Starkey Research & Clinical Blog

The effect of digital noise reduction on listening effort: an article review

This article marks the first in a monthly series for StarkeyEvidence.com.

Each month scholarly journals publish articles on a wide array of topics. Some of these valuable articles and their useful conclusions never reach professionals in the clinical arena. The aim of these entries is to discuss research findings and their implications for hearing professionals living a daily clinical routine. Some of these topics may have general clinical relevance, while other may target specific aspects of hearing aids and their application.

This first discussion revolves around an article by authors Sarampalis, Kalluri, Edwards, and Hafter entitled “Objective measures of listening effort: Effects of background noise and noise reduction”. In this 2009 study, the authors pursue the sometimes elusive benefits of digital noise reduction. A review of past literature suggests that digital noise reduction, as implemented in hearing aids, benefits patients through improved sound quality, ease of listening and a possible perceived improvement in speech understanding. Significant improvements in speech understanding are, however, not a routinely observed benefit of digital noise reduction and some studies have shown significant decreases in speech understanding with active digital noise reduction.

In a 1992 article, authors Hafter and Schlauch suggest that noise reduction may lighten a patient’s cognitive load, essentially freeing resources for other tasks. To better understand the proposed effect, imagine driving a car in an unfamiliar area. It’s common for drivers to turn their stereo down, or off, when driving in a demanding situation. This is beneficial, not because music affects driving ability, but because the additional auditory input is distracting, effectively increasing the driver’s cognitive load. By removing the distraction of the stereo, more cognitive resources are freed and the ability to focus, or pay attention to the complex task of driving is improved.

In order to better understand how digital noise reduction may affect attention and cognitive load, two experiments were completed. In the first experiment, research participants were asked to repeat the last word of sentences presented in a background of noise. After eight sentences the listener attempted to repeat as many of the target words as they could. The sentence material contained both high-context and no-context conditions, for example:

High context: A chimpanzee is an ape

No context: She might have discussed the ape

In the second experiment listeners were asked to judge if a random number between one and eight was even or odd, while at the same time listening to and repeating sentences presented in a background of noise. Both experiments incorporated a dual-task paradigm: the first asked participants to repeat select words presented in noise, while also remembering these words for later recall. The second required participants to repeat an entire sentence, presented in noise, while also completing a complex visual task.

Highlights from experiment one show:

  • performance in all conditions decreased as the signal-to-noise ratio became more difficult;
  • overall performance in the no-context conditions was lower than in the high-context conditions;
  • a comparison between performance with and without digital noise reduction showed a significant improvement in recall ability with digital noise reduction

Highlights from experiment two show:

  • performance in all conditions decreased as the signal-to-noise ratio became more difficult;
  • reaction times increased with decreased signal-to-noise ratio;
  • at -6 dB SNR, reaction times were significantly improved with digital noise reduction

The findings of this study show that the cognitive demands of non-auditory tasks, such as visual and memory tasks, inhibit the ability of a person to understand speech-in-noise. In other words, secondary tasks make speech understanding more difficult. Additionally, digital noise reduction algorithms can reduce cognitive effort under adverse listening conditions. The authors discuss the value of using cognitive measures in hearing aid research and speculate that directional microphones may provide a cognitive benefit as well.

The clinical implications of this study suggest that patients may find benefits of wearing hearing aids that go beyond improved speech audibility. Modern signal processing may provide benefits that are only now being understood. For instance, a patient may report that hearing aids have made listening easier, that their new hearing instruments seem to suppress noise more than the old ones, but routine evaluation of speech understanding may not show significant differences between the two hearing aids.

Hearing aid success and benefit has traditionally been defined with the results of speech testing, or questionnaires. If advanced technology can ease the task of listening, patients may be receiving benefits from their hearing aids that we are not currently prepared to evaluate in the clinic. Hopefully, work in this area will continue, increasing our understanding of the role that cognition plays in the success of the hearing aid wearer.

References:
Bentler, R., Wu, Y., Kettle, J., & Hurtig, R. (2008). Digital Noise Reduction: Outcomes from laboratory and field studies. International Journal of Audiology, 47:8, 447-460.

Hafter, E. R., & Schlauch, R. S. (1992). Cognitive factors and selection of auditory listening bands. In A. Dancer, D. Henderson, R. J. Salvi, & R. P. Hammernik ( Eds.), Noise-induced hearing loss (pp. 303–310). Philadelphia: B.C. Decker.

Sarampalis, A., Kalluri, S., Edwards, B., & Hafter, E. (2009). Objective measures of listening effort: Effects of background noise and noise reduction. Journal of Speech Language and Hearing Research, 52, 1230-1240.