Speech Envelope Detection vs AGC Attack & Release in Hearing Aids: Updated 2

How the “secret sauce” in the Starkey S-series and the GN ReSound Sparx/Danalogic 6090 and GN ReSound Enzo, LiNX² & Enzo² hearing aids can help ANSD, profoundly deaf, and other challenging patients…

UPDATE 2: Our friend Jason Galster discussed this in 2012 in Prescribing Compression for Severe Hearing Loss in the Starkey Evidence Blog — Do indeed sign up for new post notifications if you haven’t already. We also added in the reference to the K-AMP, which is the first WDRC hearing aid circuit.

UPDATE 1: Our friends at AudiologyOnline.com had a free, brand-neutral webinar related to this topic by Pamela Souza PhD on Understanding and Managing Severe Hearing Loss (link to recorded course) on August 12, 2014.4, B, D We highly recommend you participate in this webinar (free site registration required). Separately, the Danalogic 6090 hearing aid supplied to Britain’s NHS is a rebadged GN ReSound Sparx.C

Let’s talk about speech envelope detection, where it’s needed, what to do to enable it for the patients who need it, and why.

First off, Edgar Villchur’s Wide Dynamic Range Compression (WDRC) is a wonderful innovation for the vast number of people who have hearing loss, and is probably responsible for cutting the number of hearing aid credit returns as it vastly increases patient satisfaction. In fact, many hundreds of thousands (perhaps well into the millions) of people were helped with the K-AMP hearing aid circuit, which was the grounbreaking analog WDRC hearing aid circuit developed by Mead C Killion in 1988. However, by it’s very nature with its fast AGC attack & release times, it “squashes” the speech envelope, and that is the reason for this article.

We have identified at least four classes of people where this fast attack/release is a detriment to speech discrimination, though:

These spectrographs of the same speech sample demonstrate the effects of the AGC attack & release times on the amplitude of the spectral envelope. Note the decrease in amplitude and increase in noise present in the "fast response" compared to the enhanced amplitudes of "slow response" for this speech sample. Image from Spirakis (2011)¹

These spectrographs of the same speech sample demonstrate the effects of the AGC attack & release times on the amplitude of the spectral envelope.
▬► Note the decrease in amplitude and increase in noise present in the “fast response” compared to the enhanced amplitudes of “slow response” for this speech sample.
Image from Spirakis (2011)1

1) Auditory Neuropathy Spectrum Disorder (ANSD), or more precisely, those people who fall into the auditory dys-synchrony part of the spectrum, usually due to missing inner hair cells: Because of the distortion brought on by the dys-synchrony, these people lack the temporal resolution to detect the fine structure of speech, and instead fall back on envelope detection as an adjunct to lipreading. This lack of temporal resolution can be seen in gap detection times: Instead of them being in the 4-9 msec range across the speech range, they can be in the hundreds of msecs. Start with Auditory Neuropathy Spectrum Disorder and Hearing Aids: Rethinking Fitting Strategies1 by Susan Spirakis PhD, who also presented this at the Auditory Neuropathy Spectrum Disorder Conference 2012 in March;

2) Along the lines of ANSD, people who have significant cochlear dead zones in the mid-to-high frequencies, as detected in the Threshold Equalizing Noise (TEN) test,2 would intuitively appear to be unable to resolve the fine structure of speech: More research into this needs to be performed;

3) Profoundly deaf people, especially prelingually deafened adults, absolutely detest WDRC, as they too lack the temporal processing ability, and instead use envelope detection as an adjunct to speechreading. ReSound has it almost right in the Sparx (DanaLogic 6090 for NHS), where WDRC can be disabled with linear amplification used in it’s place. That being said, although we’re now fitting the Sparx on profoundly deaf people with excellent results, we have yet to enable WDRC, as the patients actually understand speech better with these instruments running in linear mode;

