Hearing Aid Feedback from Jaw Movement
As you correctly noted, jaw movement can cause feedback from a hearing aid. This is because the jaw mandible hinges with the temporal bone of the skull right in front of the ear canal. This hinge is referred to as the Temporal Mandibular Joint or TMJ for short. The TMJ is adjacent to the outer portion of the ear. That part of the ear canal is not bone it is cartilaginous. Being made of cartilage instead of bone it is flexible. Therefore, when the jaw is opened or the mandible moves forward it distorts the shape of the ear canal. Mainly, it opens up the anterior (forward facing) wall of the ear canal.
The outer part of the ear canal is also where the hearing aid sits when it is in place within the ear canal. Hearing aids are typically custom made to snugly fit the shape of the ear canal with the jaw closed. Since the ear canal is flexible cartilage and the shell of the hearing aid is inflexible plastic, the movement of the TMJ creates a gap between the anterior wall of the canal and the anterior wall of the hearing aid. This gap provides a perfect opening for amplified sound inside the ear canal to leak out to the hearing aid microphone. Furthermore, continued jaw movement can also slowly work a small hearing aid slightly out of position so that even when the jaw comes back to its resting place the hearing aid has been moved enough to create a permanent gap between the shell and the ear canal.
Acoustic feedback (usually a whistling or howling sound) occurs when a sound passes through the hearing aid from the microphone (input) to the receiver (output) and then finds a pathway back to the microphone. When this old sound cycles through the hearing aid repeatedly it picks up amplification on each pass and eventually gets strong enough to make the components of the aid ring. That is the whistling caused by feedback. As mentioned above, the gap between the ear canal and the shell of the aid provides just a feedback pathway. If the gap is only there during jaw movement the feedback will come and go. But once the gap becomes constant the feedback will not stop until the aid is either turned down or pushed back into the ear.
There are a couple of things you could try to alleviate the problem. If the aid is a small In-The-Canal device or a smaller Completely-In-The-Canal device you can have a small piece of Lucite added to it. That is called a canal lock or a variation of that is called a helix lock depending on what part of the ear it hooks into. The purpose of both types of locks is the same. They provide an extra contact point with the outer ear. That contact point will usually hold the aid in place in the canal even during jaw movement. If the aid is slightly slipping out due to jaw movement, this may be enough to hold it in place and stop the feedback pathway from opening. That is the simple approach. The next level up from that would be to engage a feedback canceller in the aid if it has one. However, that may prove to be a two edged sword. An adaptive feedback manager will stop the acoustic feedback even in brief interval, if it is fast enough. Some are and some are not. However, any feedback manager is going to alter the processing of the hearing aid leading to altered sound quality. Obviously, sound quality is paramount to a musician while they are playing. So this may cause rejection of the aid because the cure is actually worse than the disease. In fact, the second part of your question about the aid shutting down when you play could be because a feedback canceller is working to break up the feedback pathway. But that would be a pretty extreme example.
The shutting down phenomenon you describe could have a couple of sources. For example, the microphone stage of many small hearing aids is limited to 105 dB SPL or less. Regardless of how much output the aid can provide, it cannot faithfully reproduce an input higher than that level. It is quite possible that your trumpet exceeds that level, thus saturating the input stage and limiting the aid causing the perception it is cutting out. Another possibility is that one or more adaptive features may be turned on that can interact badly in the presence of music. If a phase canceling feedback suppression system is used and the aid cuts out from a prolonged single note on the trumpet, the system may be mistaking the note for a feedback oscillation. If so, it is deliberately canceling it. If there is a noise reduction algorithm turned on, it may make a similar mistake and cut gain in the presence of music. Therefore, there are a number of possible causes. However, you should bear in mind, if it is a phase canceller, etc. that feature may only be turned on because you are wearing hearing aids that are too small for your hearing loss. Im not saying that is the case. However, it sounds like it is a possibility. One advantage of a sophisticated digital aid is that it allows the fitter to push the technology a bit to give you something you want (like a very small aid maybe) when it would otherwise be impossible with traditional technology. Normally, such adaptive features would be disabled for music because they can reduce fidelity. But if you are wearing a device that is smaller than it should be, the audiologist who fit you may have had to engage some such features to accommodate your needs. This leads to the final possible solution.
Consider wearing a larger hearing aid when playing music. Larger aids use larger and better isolated components capable of greater power which leads to improved overall stability of the device. Furthermore, they move the microphone away from the ear canal and provide more physical contact between the ear canal wall and the shell of the hearing aid. All of these things reduce the risk of feedback considerably regardless of jaw movement. When there is less likelihood of feedback fewer adaptive features are required to control it while maintaining high sound quality. Therefore, fidelity for music will improve. Larger components can also be used to allow higher input levels before limiting at the input stage becomes a problem. The ability to handle higher inputs before limiting is often key to musical sound quality for musicians. Depending on the shape and degree of hearing loss you have, it may be possible for you to wear a completely open Behind-The-Ear instrument. They provide excellent sound quality for people with the right hearing loss and they are very cosmetically appealing. They are often less noticeable than even a small hearing aid inside the ear canal.
To visit the Unitron website, visit www.unitronhearing.us.