17_Feedback Management Flashcards
How does accoustic feedback occur?
Receiver output leaks back (through vent or leak) to the microphone input and is re-amplified (gain applied to forward path)
Amount of attenuation (during feedback path) is dependent on frequency
If you hold a hearing aid in your hand, when does feedback become audible?
Feedback becomes audible when the hand is cupped around it. There is still feedback when the palm is open, but it is not strong enough to be audible
Why is feedback tonal?
Tonal quality depends on how the fingers are positioned when cupping the hearing aid.
- changing the feedback path characteristics (such as finger placement) results in a different set of frequency components meeting the feedback conditions (amplitude and phase)
What is “loop gain”?
Gain (forward path) + attenuation (feedback path)
Positive loop gain = feedback gets stronger with each iteration through the loop, leading to an audible squeal
E.g. Gain (30 dB) + attenuation (-20 dB)
Input 60 -> output 90 -> feedback 70
(70 > original input of 60 -> positive loop gain, which continues to grow)
What are the 2 conditions for feedback oscillation?
The amplitude condition
The phase condition
Describe the amplitude condition of feedback oscillation
- audible feedback can only occur if the amount of amplification through the direct path is greater than the amount of attenuation through the feedback path
- loop gain > 0
- REAG > attenuation
Describe the phase condition of feedback oscillation
For the amplitude condition, the input and the feedback signals are assumed to be IN PHASE
- if they are not in phase, then the loop gain has to be much greater than 0 dB in order to induce audible oscillations
At what frequencies is feedback usually a problem (low or high)?
Usually high frequencies b/c most HAs have more gain in high frequencies (amplitude criteria)
What are the 3 stages of feedback?
- no audible feedback
- sub-oscillatory feedback
- sustained feedback
Is feedback “all or none”?
No - sub-oscillatory feedback may still have a significant effect on quality of output
How does sub-oscillatory feedback compare to the sustained feedback condition?
The gain response at some frequencies is close to threshold, but not over, so the HA is not squealing, but it still has an impact on quality
e.g. might be slight whistling at some high frequency syllables?
Name 3 electronic means of managing and controlling feedback
- gain reduction
- damping of high frequency peaks
- bandpass filtering
- notch filtering
- phase shifting
- frequency shifting
- adaptive feedback cancellation
Name 3 acoustic means of controlling feedback
- tighter fit
- change to a ore occluded dome
- close the vent
What is a notch filter?
A digital filter with a bandstop response with an extremely narrow bandstop
- can be employed to control feedback if you know the offending frequency (will have to change if the frequency changes = roving notch)
- has the disadvantage of affecting the surrounding frequency region
- used as secondary feedback mitigation option, after adaptive feedback cancellation
What is adaptive feedback reduction?
AKA: adaptive phase- or feedback path- cancellation
When the HA tried to mimic the response of the external feedback path to cancel its effects.
Advantages:
- change in environment -> change in feedback transfer function
- b/c it is an estimate, regardless of the stage of feedback, it is effective in both sustained and sub-oscillatory feedback modes
Effective, but not perfect
