Abstract
When the external auditory cannal is closed with an ear-plug, the absolute threshold values of bone-conduction hearing become smaller when frequency is lower than about 2000 cps and these threshold shifts have two peaks at about 150 and 800 cps, the values of these peaks seem to change with the age of the objects. The threshold shifts of a man, 21 years old , are about 30 db at 100 cps, 40 db at 150 cps, 25 db at 300 cps, 22 db at 500 cps, 20 db at 800 cps, 15 db at 1000 cps and 7 db at 1500 cps, and these values of an older person are smaller only at lower frequencies. The threshold values do not very by closing the external auditory cannal with an ear-plug in the person who has the perforated ear-drum and the karies of the malleus. The one peak at 150 cps can be explained by assuming the eardrum to be a system of mass and resistance which resonates with the cavity of the auditory cannal. The other peak at 800 cps is due to the resonance in the middle-ear mechanisms.
