Open fit hearing instruments can be fit with a multitude of solutions for coupling to the ear canal. Manufacturers have different strategies for accounting for these solutions, and it may be unclear to clinicians what the result of choosing different parameters in fitting software is. ReSound’s Jennifer Groth, M.A., & Erica Koehler, Au. D., provided tips for open and occluding fittings at the 2013 AudiologyNOW! conference.
In 2003 the open fit option revolutionized how we fit hearing aids today for one of the most common hearing losses. Dome options include:
- Open domes: least occluding and ideal for normal hearing below 1kHz.
- Power/double domes: occlusive. Ideal for those that have hearing greater than 40dB in the low frequencies and need 10dB or more of low frequency gain.
- Hollow molds: more occlusive than an open dome but less occlusive than a double dome or solid mold.
- Solid molds: provides the most low frequency gain, 3-11dB at 250-1kHz.
Acoustic mass determine occlusion:
Coupling choices also impact canal resonance:
How fitting ranges are calculated:
- Electroacoustic specs in coupler
- Real-ear-to-coupler difference +MLE (CORFIG)
- Fitting prescription
- Add CORFIG to coupler FOG to get max insertion gain.
- Plug audios into prescription.
- Build in safety margin based on MSG to allow VC use and fine-tuning.
Remember, fitting ranges are not standardized! The first fit matters more than the fitting range.
In conclusion, make sure that the physical configuration of what you are fitting matches the software configuration and if in doubt, and even if not, measure.