Limitations of the Envelope Difference Index as a Metric for Nonlinear Distortion in Hearing Aids.
James M Kates
PMID 31356388WHAT IT FOUND
The Envelope Difference Index is not a reliable way to judge nonlinear distortion in hearing aids.
Linear frequency changes can raise it as much as clipping does, so it cannot separate linear from nonlinear processing.
Key findings
01The EDI is a broadband envelope measure and cannot separate linear from nonlinear envelope modifications.
02For simulated hearing-aid linear amplification, the EDI reached 0.524 on a 0-to-1 scale even when no nonlinear distortion was present.
03Spectrum equalization reduced EDI values and gave the closest agreement with cepstral correlation, but this agreement is not guaranteed for other nonlinear processing.
STILL TO COME
How it was doneWhat they foundWhat it means for SLPs
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What it does not show
The work used simulated signals and assumed normal hearing for the broadband comparisons, so it does not show how real listeners perceive hearing-aid speech quality. It tested only nine of the ten standard audiograms and excluded severe loss outside the HASQI validation range. The best agreement after spectral equalization is not guaranteed for other nonlinear hearing-aid processing. The comparison used a male talker sentence, so other speech materials may behave differently.
Declared interests
The supplied metadata lists NIH extramural and non-U.S. government research support. The article text does not include an author conflict-of-interest declaration.
The easy way to misread this
Do not read a high EDI value as proof of nonlinear distortion in a hearing aid. Linear frequency changes can raise the EDI without nonlinear distortion, and the paper shows this with spectral tilt and linear amplification.