Frequency Modulation System and Bone Conduction Hearing Aid: Electroacoustic Verification.

A protocol has not yet been developed to perform electroacoustic measurements and behavioral tests to fit the frequency modulation (FM) system in bone conduction hearing aid (BCHA) users. Electroacoustic verification, with "FM transparency" achieved, ensures user audibility of FM transmitter and hearing aid signals.  To propose and validate a protocol for electroacoustic verification of the FM system coupled to the BCHA.  Twenty-four sets of FM system and BCHA were submitted to electroacoustic verification, using a receiver and a plastic adapter to connect the BCHA to a 2cc coupler in the hearing instrument analyzer. The measurements were performed in the acoustic box, at 65 dB sound pressure level (SPL), with International Speech Test Signal (ISTS), first to the BCHA microphone and then to the FM system microphone, to determine the transparency, in which equivalent inputs for both microphones result in equivalent outputs. The FM gain or volume has been adjusted to try to gain transparency for the outputs of the two input devices.  Transparency was achieved for all sets evaluated, but in some combinations, adjustments to the FM receiver gain over the manufacturer's default setting were required.  The proposed protocol proved to be effective for the electroacoustic verification of the FM system coupled to the BCHA.