An audio signal that is output from a sound source circuit (14) is caused to flow through a coil (16) in a transducer for vibrating a soundboard, and the vibration of the soundboard generates an acoustic signal. A microcomputer (30) calculates a temperature of the coil (16) with high accuracy by inputting an ambient temperature (Ta) detected by an ambient temperature sensor (21) and a voltage (V) applied to the coil (16) and executing computation based on a thermal equivalent circuit of an acoustic signal converter using the ambient temperature and the voltage that are input thereto, the computation including calculation of an amount of electric power consumed in the coil (16) using the input voltage.