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
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
What is phase cancellation?
- Feedback path is estimated through analysis of HA input and output (HA tries to identify sounds that have re-entered the system)
- Earlier generations created a signal 180° out of phase with FB signal (Phase inverted signal cancelled FB signal, without gain cut; Problem: person heard generated tones when HA thought that tonal signals (e.g., microwave beep) were feedback)
- Newer generations use filters to cancel feedback
Can allow about 10-15 dB extra gain before feedback (added stable gain)
What is frequency shifting?
- The frequency of amplified sounds is slightly shifted (10-20 Hz).
- Amplified sound re-entering the mic will not be in phase with input signal (it will be out of phase; no oscillation)
- No gain cut
- Not usually detected by the user
Does feedback stay static/constant?
- Feedback is not Static
- Spectral, temporal, and amplitude characteristics of feedback path can change rapidly
- Signal or filter created by the feedback suppression system must be an accurate match of the feedback path and must also change rapidly
- Not all hearing aids are created equal with regard to effectiveness of feedback suppression
What is entrainment?
- The HA thinks that a tonal sound in the environment is feedback, so it tries to cancel it (examples: musical note, microwave beep, elevator ding, etc).
- The patient may hear a warbling-like sound or chirping
- This problem was worse on previous generations of digital hearing aids
- This means the HA isn’t categorizing feedback correctly
How do you prevent entrainment?
- In bilateral wireless fittings, the signal is compared between aids
- If we are wearing a bilateral HA, the tone should be presented on each side exactly the same
How effective is feedback suppression?
- Using REM, Ricketts et al.(2008) measured the amount of additional gain before feedback for 16 patients wearing open fits.
- Compared 6 feedback suppression algorithms from 5 manufacturers
- On average, results ranged from 0 to 20 dB of added gain before feedback across feedback suppression algorithms.
- Large individual variability of up to 16 dB range within the same HA model
Large variability in ____ between manufacturers, even at their best!
achievable gain
What is the 5 step process of running the feedback test (part of the REM)?
A feedback test can be performed through the HA programming software
1) Patient wears the hearing aids
2) Room must be quiet (ambient noise reader on verification equipment)
3) Signals are delivered through the HA
4) HA measures feedback path
5) Results show the max gain that the patient is able to get (max stable gain; how much gain is possible) - this is different than how much gain is prescribed to the patient
Explain this picture (red and black lines)
- Red is how much gain you measured and set with prescription
- The black line is how much gain you are going to attain before running into feedback (output limit of the HA)
- This means you can turn this HA up quite a bit before running into feedback
With a ____ there is a better chance of no feedback
closed dome
If feedback is not an issue, should you still run the feedback test?
Yes, because it will still give you information (unless the manufacturer specifies not to)
What does the occurance of feedback depend on?
When we are concerned with feedback, we must think about:
1) Gain
2) Venting
3) Attenuation of the shell
What 5 types of patients who previously had a feed-back free fit suddenly may complain of feedback?
1) Cerumen?
2) Weight loss? – may need to make a new earmold
3) Defective hearing aid? – run an ANSI test
4) Defective/Damaged shell or earmold?
5) Damaged ear hook or tubing? – has the tubing hardened or is there a chip in the ear hook
