The voltage applied to a power converter is detected by a voltage detector, a function generator produces a voltage proportional to this detected voltage, and a high-pass filter detects an AC component superimposed on the voltage. The acoustical improvement is achieved by providing a superior sound reflecting surface on the film or skin of the pad that engages with the tonehole. The fixed body includes: a first power-transmitting electrode and second power-transmitting electrode ; and an AC power supply to supply AC power to the first power-transmitting electrode and second power-transmitting electrode. Both marking and diagnosing may be performed at any location within the CDN or at any host computer connected to the CDN or receiver. The second stitch line is adapted to remain intact and being capable itself of maintaining the protection device in the initially deployed condition subsequent to rupture of the first stitch line and throughout inflation and deployment of the protection device.