The present invention relates to a piezoelectric resonator system comprising a plurality of piezoelectric resonator sensors C', C''. Each sensor C', C'' comprises an oscillator circuit 29', 29'' which is surrounded by its own individual conducting shield 44', 44''. Each shield 44', 44'' is connectable to one pole of the power supply for its respective sensor C', C''. These shields 44', 44'' prevent electromagnetic radiation from the sensors C', C'' from being transmitted through air.