Effect of static clenching on perception of bone-conducted sounds presented to the facial parts: Assessments by hearing threshold and ear-canal sound pressure

BC transducers are usually presented to the mastoid or condyle processes; however, audio devices using BC to the face, such as smart glasses, have been developed recently. The face has a complex structure consisting of many bones and muscles intricately intertwined. Furthermore, unlike the mastoid and condyle processes, the shape and hardness of the face are constantly changing. Therefore, BC sounds presented to facial parts are likely to show different characteristics from those presented to conventional parts. We examined effects of static clenching on BC perception presented on the facial parts (jaw angle, zygomatic) and ordinary parts (mastoid, condyle) using normal hearing subjects wearing earplugs. Hearing thresholds and ear-canal sound pressure (ECSP) were measured under three conditions: Closed-mouth, Open-mouth (no clenching), and Clenching. The hearing threshold increased in Clenching condition at low frequencies, whereas there was little change at high frequencies. The effect was roughly the same in facial parts and ordinary parts. In the low--frequency range, clenching may have increased physiological noises in the external auditory canal caused by blood-flow and/or muscle contraction, interfering with the perception of BC sounds.