An acoustic attenuator includes an intake air duct having an intake air duct opening leading to an outside environment and a blower fan opening leading to a blower fan. The method includes generating multiple frequency signals having frequencies selectable by control signals, combining the multiple frequency signals into a combined frequency signal having multiple frequency components, and applying the combined frequency signal as an excitation signal to a sensing element. With the power supply device, even though the fuel-cell is used as a power source, a DC power supplied to the load devices can be gradually changed and therefore reduce environmental load. Consequently, the vehicle may be more effectively steered as a result not interfering with its tracking or bringing about creep thereof. A sliding block on bottom of the elastic pressing member is mounted in a sliding groove of the main body while sliding distance of the sliding block is restricted by a stopping rod in a stopping hole. Besides, an RFID readerwriter device, which includes a transmission circuit transmitting a signal via an antenna, and a conversion circuit receiving a signal representing a size of the transmitted signal via the antenna, and converting into distance information based on the received signal, is provided.