Maximum Stable Gain and Added Stable Gain
This post is a continuation from the post last week that identified three different hearing aid gains that people fitting hearing aids should be aware of. One consisted of the common gain resulting from the difference between the output (amplification) of the hearing aid measured against level of the input signal. The other two were MSG (maximum stable gain) and ASG (added stable gain). This post examines MSG and ASG.
MSG as it Relates to Acoustic Feedback
As described in hearing aid literature, maximum stable gain (MSG) is the maximum possible real-ear gain that can be applied to a hearing aid system without rendering it unstable (feedback oscillation or degraded sound quality due to feedback oscillation). Maximum stable gain has also been referred to as actual feedback limit, maximum feedback-free gain, maximum loop gain, maximum allowable gain, maximum available gain, critical gain, open-loop gain (OLG), and probably other terminology as well. OLG differs from the others in that it is defined as a specific technique to measure the MSG; it will not be included in this discussion.
MSG Measurement (the other MSG)
One method used to measure MSG (not monosodium glutamate, a flavor enhancer commonly added to Chinese food, canned vegetables, soups and processed meats) is to program the hearing aid to linear gain, and disable all adaptive features. To determine MSG, systematically increase gain in each available hearing aid band. Start with the lowest frequency band and increase gain until feedback is detected, or maximum output is reached.
Figure 6. Insertion gain, representing MSG, as a function of frequency for four different brand hearing aids.
When feedback occurs, reduce the band gain to a stable level, just below the point of feedback. Continue this sequence of gain adjustments by frequency band until each has reached its maximum stable gain or maximum available output. Set the instrument gain to the MSG limit and measure its performance using conventional real-ear measurement procedures. Figure 6 illustrates the real-ear (insertion gain) comparison of the MSG of four different brand hearing aids. It is obvious that they all provide different levels of MSG.
A variety of signals have been used to measure MSG. Using white noise makes sense from a practical point of view because the patient does not care at what frequency gain occurs, and attempting to make this measurement on a band-by-band basis is time consuming. Tonal signals (pure tones) and spectrally-colored signals (music, speech) have been used as well. A desired goal of hearing aid design is to provide a high MSG compared to aids with feedback reduction.
Measures of Maximum Stable insertion Gain (MSG) are valuable in that they represent the effectiveness of a hearing aid’s feedback-cancellation algorithm as well as the quality of the instrument’s mechanical design.
Added Stable Gain (ASG)
Added Stable Gain (ASG) is the difference between a hearing aid’s MSG with its feedback-cancellation algorithm turned off and its MSGF with its feedback-cancellation algorithm turned on. This has been identified in the literature also as headroom and gain margin. Essentially, it is the added gain available to a hearing aid user when the hearing aid’s feedback-cancellation algorithm is activated. The higher the ASG, the wider the range of hearing losses that can be fit and the greater the safety margin from acoustic feedback when worn.
Not all feedback-cancellation (FBC) algorithms used in hearing aids provide the same amounts of feedback reduction. Manufacturers generally have their own proprietary algorithms. The consequence of this is that the ASG can vary significantly from one hearing aid brand/FBC algorithm, to another. Those who fit hearing aids would be helped if a reliable and valid measurement of ASG could be used to determine the benefit expected.
Figure 7. The relationship between added stable gain (ASG) and maximum stable gain (MSG) is illustrated, where MSG real-ear added gain is plotted as a function of frequency with the feedback canceller (FBC) activated and deactivated. The difference between them is the added stable gain (ASG).
ASG measurements are used to show differences between MSG without (off), and then with, feedback cancellation (on). The difference between the measurements (Figure 7) identifies the added stable gain (ASG). This measurement value is best employed if expressed relative to a reference algorithm averaged over all subjects, ears, and vent conditions.
ASG Measurement
A clinically viable method of measuring ASG involves gradually raising the broadband gain of a hearing aid until the MSG (with FBC off) is obtained. Record the real-ear aided response. Then, with the FBC algorithm activated, increase the overall gain until the MSG has been reached. Record this second measurement. The difference in gain between the two measurements is the ASG.
Using this ASG measurement method has some disadvantages {{1}}[[1]] https://starkeypro.com/resources/starkey-evidence/research-resources/added-stable-gain[[1]]:
- MSG with FBC “on” is not always reached before the maximum output of the hearing aid is reached, resulting in a ceiling effect.
- Either the hearing aid user or the clinician subjectively observes the presence of feedback. An objective criterion would be better.
- Extensive gain manipulation is required by the experimenter, which is time consuming and can compromise reproducibility.
- Patients may be subjected to annoying feedback and/or excessive gain.
An ASG benchmarking protocol that describes a method to address the above-mentioned shortcomings has been included in a patent by Merks{{2}}[[2]] Merks, I., Starkey Laboratories, Inc. (2006). Method and Apparatus for Measurement of Gain Margin of a Hearing Assistance Device, U.S. Pat. App. 20070217638[[2]]. The benchmark technique described suggests the following advantages over traditional methods: 1) it is free from ceiling effects, 2) it can be conducted on a KEMAR, 3) it is mostly automatic, and 4) it does not require measurements at gain settings close to audible feedback.
