[Cochlear microphonic potential (CM) recordable at non-shielded bedside--with reference to the development of a new...

By making several improvements in current hearing aid earphones, a new earphone (NC-3) for CM measurement in electrocochleography (ECochG) has been developed. A silicone tube with a 1.5mm inside diameter, 175mm in length, was attached to the acoustic hole side of the earphone. The earphone proper was shielded with aluminum foil and one end of the foil was connected to a low noise cable. Human forearm was used as a dummy ear and the electrode HN-5 was fixed thereon, and a sound stimulus of 90dBnHL was delivered by the earphone (NC-3). No measurable artifacts, such as electromagnetic conductions and the CM-like mechanical vibrations generated by the acoustic output system, were recorded. By placing the earphone (NC-3) from a right-angle to a diagonal direction toward the electrode circuit, electromagnetic conduction contamination was prevented. With an extratympanic procedure using the electrode HN-5, ECochG-CM was recorded from normal hearing subjects in both a shielded sound-proof room and a non-shielded ordinary but quiet room. Short tone bursts at 1 and 4kHz were employed as acoustic stimuli and delivered by the earphone (NC-3). In the non-shielded quiet room, no electromagnetic conduction contamination or mechanical vibration was observed and there were no differences in CM responses between the two rooms. These results suggest that this earphone (NC-3), making CM recordings possible at the ordinary bedside without shielding, may contribute significantly to the subsequent spread of ECochG-CM measurement, by compensating for the disadvantages of a loudspeaker.