7 - Feedback Management Flashcards
What is feedback? Whens it problematic and when is it not?
- Feedback occurs when amplified sound exiting the receiver, earmold, or earpiece makes it’s way back to the microphones, and is re-amplified by the hearing aid (again and again and again and again)
- This is a loop response
- Problematic (and audible) if the output signal exceeds the original input signal
- Not problematic (or audible) if the output signal has been attenuated and does not exceed the original input signal
If there is enough ____, feedback is typically not a problem
attenuation
What are the 2 types of feedback?
1) Acoustic (external)
2) Mechanical (internal)
What is acoustic (external) feedback?
- Sound leakage around earmold/shell, through vent, or around a probe tube during REM (sound getting re-picked up by the microphone)
- If we hear the HA squealing as its sitting in the ear, that is acoustic feedback
What is mechanical (internal) feedback?
Poor physical isolation of the mic and receiver within the hearing aid casing
What are 2 conditions for acoustic feedback to occur (feedback will occur if…)?
- Feedback occurs when the amplified signal returns to the microphone and is re-amplified if:
- The feedback signal is in phase with the input signal
- The feedback signal is greater in amplitude than the input signal
Example of audible feedback (6 steps)
Example of non-audible feedback (6 steps)
What are the 6 challenges of acoustic feedback?
1) Limits achievable gain
2) Distorts the signal and degrades hearing aid performance
3) Reduces battery life
4) Is a source of embarrassment for the user
5) Decreases hearing aid benefit
6) Feedback reduction was reported to be among the hearing aid features most highly related to hearing aid satisfaction
What is sub-oscillatory feedback?
- Non-Audible Feedback
- Low levels of oscillation may not be audible (i.e., no whistling) but may still degrade hearing aid performance
- Do REMs, appropriately fitting shell and dome will help with this
What does feedback create?
- A sharp peak in frequency response
- The gain is getting turned up in small increments
- At high levels of sound, patients may say that it doesn’t sound right (distorted, too sharp, boomy)
What 4 ways do you reduced audible acoustic feedback?
1) Earmold/custom earpiece/shell/dome modifications
2) Reduce peaks using filtered earhooks (smooth out the frequency response)
3) Cerumen management
4) Digital feedback management
In what ways can you modify an earmold/custom earpiece/shell/dome?
- Reduce vent size (change open dome to closed dome)
- Canal length modification
- Coating
- Extended receiver tubing on custom aids
- Thick wall tubing (heavy wall tubing)
- Retake ear impression (need to capture the second bend)
- Change material or earmold/shell style
What does the occurrence of feedback depend on (3)?
What are the overriding factors in feedback likelihood?
1) Gain
2) Venting
3) Attenuation of the shell
Explain the feedback path
- The feedback path is dynamic. It can change depending on:
- holding a telephone close to the HA (creates a wall that sends the sound back to the microphone)
- wearing a hat or headscarf, pulling up a hood
- sitting close to a wall, resting head near another surface
- Varying sources of feedback have different spectral, temporal, and amplitude characteristics
- Feedback does not occur at a single frequency
- Typically between 1500 - 3000 Hz