Is Feedback Optimization Always Necessary?

Question

When fitting open canal or Receiver-In-Canal (RIC) hearing aids, we see a large reduction in high frequency gain in comparison with other aids after running feedback optimization. This is seen even if the aid is fit well and no feedback was observed prior to the optimization. Is it necessary to run feedback optimization in all hearing aids, even when the aids are suitably fitted, as this often means a reduction in the headroom!?

Answer

Thanks to impressive digital feedback reduction applications it is possible to provide as much as 20 dB of added stable gain (ASG) when fitting people with today's digital hearing instruments. ASG is the amount of in-situ gain possible before acoustic feedback with the feedback algorithm activated versus deactivated. These algorithms are an important reason why our industry has experienced a transformation from custom instruments to behind-the-ear (BTE) technology over the past five years. That said, just because an "open fitting" application is being employed, it does not mean that a reduction in gain will be either necessary or implemented following a feedback test. And while the selection of the appropriate dome or tip is important in maximizing the fitting, there are some variables which first need to be considered: the wearer's hearing loss, the gain/output capabilities of the hearing instrument itself and primarily the type of digital feedback being used.

There are two primary methods of feedback (or "phase") cancellation being used today - static and adaptive. If static phase cancellation is being used, a feedback test is mandatory in order to set the filter parameters. From here, the fitting can take a number of different directions depending on the manufacturer and/or product: With some products the results of the feedback test must be implemented or the software will not allow the fitting to continue. With other products the feedback results can be ignored and the professional may continue with the fitting. Adaptive phase cancellation (like the FeedbackBlocker™ by Siemens) offers a more flexible application;in this system a feedback test is optional and allows the professional more freedom in that portion of the fitting. Because feedback may be more likely in an open fitting, Siemens does recommend running the test, but it is purely optional and intended only to maximize the gain available after First Fit, not to "set" a feedback filter. In our experience there seems to be a fairly even split on how professionals treat the feedback test using our adaptive algorithm: some run the test religiously, some never run the test and some run it depending on the situation. All groups indicate a high degree of fitting success, so it appears that the beauty of such a system truly is in the eye of the beholder! The bottom line is that each fitting is unique (just like each individual) and it is the professional's responsibility to verify the proper gain and output settings as well as the many other adaptive algorithms to ensure the best outcome for each person.

Dr. Hernandez received his Bachelor of Arts in Speech Pathology & Audiology from Northern Arizona University, his Master of Arts in Communication Disorders & Speech Sciences from the University of Colorado, Boulder and his Doctorate in Audiology from Salus University. He worked clinically in Boulder/Denver for three years before joining Siemens Hearing Instruments in 1997 and is currently employed as a Senior Training Manager. His responsibilities include training staff & professionals across the country on advanced amplification technology, fitting techniques & programming software.