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acoustic feedback reduction

Introduction. Acoustic Feedback PrincipleAcoustic Feedback Reduction StrategiesAccessing Acoustic Feedback TestsSummary. Acoustic Feedback Principle . . . Input signal. Feedback signal. Output signal. . . . . . . . . . . . . . . . . . . . . . . (In phase) . . . Input signal. Feedback signal. Feedback problems.

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acoustic feedback reduction

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    1. MCHAS Modernising Childrens Hearing Aid Services Acoustic Feedback Reduction

    2. Introduction Acoustic Feedback Principle Acoustic Feedback Reduction Strategies Accessing Acoustic Feedback Tests Summary Feedback occurs when enough amplified sound leaks from the ear canal to the hearing aid microphone. Feedback occurs when enough amplified sound leaks from the ear canal to the hearing aid microphone.

    3. Acoustic Feedback Principle

    4. Feedback occurs when: the ear mould is damaged or a poor fit if the ear canal is occluded with wax when the hearing aid is turned up too much. Feedback reduction algorithms try to ensure that the hearing aid is never turned up too much by: the clinician the aid wearer automatically by the hearing aid itself. It can occur when the ear mould is damaged or a poor fit, when there is too much wax in the ear, or simply when the hearing aid is turned up too much. Hearing aid feedback reduction algorithms can only overcome the last cause. They try to ensure that the hearing aid is never turned up too much by either the clinician, the aid wearer or automatically by the hearing aid itself. It can occur when the ear mould is damaged or a poor fit, when there is too much wax in the ear, or simply when the hearing aid is turned up too much. Hearing aid feedback reduction algorithms can only overcome the last cause. They try to ensure that the hearing aid is never turned up too much by either the clinician, the aid wearer or automatically by the hearing aid itself.

    5. Feedback Management Strategies This is desired frequency gain response, but there is feedback. What would you normally do?This is desired frequency gain response, but there is feedback. What would you normally do?

    6. Gain Reduction Strategies Traditionally with older unsophisticated hearing aid it would be common practice to decrease the gain to eliminate feedback. Poor solution as will result in inadequate loudness, audibility and intelligibility. Traditionally with older unsophisticated hearing aid it would be common practice to decrease the gain to eliminate feedback. Poor solution as will result in inadequate loudness, audibility and intelligibility.

    7. Gain Reduction Strategies Better solution is to decrease the gain at only those frequencies where feedback oscillation is a possibility this is normally around the peaks in the gain curve. Can immediately see the benefit of having a multi-channel instrument. The greater the number of channels the more specific the frequency reduction. If the instrument has only 2 channels then gain maybe decreased over an unnecessarily wide frequency range.Better solution is to decrease the gain at only those frequencies where feedback oscillation is a possibility this is normally around the peaks in the gain curve. Can immediately see the benefit of having a multi-channel instrument. The greater the number of channels the more specific the frequency reduction. If the instrument has only 2 channels then gain maybe decreased over an unnecessarily wide frequency range.

    8. Gain Reduction Strategies The maximum gain that can be achieved in different frequency regions is limited. Need to know what the maximum amount of gain (in each frequency) is, before feedback will occur. This safe value can be set by: the clinician at the time of fitting the fitting software at the time of fitting (in-situ measurements) the hearing aid whenever the hearing aid is worn Remembering that feedback is more likely when: 1) volume is increased 2) in WDRC aids at low input levels because thats where the maximum amount of gain will be We really only want aid to implement this reduction in gain when feedback oscillation occurs, so either you or the clinician needs to know what the maximum amount of gain in each frequency is before feedback will occur. The clinician at the time of fitting can select maximum gain the just avoids oscillation, or can increase the knee point until oscillation stops. Some fitting software automatically raises the gain in each channel until it detects oscillation occurring. Some hearing aids will automatically reduce the gain in the channel whenever it detects oscillation occurring in that channel.Remembering that feedback is more likely when: 1) volume is increased 2) in WDRC aids at low input levels because thats where the maximum amount of gain will be We really only want aid to implement this reduction in gain when feedback oscillation occurs, so either you or the clinician needs to know what the maximum amount of gain in each frequency is before feedback will occur. The clinician at the time of fitting can select maximum gain the just avoids oscillation, or can increase the knee point until oscillation stops. Some fitting software automatically raises the gain in each channel until it detects oscillation occurring. Some hearing aids will automatically reduce the gain in the channel whenever it detects oscillation occurring in that channel.

    9. Static Notch Filters Fixed filters which limit the gain only around the frequencies where feedback oscillation is occurring. Filters can only be changed by running a new feedback check. Limitation: Feedback oscillation frequency does not remain fixed over time Gain Reduction Strategies Many DSP hearing aids implement a notch filter which limits the gain only around the frequencies where feedback oscillation is occurring. Notch filter is implemented at clinic by HA software so only addresses feedback problems in this one environment and circumstance. Moving jaw, wearing hat, using phone, putting hand over ear all change the phase of the signal and hence the characteristics of the leakage path, so the oscillation frequency will also change.Many DSP hearing aids implement a notch filter which limits the gain only around the frequencies where feedback oscillation is occurring. Notch filter is implemented at clinic by HA software so only addresses feedback problems in this one environment and circumstance. Moving jaw, wearing hat, using phone, putting hand over ear all change the phase of the signal and hence the characteristics of the leakage path, so the oscillation frequency will also change.

