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Integrated Active Noise Control and Noise Reduction in Hearing Aids

In every day life conversations and listening scenarios the desired speech signal is rarely delivered alone. The listener most commonly faces ascenario where he has to understand speech in a noisy environment. Hearing impairments, and more particularly sensorineural losses, can cause areduction of speech understanding in noise. Therefore, in a hearing aidcompensating for such kind of losses it is not sufficient to just amplify the incoming sound. Hearing aids also need to integrate algorithms that allow to discriminate between speech and noise in order to extract a desired speech from a noisy environment. A standard noise reduction scheme in general aims at maximising the signal-to-noise ratio of the signalto be fed in the hearing aid loudspeaker. This signal, however, does not reach the eardrum directly. It first has to propagate through an acoustic path and encounter some perturbations which are, by design, neglected in standard noise reduction schemes.

In an open fitting setup, there is no earmold to prevent ambient sound from reaching the eardrum or to prevent the sound amplified by the hearing aid from leaving the earcanal. This signal leakage through the open fitting combined with the attenuation in the acoustic path between the hearing aid loudspeaker and the eardrum, \textit{i.e.}, the so-called secondary path, can then override the action of the noise reduction in the hearing aid. Active noise control can be used to compensate for the effects of this signal leakage. The principle of active noise control is to generate a zone of quiet based on destructive interference, in this case at the eardrum. In the hearing aids framework, however, active noise control alone is not sufficient. It has to be performed together with the noise reduction algorithm.

This thesis first presents an integrated active noise control and noise reduction scheme for hearing aids to tackle secondary path effects and effects of noise leakage through an open fitting. Integratingactive noise control and noise reduction in a single set of filters allows to compensate for the signal leakage and the secondary path effects.The implementation of the integrated active noise control and noise reduction scheme in hearing aids, however, comes with a number of problems primarily due to the dimensions of the devices.

Firstly, the integrated active noise control and noise reduction scheme does not allow tobalance between the noise reduction and the active noise control. In some circumstances it would be useful to emphasise one of the functional blocks. Secondly, the integrated active noise control and noise reductionscheme relies on an ear canal microphone which should ideally be located at the eardrum. In practice, however, the ear canal microphone is distant from the eardrum and the sound reaching the eardrum is basically unknown and uncontrolled. Finally, the use of active noise control in hearing aids, is limited by the size of the devices and the number of microphones available on each device. The number of noise sources that can be compensated for by the active noise control is limited by the number of microphones available. Also the small separation between the microphones and the loudspeaker results in a short acoustic propagation time and hence small causality margins. 

In order to solve these problems, variations on the integrated active noise control and noise reductionscheme are also presented in this thesis. Firstly, changing the original problem to a constrained problem leads to weighted integrated active noise control and noise reduction schemes. A first weighted integrated active noise control and noise reduction scheme is derived that allows to emphasise either the active noise control (providing an improved signal-to-noise ratio) or the noise reduction (providing a lower speech distortion). A speech intelligibility weighted integrated active noise control and noise reduction scheme is then derived that allows to focus on reducing speech distortion at the eardrum or on minimising the residual noiseat the eardrum. Secondly, an integrated approach to active noise control and noise reduction that is based on an optimisation over a zone of quiet generated by the active noise control is then proposed. This approach allows to overcome the ear canal microphone location problem. Finally,a binaural approach is introduced that allows to access extra microphones from the contra-lateral hearing aid and to design a scheme with increased causality margin.
Date:10 Jan 2008  →  20 Jun 2011
Keywords:Active noise, Hearing aids
Disciplines:Other engineering and technology
Project type:PhD project