Acoustic Feedback Reduction

1. Acoustic Feedback Reduction MCHAS Modernising Children’s Hearing Aid Services

2. Introduction • Acoustic Feedback Principle • Acoustic Feedback Reduction Strategies • Accessing Acoustic Feedback Tests • Summary

3. Acoustic Feedback Principle Input signal Output signal Input signal Feedback signal = (In phase) Feedback problems Feedback signal GN Resound

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.

5. Feedback Management Strategies 40 Desired 30 Hearing aid gain (dB) 20 10 0 125 4000 250 500 1000 2000 8000 Frequency (Hz) Dillon 2001

6. Desired Volume decreased Gain Reduction Strategies Dillon 2001

7. Gain Reduction Strategies Dillon 2001

8. Gain Reduction Strategies • 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

9. Gain Reduction Strategies • Active Notch Filters - Also called Roving Notches - HA actively monitors it’s 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)

10. 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.

11. External leakage path + - Internal feedback path Feedback Cancellation Dillon 2001

12. 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)

13. 1. Max gain with DFS 2. Max gain without DFS 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. 2 1 Taken from actual Danalogic 163D fitting with 2 mm vent (GN Resound)

14. 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

15. 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.

16. SPIRIT II DIRECT & POWER

17. DANALOGIC 283D

18. PRISMA 2M & DSP • Optimize will insert static notch if necessary • Adaptive sets up active notch filter(s) • If feedback occurs aid checks for presence after 30 secs and 5 minutes. Notch(es) will be removed if feedback signal no longer present. • Can implement one notch per channel (up to 4)

19. STRATA A312

20. Phonak Aero & Supero

21. NHS DSP Hearing Aid FB Strategies

22. Thank-you Questions? MCHAS Modernising Children’s Hearing Aid Services