4) There’s some tantalizing new research coming out of Nina Kraus’ “Brainvolts” Lab at Northwestern, noted for their neurobiology angle to audiology, by Pamela Souza PhD that correlates working memory ability and long term memory to AGC attack/release times. From 20Q: Cognition Measures -They Might Change the Way You Fit Hearing Aids3

10. You said something about working memory being important in cases where a particular type of hearing aid processing might not be appropriate. What did you mean by that?
I’m glad you asked, because I think this is one of the most interesting and potentially valuable reasons to think about working memory. There is evidence in the research literature that working memory affects the way an individual may respond to hearing aid processing. Let’s focus on two different aspects of hearing aid processing: fast-acting wide dynamic range compression (WDRC) and frequency lowering. Both of these are sometimes used in digital hearing aids, and both offer potential for improved speech-sound audibility and recognition. But both also alter the acoustic properties of the signal in a way that may not be suitable for every patient.

11. OK, I’m interested. I sometimes fit products that have a fast compression release time. Should I be thinking about working memory when I do?
First, let’s think about what a short release time (with a low compression kneepoint) will do to speech. The hearing aid gain is adjusting very quickly, so the hearing aid improves consonant audibility but at the expense of more alteration of the signal. The longer the release time, the more similar the amplitude variations (or “envelope”) of the output signal will be to unprocessed speech. To understand where working memory comes in, consider one study in which the experimenters measured working memory, attention and reaction time (Lunner & Sundewall-Thoren, 2007). They fit their patients with two-channel WDRC with two different compression settings: a fast WDRC with a 40 ms release time and a slow WDRC with a 640 ms release time. Patients wore each compression setting for 10 weeks and speech recognition was measured in unmodulated and modulated noise. For the modulated noise, patients with better working memory did better with fast-acting WDRC. In fact, the cognitive score explained 39% of the variance in performance, while the amount of hearing loss explained only 3%! So, this really supports the idea that in a complex listening task (in this case, the noise is modulated so that you need to “glimpse” the signal of interest through the noise) and when fast WDRC is altering the signal, cognition matters. That reinforced earlier work which showed a similar relationship between cognitive ability and release time (Gatehouse, Naylor, & Elberling, 2006).

Hearing aid programming:

This section features hearing aids engineered and built by GN ReSound and Starkey: If anyone knows of other instruments which don’t squash the speech envelope, please let us know.

The ReSound Sparx provides the choice of flashing linear or WDRC firmware in the selection screen. We like the Sparx, as it’s a truly linear “Big Boomer” that nicely emulates the “analog sound” longtime severely & profoundly deaf hearing aid users need, plus it has the advantages over true analog circuits of good digital feedback reduction, three programs, and basic noise reduction. However, the two disadvantages are a one year warranty, and more importantly even current Sparx production lacks nanocoating, which reduces reliability.

Here are the relevant Aventa screens:

Sparx selection screen in ReSound Aventa 2.95 programming software, where true linear or WDRC is selected.

Sparx selection screen in ReSound Aventa 2.95 programming software, where true linear or WDRC is selected.
Click to enlarge in a new window

Sparx selection screen in ReSound Aventa 2.95 programming software, where true linear or WDRC is selected. Click to enlarge in a new window

Sparx gain adjustment screen in ReSound Aventa 2.95 programming software, where true linear has been selected on the left side of the screen, and WDRC is selected on the right. Note that for the linear firmware, the gain is constant for all input levels and the compression ratio CR is a constant 1.0; while for the WDRC firmware the gain is much lower for 80dB SPL input (black line) than for 50dB SPL (green line), and the compression ratio CR is >1.0
Click to enlarge in a new window

The non-wireless Starkey S series aid is the final one built where the AGC attack & release times can be lengthened to 5 seconds, and is available in all styles except super power. However, since patients who have ANSD or issues also have extra difficulty extracting and properly perceiving speech in noisy environments, we only recommend the to S-11IQ version, as it has the strongext noise reduction.

