When custom devices are constructed with the receiver and microphone in the same housing, mechanical feedback pathways exist for the structural transmission of sound energy. By removing the microphone from the instrument case, the mechanical feedback pathway is reduced due to the increase in distance between components and decrease in points of contact between microphone, shell and receiver.
The benefits of microphone externalization in Remote Microphone open configurations are masked by the direct feedback pathway of sound leaving the ear canal. However, the closed Remote Microphone configuration reduces the primary acoustic feedback pathway by occluding the ear canal and decreasing the transmission of structurally transmitted feedback via external microphone placement. To that end, a tightly sealed custom shell Remote Microphone hearing instrument can provide power comparable to an ITE device in the size of a CIC.
This concept was investigated during the development of Remote Microphone CIC products. Twenty-six ears were fit with custom Remote Microphone CIC products and traditional CIC products. The devices were closely matched in terms of the shape, size and the fit of the shell. A measurement of maximum stable gain was obtained by setting the gain handles of the fitting software to a flat level and gradually increasing the gain until the point of feedback. Both the traditional CICs and RM CICs were tested using this technique. A baseline insertion gain measure was obtained to ensure that differences in device calibration did not influence the data. These findings suggest that, on average, one might expect a 9 dB increase in maximum gain before feedback in RM CIC compared to traditional CICs.
While the increase in gain before feedback is related to the microphone relocation, it is considered that other factors such as the fit of the device and the venting of the device are probably also factors.