A plectrum for a stringed musical instrument having a plurality of conductive strings is provided with a non-conductive body and a conductive tip. The conductive tip is sized so as to fleetingly contact a string when the string is plucked with a plectrum. The tip is electrically connected to a monitoring circuitry which provides a triggering signal each time the tip contacts any of the strings. A transmitter and receiver arrangement is provided to monitor the contact of the tip with the strings and generate the triggering signal. The triggering signal is in turn received by a signal processing apparatus which modifies the audio signal output from the stringed musical instrument under control of the triggering signal.