    10. Gain Reduction Strategies Active Notch Filters - Also called Roving Notches - HA actively monitors its own output for feedback and creates a notch filter at the frequency it detects feedback. - Can implement two or three spikes - More effective than static notches but are limited and may be slow to adapt (Hayes, 2003) A better solution is for the hearing aid to actively monitor its own output for feedback and create a notch filter at the frequency it detects feedback. A better solution is for the hearing aid to actively monitor its own output for feedback and create a notch filter at the frequency it detects feedback.

    11. Feedback Cancellation Most sophisticated strategy as there is no gain reduction. HA detects signal leaking back to the microphone and automatically cancels this signal by adding some equal but opposite signal.

    12. Feedback Cancellation The second intentional feedback path leaks back a signal that is equal to (same frequency and intensity) but opposite in phase to the feedback signal. When the two signals are added the feedback signal is cancelled. The second intentional feedback path leaks back a signal that is equal to (same frequency and intensity) but opposite in phase to the feedback signal. When the two signals are added the feedback signal is cancelled.

    13. Feedback Cancellation Highly adaptive Can control several feedback spikes simultaneously without sacrificing sound quality However, battery life, sampling time and processing power are key to its effectiveness (Hayes, 2003) Hayes (2003) p47 In real time feedback suppressio, multiple narrowband detectors can be deployed to monitor the presence of oscillations. Each detector reacts to feedback in its own designated band without draining digital processing power and battery lifeHayes (2003) p47 In real time feedback suppressio, multiple narrowband detectors can be deployed to monitor the presence of oscillations. Each detector reacts to feedback in its own designated band without draining digital processing power and battery life

    14. Feedback Cancellation Feedback cancellation is also known as search & destroy and can allow up to 10 dB of extra gain before feedback. Limitation: Some musical tones (highly periodic) and simple test box signals may also be suppressed. The only disadvantage to this type of feedback reduction strategy is that some musical tones (highly periodic) and test box signals may also be suppressed. The only disadvantage to this type of feedback reduction strategy is that some musical tones (highly periodic) and test box signals may also be suppressed.

    15. Accessing Feedback Tests Active gain reduction strategies Phonak Supero the active notch filter is deactivated during the fitting process and in the music program. In the Siemens Prisma 2M and DSP notch filters are removed after 5 mins to see if feedback is still present. Starkey Strata there is an automatic gain reduction in the high channel. Feedback Cancellation Danalogic 283D use this strategy. The Digital Feedback Suppression (DFS) can be deactivated in the software. Spirit II Direct and Power This means pure tones could be used (as long as noise cancellation was off for danalogic) during fitting. What it does mean though is that teachers in the field must be aware that if they use pure tones they will get very strange results as the adaptive feedback control will be active and it will suppress the signal. This means pure tones could be used (as long as noise cancellation was off for danalogic) during fitting. What it does mean though is that teachers in the field must be aware that if they use pure tones they will get very strange results as the adaptive feedback control will be active and it will suppress the signal.

    16. Accessing Feedback Tests. All feedback tests can be accessed by clicking on a software icon or going into one of the pull down menus. All tests require noise free environments. There may be a number of different options available. Instructions normally accompany any procedure but if you have difficulties try reading the instructions in the help menu. Noise free environments implications for children. Automatic tests vary in time taken to complete.. Implications for kidsNoise free environments implications for children. Automatic tests vary in time taken to complete.. Implications for kids

    17. See old presentation for notesSee old presentation for notes

    18. Adaptive feedback cancellation. Programmeable within each memory. Now has fast and slow (variable sampling rate). Slow for music (too slow to respond) and fast for normal environments as set by clinician.Adaptive feedback cancellation. Programmeable within each memory. Now has fast and slow (variable sampling rate). Slow for music (too slow to respond) and fast for normal environments as set by clinician.

    19. Static and active notches. Works similarly to Triano. Checks for presence of feedback 30 seconds after initial occurrence. Following implementation of notch(es) will be removed after 5 mins. If fdbk still present will leave them until until aid is switched off. Able to implement one notch per channel (4 channels). Can be set individually within each programme. Select Optimize run. Software will check max gain available before feedback. Static and active notches. Works similarly to Triano. Checks for presence of feedback 30 seconds after initial occurrence. Following implementation of notch(es) will be removed after 5 mins. If fdbk still present will leave them until until aid is switched off. Able to implement one notch per channel (4 channels). Can be set individually within each programme. Select Optimize run. Software will check max gain available before feedback.

    20. If feedback at time of fitting run feedback manager. Will adjust TK and gain if possible, or will implement notch of 12 or 18dB. If activate adaptive feedback management will reduce channel gain by 6dB. If feedback at time of fitting run feedback manager. Will adjust TK and gain if possible, or will implement notch of 12 or 18dB. If activate adaptive feedback management will reduce channel gain by 6dB.

    21. Phonak Aero & Supero To override the gain limits set in an individual channel, click at the desired position above the black marker for Feedback threshold (not generally recomm). The gain limit is set 4 dB below feedback threshold for the frequencies 2-4 kHz and 2 dB below for lower or higher frequencies. To override the gain limits set in an individual channel, click at the desired position above the black marker for Feedback threshold (not generally recomm). The gain limit is set 4 dB below feedback threshold for the frequencies 2-4 kHz and 2 dB below for lower or higher frequencies.

    22. NHS DSP Hearing Aid FB Strategies

    23. MCHAS Modernising Childrens Hearing Aid Services Thank-you Questions?

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