The absolute threshold value of bone-conduction hearing has been measured by observing the force transmitted from an electro-magnetic vibrator on the head skin by BaTiO_3 ceramic between the vibrator and the skin. The thresholds on the mastoid(excited area 1cm^2)with the open auricle were about 800dynes at 100cps, 30dynes at 500cps, 25dynes at 1000cps, 3dynes at 2000cps and 20dynes at 4000 cps, respectively. The distinctive nodal lines were observed at about 250, 400 and 600cps, due to the resonance of skull. In the case of the closed auricle, the decrease of threshold value was about 25 db at 100 cps, 35 db at 150cps, 20db at 200cps, 10db at 400cps, and 0 db at 800cps, respectively. The above phenomena could be explained by assuming the ear-drum to be mass(19. 6mg)and resistance(3. 24g/sec)
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