Here is the relevant Inspire programming screen, which is located on the Compression tab under Fine Tuning on the left:

The AGC attack & release times for Starkey hearing aids that have this feature are adjusted in the Inspire fitting software in the Compression tab, found under the Fine Tuning tab, circled in red in the middle near the bottom.Click to enlarge in a new window

The AGC attack & release times for Starkey hearing aids that have this feature are adjusted in the Inspire fitting software in the Compression tab, found under the Fine Tuning tab, circled in red in the middle near the bottom.
Click to enlarge in a new window

References:

  1. Auditory Neuropathy Spectrum Disorder and Hearing Aids: Rethinking Fitting Strategies by Susan Spirakis PhD: Hearing Review, Fall 2012 edition
  2. Threshold Equalizing Noise (“TEN”) test, by Prof Brian CJ Moore, Cambridge University Auditory Perception Lab;
  3. 20Q: Cognition Measures -They Might Change the Way You Fit Hearing Aids  by Pamela Souza PhD with Gus Mueller
  4. AudiologyOnline Course #23839 (Live), #24805 (Recorded): Understanding and Managing Severe Hearing Loss Tuesday, August 12, 2014 at 1600 UCT (12:00 pm EDT). Course presenter: Pamela Souza PhD.
  5. Using multichannel wide-dynamic range compression in severely hearing-impaired listeners: effects on speech recognition and quality: Pamela Souza PhDLM Jenstad, and R Folino.  Ear and Hearing 26(2), 120-131 (2005)
  6. Severe Hearing Loss – Recommendations for Fitting Amplification, by Pamela Souza PhD. AudiologyOnline, Jamuary 19, 2009
  7. Prescribing Compression for Severe Hearing Loss, by Jason Galster in the Starkey Evidence Blog, April 1st, 2012.

Bootnotes:

A) We at The Hearing Blog highly recommend every hearing professional have a copy of the TEN test in their bag: It’s inexpensive, it’s helpful for troubleshooting problem hearing aid fittings, and it can be used as a screening tool for ANSD when a clinical tympanometer is not available. It comes in two flavors: The TEN(HL) test which is used with TDH headphones, and the TEN(ER3) which is used with The Big Bald Guy’s insert earphones — We recommend keeping copies handy of both versions. If you send Professor Moore a USPS money order, you’ll have your CD’s in about a week — Click here to order the TEN test materials from him. The TEN test instruction manual for the Frye Fonix FA-10 & FA-12 audiometer is available at this link; other clinical audiometers are configured similarly.

B) AudiologyOnline Course #23839 Abstract:
Among all individuals with hearing loss, patients with severe hearing loss are the most significantly impaired, and the least successful with hearing aids. And there is no “typical” patient – communication ability varies widely, as does hearing-aid success. In this presentation we explore the abilities, challenges and possible solutions for adults with pure-tone thresholds between 60 and 90 dB HL. We will discuss common etiologies, pure-tone sensitivity, dynamic range, assessment of dead regions, and objective and subjective speech-recognition abilities. We will also consider fitting options, including a (sometimes surprising) sample of the technology use by patients in this group.

C) We just found out from our British colleague Gemma Leadbetter of the Dumfries and Galloway Royal Hospital clinic that the Danalogic 6090 hearing aid supplied to their NHS is a rebadged GN ReSound Sparx. She writes, “What an interesting article: It explains perfectly why my prelingually deafened patients can’t get enough of these aids… Linear all the way!

D) AudiologyOnline.com is perhaps the very best resource for audiologists, hearing aid professionals, and the hearing impaired community in general. Their articles and webinars are all free (free site registration required); and if you need professional CEU’s their $99 annual “all you can eat” price is the best bargain in hearing healthcare. If you haven’t registered with them yet, here’s the link to do it now.
~

← First Person Report: Not Everyone Can Adjust to Digital Hearing Aids After A Lifetime Of Analog(ue) [Article deleted] →

About the author

Dan Schwartz

Electrical Engineer, via Georgia Tech

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