Acoustic Feedback Reduction MCHAS Modernising Childrens Hearing Aid
Acoustic Feedback Reduction MCHAS Modernising Children’s Hearing Aid Services
Introduction ØAcoustic Feedback Principle ØAcoustic Feedback Reduction Strategies ØAccessing Acoustic Feedback Tests ØSummary MCHAS Modernising Children’s Hearing Aid Services
Acoustic Feedback Principle Input signal Output signal Input signal (In phase) Feedback signal = Feedback problems GN Resound MCHAS Modernising Children’s Hearing Aid Services
ØFeedback 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. ØFeedback 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. MCHAS Modernising Children’s Hearing Aid Services
Hearing aid gain (d. B) Feedback Management Strategies 40 Desired 30 20 10 0 125 250 500 1000 2000 4000 8000 Frequency (Hz) Dillon 2001 MCHAS Modernising Children’s Hearing Aid Services
Gain Reduction Strategies Desired Volume decreased Dillon 2001 MCHAS Modernising Children’s Hearing Aid Services
Gain Reduction Strategies Dillon 2001 MCHAS Modernising Children’s Hearing Aid Services
Gain Reduction Strategies ØStatic 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 MCHAS Modernising Children’s Hearing Aid Services
Gain Reduction Strategies ØActive 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) MCHAS Modernising Children’s Hearing Aid Services
Feedback Cancellation ØMost sophisticated strategy as there is no gain reduction. ØHA detects signal leaking back to the microphone and automatically cancels this signal by adding some equal but opposite signal. MCHAS Modernising Children’s Hearing Aid Services
Feedback Cancellation External leakage path + - Internal feedback path Dillon 2001 MCHAS Modernising Children’s Hearing Aid Services
Feedback Cancellation ØHighly adaptive ØCan control several feedback spikes simultaneously without sacrificing sound quality ØHowever, battery life, sampling time and processing power are key to its effectiveness (Hayes, 2003) MCHAS Modernising Children’s Hearing Aid Services
Feedback Cancellation Ø Feedback cancellation is also known as ‘search & destroy’ and can allow up to 10 d. B of extra gain before feedback. Ø Limitation: Some musical tones (highly periodic) and simple test box signals may also be suppressed. 2 1. Max gain with DFS 2. Max gain without 1 DFS Taken from actual Danalogic 163 D fitting with 2 mm vent (GN Resound) MCHAS Modernising Children’s Hearing Aid Services
Accessing Feedback Tests ØActive gain reduction strategies – Phonak Supero the active notch filter is deactivated during the fitting process and in the music program. – In the Siemens Prisma 2 M 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. ØFeedback Cancellation – Danalogic 283 D use this strategy. The ‘Digital Feedback Suppression’ (DFS) can be deactivated in the software. – Spirit II Direct and Power MCHAS Modernising Children’s Hearing Aid Services
Accessing Feedback Tests. ØAll feedback tests can be accessed by clicking on a software icon or going into one of the ‘pull down’ menus. ØAll tests require noise free environments. ØThere may be a number of different options available. Instructions normally accompany procedure but if you have difficulties try reading the instructions in the help menu. MCHAS Modernising Children’s Hearing Aid Services
SPIRIT II DIRECT & POWER MCHAS Modernising Children’s Hearing Aid Services
DANALOGIC 283 D MCHAS Modernising Children’s Hearing Aid Services
PRISMA 2 M & DSP Ø Optimize will insert static notch if necessary Ø Adaptive sets up active notch filter(s) Ø If feedback occurs aid checks for presence after 30 secs and 5 minutes. Notch(es) will be removed if feedback signal no longer present. Ø Can implement one notch per channel (up to 4) MCHAS Modernising Children’s Hearing Aid Services
STRATA A 312 MCHAS Modernising Children’s Hearing Aid Services
Phonak Aero & Supero MCHAS Modernising Children’s Hearing Aid Services
NHS DSP Hearing Aid FB Strategies Hearing Aid Limits gain in Static Adaptive channels Notch Filter Phonak Aero 211 & 311 AZ Phase Cancellation Phonak Supero Oticon Spirit II Direct & Power Siemens Prisma 2 M & DSP Starkey Strata A 312 * Active high channel gain reduction Danalogic 283 D MCHAS Modernising Children’s Hearing Aid Services
Thank-you Questions? MCHAS Modernising Children’s Hearing Aid Services
- Slides: 22
