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
How do you reduce acoustic feedback?
- Digital Feedback Management
- Does not replace the need for carefully made ear impressions and appropriately fitting earmolds, custom tips, or shells (it is the supplement to that).
What are 5 strategies HAs use for digital feedback management?
1) Static gain cut
2) Adaptive gain cut
3) Adaptive notch filters
4) Phase cancellation
5) Frequency shifting
What feedback suppression strategies do modern HAs use?
1) Notch filtering
2) Phase cancellation
3) Frequency shifting
Often all these approaches are used in the same HA model
Depending on the model, the programming software may offer:
1) choice of static or adaptive cancellation
2) basic or advanced digital feedback suppression settings.
What are 2 questions we need to ask when looking at feedback management programming?
1) Is the risk of feedback the same in all environments / programs?
2) Should active feedback management be set differently for various programs (assuming the software allows you to do so)?
____ were made possible because of feedback cancellation through phase cancellation
Open fittings (RIC and RITA)
How do you reduce feedback with gain cut?
- Gain cut in the frequency region where feedback is occurring
- Hearing aid detects feedback, reduces the gain
- May be permanent (fixed at the time of programming)
- May occur only when gain is increased (e.g., raised VC or low inputs with WDRC)
- May occur only when hearing aid detects sustained oscillation in the input signal
Too much gain cut affects ____. Why?
speech audibility
At a certain point the HA cut the gain because there was feedback (but this can also potentially cut any speech spectrum that is falling in that range)
What are notch filters?
- HA creates a notch filter to reduce gain around the frequency where it detected sustained oscillation.
- Adaptive notch filter: frequency of notch changes.
- Audibility of speech may be compromised if the notch has a wide bandwidth.
- Speech isn’t compromised if we get a narrow bandwidth
-
1 - Review & Fit51
-
2 - Speech Testing35
-
3 - Ear Impressions52
-
4 - Compression62
-
5 - Digital Technology39
-
6 - Noise Reduction63
-
7 - Feedback Management37
-
8 - Signal Processing83
-
9 - Pediatrics & Geriatrics83
-
10 - Cerumen Management119
-
11 - Fine Tuning67
-
12 - CROS & BiCROS55
-
13 - Wireless Accessories & ALDs65
