the second s signal no 1 of the second frequency f 2 however the height of the first r signal no 1 of the first frequency f 1 is higher than that of the second frequency f 2 i e the second r signal no 1 of the second frequency f 2 is largely attenuated compared with the first r signal no 1 of the first frequency f 1 similarly the second r signals no 2 4 are largely attenuated i e reduced here attenuation loss p i e absorption loss of the ultrasonic wave is obtained by the following formula p u221d u2147 mr f u2062 u2062 1 m 33 0 2 u2062 t u2062 f 2 u2a2f 10 12 mf k 2 u2062 u03c0 u2062 u2062 f 2 u2062 u03c0 u2062 u2062 f k f2 k 1 92 u2a2f g 0 g u2a2f h 1 3 u2a2f 10 5 f3 here m represents absorption coefficient r represents transmission distance m represents a predetermined coefficient f represents a frequency t represents a temperature g 0 represents a saturated vapor pressure g represents a total air pressure and h represents a humidity from the above formula f 1 the attenuation loss p depends on the frequency f as the frequency f of the ultrasonic wave becomes larger the attenuation loss becomes larger further the attenuation loss p depends on not only.