I’ve had Resound Prego HAs for 6 months. They rarely give spontaneous feedback but at times I can induce feedback, especially in the right HA, by cupping my hand over the ear. Most of the time this cupping will not cause feedback but sometimes it will. I just wonder what factors might affect this inconsistency?
How occlusive a fit do you wear? I wear a very occlusive fit and so normally need little feedback control since little or no sound can escape my ear canal to the external HA microphones. But if my custom mold is not properly positioned and there is some space between the side of the mold and my ear canal, that can generate feedback.
P.S. Since the amount of feedback varies with sound frequency, another explanation could be the relative strength of various frequency components at the time that you generate feedback, i.e., the 3 to 4 kHz sounds that are especially likely to generate feedback just happen to be exceptionally strong in the sound that you are listening to when you happen to bring your hand near your ear, etc.
The earpiece covers are not domes but two thin flaps of silicone that form a cone. I don’t know what they are called, can you tell me?
So is it the sound from the speaker in the ear leaking out to the external mic that causes feedback?
Yes on the sound leaking out. That’s why when you bring your hand near your ear you get the feedback. Sound that was escaping into space is now reflected back towards your external HA mics. Feedback is a loop. And as Sierra “edyookated” me, paradoxically it’s the softer (lower volume) level sounds in the feedback-sensitive frequency range that cause the problem - because they are amplified more in going from mic to receiver than louder sounds of the same frequency, which may not be amplified in passing through your HA’s at all. Say the amplification is adding 30 dB of level each time thru your HA, so 20 dB ->50 dB (+30) -> 80 dB -> PAIN! whereas for higher volume level initial input 60 dB -> 60 dB -> 60 dB (your HA has what’s called a gain table based on your audiogram - you lose sensitivity to softer sounds at a certain frequency but can still hear louder sounds - so your HA adds gain (dB) to sounds that would be too soft to otherwise hear but adds little or no gain to louder sounds while still trying to keep a distinguishable range between the amplified soft sounds (now louder) and moderate to very loud sounds at the frequency for which your hearing is being “repaired.” So you can still hear a difference between “soft” sounds, moderate, and loud sounds but now everything is scrunched together somewhere in the moderate to loud range (it’s called COMPRESSION) whereas when you had good, normal hearing you used to enjoy the wide rangge of difference between very soft and very loud - but now, without your HA’s, “very soft” is dead silence… But it’s the relatively great amplification of the soft sounds that causes the feedback loops.
Update: based on the gain table, 80 dB -> PAIN in a feedback loop is not quite right but the sequence captures the general idea, so I’ll let the text stand as is. HA feedback is basically the same sort of thing as auditiorium feedback where the output of the auditorium sound is going back into the speaker’s microphone and getting re-amplified and resulting in unpleasant output louder than you want to hear.
Thanks, Jim. So feedback without hand cupping would be caused by generous leakage of sound from the mic out between domes (or whatever those thin flappy cone things are called) and the ear canal? Solution would be a better seal? Ideal would be complete seal?
My first HAs from two iterations back had custom silicone molds with a hole drilled in them, I guess to let out internal sounds like chewing (or thinking hard?). With my Resound HAs do these sounds escaped around the edges of the seals or back through the speaker, or what?
Your provider can also adjust the HA’s electronic feedback control in the fittings settings. When fitting, there is a test for feedback. A spectrum of sound frequencies is played into your ear from the receiver and the fitting software measures from what’s picked up by your external mics what the propensity for feedback is at each frequency and adjusts the allowed gains for soft, moderate, and loud sounds accordingly. Problems arise if you change your fitting (without changing your feedback control settings) or your fitting no longer fits your ear properly or is not properly positioned in your ear, etc. But if you regularly see a provider, you could mention your problem to the provider and let him/her use their expertise to handle it. I am barely knowledgeable on these things so take any advice I provide with a grain of salt and check it out with your provider.