Sonic Innovation's commitment to design, develop, manufacture and market advanced digital hearing aids and hearing aid components continues, as does their dedication to dispenser and consumer satisfaction.
These commitments are evident in their ongoing advanced research. In June, 2007 Sonic Innovations presented two research posters at the 153rd Acoustical Society of America meeting in Salt Lake City, Utah. These posters have recently been posted on Sonic Innovation's website and are available for viewing. The two posters discuss the topics of validating the Noise Reduction Index (NRI) and comparing speech intelligibility in noise for single microphone hearing aids incorporating 9-channel, 16-channel, and 24-channel noise reduction.
Validation of the Noise Reduction Index (NRI)1
Conventional wisdom asserts that a hearing aid cannot fundamentally alter (improve) the input signal-to-noise ratio (SNR) using a single-microphone signal processing algorithm such as digital noise reduction (DNR). Sonic Innovations presents research exploring the validity of the Noise Reduction Index (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 reduction on a concert stage, an ear trumpet, and a multi-channel hearing aid with a digital noise reduction algorithm both engaged and disengaged.
The concern among some in the engineering community is that the NRI lacks neutrality and transparency as a means of testing SNR. The belief, held by some professionals, is that hearing aids are unable to improve SNR of signals composed of speech and noise when those signals are received through a single input device. The apparent contradiction of mixed signals delineated through a single source microphone has led some engineers to question the utility of the NRI as a viable test methodology.
Results of this study 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. As stated in the poster, NRI "has been shown to be neutral and transparent, reflecting only the changes to the input SNR...The NRI does not provide information as to how the device changes the SNR, only that the SNR has changed."
Speech Intelligibility in Noise with Single-Microphone Noise Reduction Implemented in 9-, 16-, and 24-Channel Compressors2
The second Sonic Innovations poster discusses the long history of theoretical benefit from single-microphone noise reduction and limited evidence of measurable performance improvements. The ability to improve signal-to-noise ratio (SNR) is only possible if the gain can be independently adjusted across frequency to selectively amplify regions with a positive SNR, and decrease the gain in regions with a negative SNR. Compression channel structures can change the ability to separate positive and negative SNRs. To test this hypothesis, researchers tested three signal processing algorithms that varied in the frequency bandwidth of the compression channels. If performance improvements are dependent upon adequate frequency resolution, then the system with the finest resolution is more likely to demonstrate benefit.
The research concluded that the hypothetical "increase in noise reduction benefit with [an] increasing number of compression channels was not statistically significant with this small sample, but the mean [HINT] scores trend in the appropriate direction." 1
Although the hearing benefits of an increasing number of compression channels are small when compared to the benefits of digital microphone directionality, the authors note as Plomp says "every dB counts."
1Ghent, R., Nilsson, M., Bray, V. (2007, June). Validation of the noise reduction index (NRI). Paper presented at the meeting of the Acoustical Society of America, Salt Lake City.
2Nilsson, M., Hicks, M. Ghent, R. and Bray, V.(2007, June). Speech intelligibility in noise with single-microphone noise reduction implemented in 9-, 16-, and 24-channel compressors. Paper presented at the meeting of the Acoustical Society of America, Salt Lake City.
For further information on these poster sessions and to read the poster reports in full, please visit www.sonici.com/literaturedownloads.
For more information on Sonic Innovations please visit www.sonici.com or Sonic Innovations Web Channel at Audiology Online.
Sonic Innovations Adds Two New Poster Reports To Product Research Archive
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