A pressure-responsive signalling valve for use on pneumatic tires is adapted to generate a highly efficient acoustical signal whenever the pressure within the tire falls outside of a predetermined range. The emitted signal is detected by means of an acoustical receiver which, in turn, is coupled to an alarm typically in the form of a warning light or audio device conveniently located within the vehicle to alert the driver. The acoustical valve employs a rolling or flat seal disc diaphragm and a flat sealing arrangement providing a positive sealing action, minimal friction, low hysteresis and extremely precise pressure response characteristics.