Finding the Optimal Head Position for Bone Conduction Sound Measurements when Using a Skin Microphone

Abstract

Abstract The global problem of hearing loss, which affects roughly 1.5 billion people, has been addressed for hundreds of years. In the 1970s, the concept of bone anchored hearing aid was developed, transmitting vibrations directly to the skull bone and is estimated to be implanted in over 300 000 patients worldwide, mainly who are suffering from conductive or mixed hearing loss. Now there are several models of implantable bone conduction hearing aids on the market, and there is a need to examine their sound quality in place. The aim of this study was to determine the optimal position when using a skin microphone to measure bone conducted sound induced in the skull bone. The optimal position could then be used in a subjective assessment of the sound quality when listening to a person’s bone conducted sound. The same position could possibly be used in a verification method when fitting bone conduction devices, similar to real ear measurements for air conduction hearing aids. In the physical measurements, a skin microphone based on an electric condenser microphone from Sonion was used to measure bone conducted sound. In the tests, two different transducers were used as sources of the bone conducted sound; the au diometric transducer Radioear B81 and an Intenso hearing aid on softband. Tones at specific frequencies and levels were applied from a clinical AC40 audiometer from either a speaker as sound field exposure to the Intenso sound processor or directly to the skull from the B81. The measurements were divided into three different trials. In the first trial positions on the forehead and around the ear were examined, as well as the effect of hair between the microphone and the skin. In the second trial, higher frequencies and additional positions on the head were tested. In the final trial, the most promising positions were chosen and tested on four test subjects. A test-retest was performed on one of these four subjects. The results showed that the hair had an negligible effect on the skin microphone for these test subjects and that the ipsilateral eyebrow was the most stable position between subjects for both devices when comparing the left and right sides. Each subject had different optimal positions, primarily positions on the ipsilateral side of the head. Another criterion investigated the positions with the lowest standard deviation between measurements for all subjects, where the ipsilateral eyebrow for the B81 and the ipsilateral positions on the head for the Intenso had the lowest value. Future research could examine how much the sound leakage and pressure of the skin microphone affect the measurements.