PP101 - Acoustic Impact of Degree and Configuration of High-Frequency Hearing Loss on Hearing Aid Fitting

Abstract:
This study investigated how the degree and configuration of high-frequency hearing loss (HFHL) affect hearing aid fitting acoustically. Software simulations and real ear measures on Kemar were completed using four groups of HFHLs, each with five low-frequency thresholds and five HFHL configurations with domes, c-shells, and slimTips for three fitting formulae. Results showed significant impact of the degree and configuration of HFHL on the gain and output. Further effects of multiple aspects were reported. Recommendations regarding appropriate venting and fitting formula choices for different HLs were discussed, providing good acoustic insight for a better fitting outcome.

Summary:
A common hearing aid fitting difficulty for high-frequency hearing loss (HFHL) is balancing issues of occlusion effect, feedback, and adequate gain for speech clarity. Proper vent size is used to address these issues (Dillon, 2012). Studies have found that occlusion effect decreases with increasing vent diameter (e.g., Kuk et al., 2005), and it was recommended to use the degree of HL at 500Hz to determine vent size (Kuk and Keenan, 2006). The degree and configuration of HFHL are also important for vent size choices due to different amplification applied to each frequency based on the HL, which, however, is not systematically investigated. Also, clinically there is no guideline regarding choosing specific acoustic options to fit a HFHL. This study evaluated the acoustic effects of different degrees and configurations of HFHL with considerations of different vent sizes and fitting formulae.

Software simulations and real ear measures on Kemar were completed using four groups of HF sensorineural HLs: mild, moderate, moderately severe, and severe. Each group included five low-frequency (LF) thresholds (0, 10, 20, 30, 35 dB), and five configurations of HFHL (HL range at 2-8, 3-8, 4-8, 6-8, 8 kHz) for each LF threshold, with a total of 25 combinations of HLs in each group. Gain for different inputs and maximum gain before feedback were recorded from the software for dome (open, vented, cap, & power), c-shell, and slimTip (vent sizes: occluded, .9-1.1mm, 1.2-1.6mm, 1.7-2.2mm, 2.3-3.0mm, 3.1-4.5mm, & open). NAL-NL1, NAL-NL2, and DSL-Pediatric fitting formulae were utilized for each acoustic parameter. Real ear occluded gain, aided gain for 20 dB insertion gain, and aided gain and output for different HLs, and maximum gain before feedback were recorded on Kemar for different domes and fitting formulae. Data were analyzed using Matlab for multiple aspects.

Results revealed that as LF loss increased, both LF and HF gain increased, but the increase of HF loss only led to the gain increase of HF, not LF. Overall, the smaller the range of HFHL, the less gain was needed, with significant gain reduction when affected frequencies moved from 2-8k to 3-8kHz for three formulae, especially NAL-NL1 (up to 20 dB). For feedback-activated fitting situations, there was essentially no gain limitation for mild HL and only affected soft inputs for moderate loss for all formulae and acoustic options. For moderately severe (NAL-NL2: HF=2/3-8kHz, 4kHz with LF=30/35dB; DSL: HF=6/8-8kHz) and severe (NAL-NL2: HF=6/8-8kHz, LF=20-35dB) losses, gain limit affects soft and average inputs up to 12-15dB but the gain restriction was mostly limited to 6-8kHz with occluded-2.2mm vent. When HFHL=6/8-8kHz (Moderately Severe: LF=0-35dB, Severe: LF=0/10dB) for NAL-NL2, gain limit essentially only affected soft ( < 10dB) for all venting. For moderately severe (DSL: HF=2/3/4-8kHz) and severe (NAL-NL2: HF=2/3/4-8kHz; DSL: HF=2/3/4-8kHz, HF=6/8-8kHz with LF=30/35dB), gain limit affected all inputs at 6-8kHz, but with occluded-1.1mm vent, restriction occurred for soft and average inputs only. There was essentially no gain limitation up to severe loss when using NAL-NL1 with the exception of gain limitation up to 11 dB for severe loss (LF=35dB, HF=2-8kHz) using open-2.3mm vent. More information will be discussed/reported for vent/formula selection for desired outcomes.