental - the "handling noise" from a guitar or a bass can really cause the speaker cones to flap if very low frequencies are allowed through!
I remember one well-known (and respected) maker of a triggered filter who had huge problems with the filter spontaneously changing frequency in response to inputs that he couldn't hear. When I pointed out that the over-specified input capacitor to the sidechain circuit gave the rectifier a response down to <2Hz, the value was changed to 1/20th of its original value, and the problem disappeared!
Exercises for beginners: Look up "Capacitive Reactance" and see how it changes with frequency. Look up C-R filters and find out how to calculate their response. Try designing some simple C-R filters of your own and try putting them in the output of a simple booster or fuzz circuit, and listen to the change you can make with just a couple of parts!
