Does the PolyTune Clip receive input via a built-in microphone? If so, then I assume that it is not useful for tuning in noisy environments where other sounds would interfere. If not, is it based on sensing vibrations from its attached metal clip?
Both actually: there is a microphone on the PolyTune Clip for acoustic use but as soon as you put the PolyTune Clip on an instrument (typically on the headstock) the microphone is muted and only the vibrations of the instrument are taken through a piezo sensor on the metal clip itself. Thus there will no problem trying to tune in noisy environments.