The effects of FM and hearing aid microphone settings, FM gain, and ambient noise levels on SNR at the tympanic membrane

Background: Speech understanding in noise is challenging for individuals with hearing loss. Hearing aids (HAs) alone are typically unable to resolve these listening difficulties. Frequency modulation (FM) systems or other remote microphone accessories, coupled to HA, are intended to provide listeners with a good signal-to-noise ratio (SNR), thus improving signal audibility and speech understanding. Purpose: The goal of this study was to assess variables that influence SNR at the tympanic membrane (TM) when using a remote microphone/HA combination. We examined microphone setting, transmission system gain, and background noise levels using (1) mathematical computations to manipulate variables and observe the outcomes and (2) behavioral testing. Research Design: This study used mathematical computations to estimate SNR at the TM and a mixedmodel experimental design to confirm a subset of the calculations. Study Sample: Ten children with normal hearing (mean age, 13.7 yr) and ten adults with high-frequency sensorineural hearing loss (mean age, 49.6 yr) participated. Data Collection and Analysis: Speech recognition thresholds were obtained using Bamford-Kowal-Bench sentences in the presence of noise. Participants used an FM system coupled to an HA in an FM-only and an FM 1 HA microphone condition. Results: Better performance was observed in the FM-only compared to FM 1 HA condition with the overall amount of the FM-only advantage slightly larger than the mathematical calculations predicted. Further calculations demonstrated that (1) when using an FM-only microphone setting, the SNR at the TM is determined primarily by the SNR atthe FM microphone; (2) when both HA and FM microphones are active, the SNR is determined by the highest level of the speech, which is typically at the FM microphone, and the highest level of noise at either the FM or HA microphone; (3) increasing FM gain has no impact on SNR in an FM-only condition; and (4) in an FM 1 HA condition, increasing FM gain improves SNR. The amount of improvement depends on noise levels at the FM and HA microphones. When the noise levels are similar at the two microphones, an improvement in SNR of ∼2 dB is expected. Greater improvement is expected when the level of the noise at the FM microphone can be reduced relative to the level at the HA microphone. Conclusions: When using a remote microphone system coupled with a listener's HA, several variables influence SNR at the TM. Two variables that can be manipulated by programming of either or both devices are the microphone setting and gain setting. Mathematical calculations were used to determine the specific influence of and interactions between these variables and showed the importance of (1) managing noise levels to optimize SNR; and (2) counseling clients regarding optimal use of and realistic expectations from their system. This information is useful in the clinical management of persons with hearing loss, especially with the advent and affordability of wireless microphone accessories to assist listeners in background noise.