Abstract
Absorption mechanism of porcus material for a sound pulse is analysed by means of the Fourier transformation of the incident and reflected pulses. Measurement is made by impinging a sound pulse to a specimen mounted into a pipe (Fig. 1). For a thicker specimen compared with the pulse length, one obtains discrete series of the reflected pulses spaced with a certain duration corresponded to the time propagating in the specimen (Fig. 4). When this pulse series of the reflected wave is analysed as a whole by the Fourier transformation (Eq. 2), the absorption coefficient agrees with the values by the ordinary standing wave method for the sinusoidal waves (Fig. 5). This means that the reflection coefficient of a specimen identifies with the transfer function of the specimen for a linear system (Eq. 8). On the other hand, if the power spectrums of the individual pulse in the reflected pulse series are analysed and the absorption coefficients are calculated from them (Eq. 3), then the frequency response is different from the values of the former (Fig. 6). However, if the total absorbed energies for both cases integrated over the whole frequency range are calculated (Eqs. 14-17), it results in good agreement between them. Therefore, even if the frequency characteristics of the absorption coefficient are different, its total absorbed energy is equal (Eq. 18), and then the total absorption of pulse energy is determined by the attenuation constant of the material, and the frequency characteristics of the absorption coefficient depend upon the acoustic situation in which a specimen is mounted.
