FIG. 4(b) is a graph depicting an example of the acoustic signal observed by the acoustic signal observation means 102. In the graph in FIG. 4(b), the horizontal axis represents time t, and the vertical axis represents the signal value y(t) of the acoustic signal observed at time t. The acoustic signal observed by the acoustic signal observation means 102 is hereafter also referred to as “observation acoustic signal”.

Subsequently, the acoustic property calculation means 103 compares the transmission acoustic signal and the observation acoustic signal, and calculates, from the change between the signals, the acoustic property of the acoustic signal when the acoustic signal propagates through the part of the head of the user (step S103). Examples of the acoustic property include an impulse response, and a transfer function obtained by Fourier transforming or Laplace transforming the impulse response. The acoustic property preferably includes information of how the acoustic signal reflects and/or attenuates in the living body. In the case of placing the earphone and the microphone at the opening of the ear canal and measuring the acoustic property of reflection through the ear canal, the acoustic property may be an ear canal impulse response or an ear canal transfer function.

FIG. 5 is a graph depicting an example of the impulse response as the acoustic property calculated by the acoustic property calculation means 103. In the graph in FIG. 5, the horizontal axis represents time t, and the vertical axis represents the value g(t) of the impulse response of the acoustic signal observed at time t.