Difference between the default telecoil (T-Coil) and programmed microphone frequency response in behind-the-ear (BTE) hearing aids

Background: A telecoil (t-coil) is essential for hearing aid users when listening on the telephone because using the hearing aid microphone when communicating on the telephone can cause feedback due to telephone handset proximity to the hearing aid microphone. Clinicians may overlook the role of the t-coil due to a primary concern of matching the microphone frequency response to a valid prescriptive target. Little has been published to support the idea that the t-coil frequency response should match the microphone frequency response to provide "seamless" and perhaps optimal performance on the telephone. If the clinical goal were to match both frequency responses, it would be useful to know the relative differences, if any, that currently exist between these two transducers. Purpose: The primary purpose of this study was to determine if statistically significant differences were present between the mean output (in dB SPL) of the programmed microphone program and the hearing aid manufacturer's default t-coil program as a function of discrete test frequencies. In addition, pilot data are presented on the feasibility of measuring the microphone and t-coil frequency response with real-ear measures using a digital speech-weighted noise. Research Design: A repeated-measures design was utilized for a 2-cc coupler measurement condition. Independent variables were the transducer (microphone, t-coil) and 11 discrete test frequencies (15 discrete frequencies in the real-ear pilot condition). Study Sample: The study sample was comprised of behind-the-ear (BTE) hearing aids from one manufacturer. Fifty-two hearing aids were measured in a coupler condition, 39 of which were measured in the real-ear pilot condition. Hearing aids were previously programmed and verified using real-ear measures to the NAL-NL1 (National Acoustic Laboratories - Non-linear 1) prescriptive target by a licensed audiologist. Data Collection and Analysis: Hearing aid output was measured with a Fonix 7000 hearing aid analyzer (Frye Electronics, Inc.) in a HA-2 2-cc coupler condition using a pure-tone sweep at an input level of 60 dB SPL with the hearing aid in the microphone program and 31.6 mA/M in the t-coil program. A digital speech weighted noise input signal presented at additional input levels was used in the real-ear pilot condition. A mixed-model repeated-measures analysis of variance (ANOVA) and the Tukey Honestly Significant Difference (HSD) post hoc test were utilized to determine if significant differences were present in performance across treatment levels. Results: There was no significant difference between mean overall t-coil and microphone output averaged across 11 discrete frequencies (F(1,102) = 0, p < 0.98). A mixed-model repeated-measures ANOVA revealed a significant transducer by frequency interaction (F(10,102) = 13.0, p < 0.0001). Significant differences were present at 200 and 400 Hz where the mean t-coil output was less than the mean microphone output, and at 4000, 5000, and 6300 Hz where the mean t-coil output was greater than the mean microphone output. Conclusions: The mean t-coil output was significantly lower than the mean microphone output at 400 Hz, a frequency that lies within the typical telephone bandwidth of 300-3300 Hz. This difference may partially help to explain why some patients often complain the t-coil fails to provide sufficient loudness for telephone communication.