Abstract: Devices are disclosed for use in generating an acoustical warning signal in the event of an abnormal pressure in a pneumatic tire. A spring-loaded piston is mounted for reciprocation in a cylindrical chamber along an axis perpendicular to the radius of the tire for substantially eliminating the effects of centrifugal force on the operation of the device. The piston maintains the device closed until such time as the tire pressure drops below a predetermined limit at which point a small sampling of air is released into a whistle which produces a coded acoustical signal detected by an acoustical transducer mounted on the vehicle. The transducer converts the acoustical signal into an electrical signal which is processed and used to actuate an alarm for warning the driver. Cap seals are disclosed which protect the whistle from water dirt, etc.