Some more feedback on…feedback!
I last wrote about feedback in October of 2015. There have been some significant changes in technology.
We all have memories of the sound of a hearing aid squeaking loudly in church. Many times the actual feedback tone was at a frequency the person wearing the hearing aidcould not hear. Those nearby were embarrassed for the one with the whistle and many came away from this memory with the firm resolve to never wear hearing aids “if it was going to sound like that and I was going to look like that.”
The good news is that a great deal has changed in hearing aid technology related to feedback. If fear of feedback is keeping you from hearing better remember that technology today is not your grandparent’s hearing aids!
Feedback is essentially the function of any sound system. It occurs when sound that travels through a microphone to speakers is continuously picked up by the microphone and re-amplified. We also remember the school principal or teacher at an assembly holding a microphone that got too close to a speaker. The wild squeak that came from the amplified speaker on the stage happened when the cycle of input from the microphone repeated itself rapidly until the system basically shouted “I can’t do this anymore!” It didn’t shout it in English but in electronics as a squeal.
In hearing aids, feedback happens on a smaller scale when sound leaks from the receiver back to the microphone, the microphone will continuously re-amplify the sound. This creates a feedback loop, which manifests as a high-pitched, unwanted squeal or “whistle.” Sound leaks back to the microphone when the hearing aid is loosened. This can happen any time a person chews, talks, puts on a hat or combs their hair. Loose hearing aids may also be the result of poor fitting, the fault of the professional.
Vents can also cause feedback issues. Vents are holes drilled into hearing aids that allow amplified sound to escape the ear canal. They help avoid the “occlusion effect,” an acoustic phenomenon that increases the volume perception of a person’s own voice due to sound trapped between the hearing aid and the eardrum. Without proper venting, the hearing aid wearer tends to hear their voice through their nasal cavity like plugging your ears with your fingers and talking. Try it. That’s occlusion.
Digital signal processing is now permitting us to reach our amplification goals without the limitations imposed by acoustic feedback. Early feedback suppression systems worked by reducing the degree of amplification at the feedback frequencies. Or the hearing would “notch out” the offending frequency by markedly reducing the gain around that point.
While both of these methods worked, in that more hearing aid gain was possible before the squealing point was reached, the consequence was less audibility at frequency locations where the person may have required more. Feedback was managed, but to the expense of understanding what was said.
An optimal feedback cancellation or suppression circuit will reduce acoustic feedback without any undesirable modifications of the hearing aid’s frequency response. A number of manufacturers now include this capability in their hearing aids. While each company has its own proprietary software method, they all apparently have this one feature in common.
Generally, feedback cancellation circuits continually monitor the output of the hearing aid to determine whether some portion of the amplified signal contains elements that have the acoustic characteristics of acoustic feedback. This attribute of the circuit jumps into action when it immediately recognizes a sound it just heard. As the sound, the feedback begins to repeat, the feedback circuit first determines the electronic characteristics of the feedback signal and then generates signals of opposite wave attributes that will cancel (or markedly reduce) thefeedback. Don’t let a fear of feedback keep you from hearing better.
Hearing care is health care. If you have the symptoms of hearing loss let a professional help you find out why. The hearing professional will help you sort out the technology level to meet your need, your budget, and answer your hearing need questions.
Jeffrey L. Bayliff, NBC-HIS, is owner of Hear the Birds Hearing Aid Center, Lock Haven, PA