Validation of the noise reduction index (NRI) as an estimate of the signal-to-noise ratio change of a mixed signal by a hearing aid
Robert M. Ghent, Michael J. Nilsson, Victor Bray · The Journal of the Acoustical Society of America · 2007
The NRI is an extension of an idea discussed by Licklider [J. Acoust. Soc. Am. 20, 150–159 (1948)] and implemented in practice by Hagerman and Olofssen [Acta Acoustica 90, 356–361 (2004)] to investigate the conventional wisdom that asserts that a hearing aid cannot fundamentally alter (improve) the input SNR using a single-microphone signal processing algorithm such as digital noise reduction. The present research explores the validity of the NRI by using it to estimate the change in input SNR through a variety of audio devices, including linear hearing aids (with and without directional microphones), a directional microphone designed for noise rejection on a concert stage, an ear trumpet, and a multi-channel hearing aid with a digital noise reduction algorithm both engaged and disengaged. Results indicate that the change in SNR through an audio device can be negative as well as positive, that the NRI is a robust and valid method for estimating this change, and that single-microphone noise reduction algorithms can improve the input SNR when properly designed. The test setup and methodology, along with some pitfalls to avoid when performing these measurements, will be discussed.