Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Health and Rehabilitation Sciences

Supervisor

Scollie, Susan D.

2nd Supervisor

Purcell, David W.

Joint Supervisor

Abstract

Following the fitting of amplification devices, outcomes need to be evaluated to determine benefit of intervention. Objective measurements, which generally require rapid acoustic or electrophysiological measurement without active participation of the individual, are one subset of evaluation methods.

This thesis aimed to improve the reliability, accuracy, and efficiency of a subset of objective outcome measurements: the speech-evoked Envelope Following Response (EFR), and the wideband real-ear-to-coupler-difference (wRECD), which are used in hearing aid validation and verification respectively.

The speech-evoked EFR, a neural response reflecting phase-locked activity to the envelope of a speech stimulus, can be detected using a variety of statistical indicators. Chapter 1 focused on improving speech-evoked EFR detection by comparing the sensitivity and efficiency of statistical indicators in adults and infants. Results show that indicators using phase information tend to outperform those that do not. Accuracy and speed of speech-evoked EFR detection was also found to differ between infants and adults.

The main contribution of this thesis is the proposal and validation of a clinically viable test paradigm for wRECD measurement which does not require a probe-tube microphone and is not affected by reflected wave interference. The projects evaluated the measurement of the integrated pressure level (IPL) wRECD using a Thevenin-equivalent source parameter calibrated transducer. Calibration of the transducer was found to be reliable across time. IPL wRECD improved wRECD reliability, high-frequency performance, and simultaneously assessed middle ear function using wideband acoustic immittance. Below 5 kHz, the IPL wRECD was not clinically significantly different from probe-tube microphone measurements when a generic coupling method was used. An individual’s earmold significantly impact resulting wRECD measurements due to variable lengths of tubing associated with the earmold. The current thesis proposed a method to acoustically determine the tubing length to create accurate and reproducible generic-tip-to-earmold transforms, which improved the estimation of the earmold wRECD.

In summary, IPL wRECD measurement shows promise as an alternative to probe-tube microphone wRECD measurement procedures and is expected to improve validity of hearing aid fittings, especially in the high frequencies. Similarly, speech-evoked EFRs can be used as an objective measurement to validate hearing aid fittings accurately and efficiently.

Summary for Lay Audience

Following hearing aid fitting, outcomes need to be determined to ensure benefit of the intervention. This is done using several outcome measurement tools, such as objective outcome measurements which can be measured by recording brainwaves. Objective outcome measurements are quick and do not require sustained participant cooperation.

This thesis aimed to improve the accuracy and speed of two objective measurements: the speech-evoked Envelope Following Response (EFR) and the wideband real-ear-to-coupler-difference (wRECD). The speech-evoked EFR is used to confirm that the hearing aid is providing enough sound to the individual. The wRECD accounts for individual ear canal effects on sound input to the eardrum, improving test accuracy for growing infants.

The speech-evoked EFR is a brainwave response that can be measured while the participant sleeps. It is detected by various statistical tests which determine if sound is reaching the brainstem. Chapter two compared common statistical tests used for speech-evoked EFR detection in infants and adults. Results show that statistical tests that incorporate response phase outperform those that solely rely on response magnitude.

The main contribution of this thesis is the proposal and validation of a clinically viable wRECD measurement paradigm. Current clinical measurements require a probe-tube microphone placed within millimeters of the eardrum. In contrast, this thesis proposes a new method of measurement, the integrated pressure level (IPL) wRECD. It uses a specially calibrated earpiece which houses a speaker and a microphone. The calibration procedure allows for the calculation of the sound-level at an individual’s eardrum using a measurement by the microphone in the earpiece. This thesis found that this calibration procedure was reliable over time. IPL wRECD measurement using a generic tip and the individual’s earmold was also validated. Results indicate that IPL wRECD measurements are more reliable than conventional wRECD measurement. Above 4 kHz, IPL wRECD improves on probe-tube microphone measurements. Overall, IPL wRECD is a promising alternative to current wRECD measurements and may improve validity of hearing aid fittings, especially in the high frequencies. The IPL wRECD could be used with speech-evoked EFR to provide accurate, objective measurement of infant brain responses to amplified sound from hearing aids.

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