A Novel Water Method for Reducing Air Conduction in Soft Tissue Conduction

Background: Bone vibrator (BV) stimulation applied to skin sites on the body elicits hearing by soft tissue conduction (STC). However, BV stimulation to sites far from the ear requires the delivery of higher-intensity stimulus vibrations to achieve threshold, which can then induce hearing by air conduction (AC) contamination. This problem limits the study of STC thresholds at sites more distant from the ear. Objective: To overcome this problem, we evaluated the possibility of delivering STC vibratory stimuli to body sites in a water bath, based on the different acoustic impedances between air and water, which produces a 30 dB reduction in transmission from water to air. Methods: A standard clinical BV delivered vibration stimuli (tonal and speech stimuli) applied directly to two body sites: finger and foot. BV and body sites were immersed in a water bath. One control involved both stimulation site and BV both in water, but not in contact. In an additional control, the BV was in the bath, while the stimulation site was out of the bath. Results: STC hearing of both pure tones and speech could be elicited at stimulus intensities below those induced by control stimulation (body site and BV both in water, but not in contact; BV in bath, stimulation site out of bath). STC thresholds at the finger site were lower than those at the foot. Conclusions: The current results suggest that water-immersion method enables study of STC hearing in response to higher-intensity vibrational stimuli, and at body sites more distant from the ear, without contamination by AC hearing.