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Wind noise in hearing aids
Harvey Dillon, Richard Katsch, Inge Roe,
National Acoustic Laboratories,
Australian Hearing,
With the support of GN Resound, Oticon, Phonak, & Widex
Dillon, Roe & Katsch, NAL.
The problem
Wind + hearing aid = noise
But why, and how, and how bad is the problem?
Dillon, Roe & Katsch, NAL.
Turbulence
f = US/L (Hz)
L
U
S = Strouhal number
Dillon, Roe & Katsch, NAL.
Spectrum of noise behind wire
50
60
70
80
90
100
110
25 50 100 200 400 800 1600 3150 6300
Frequency (Hz)
1/3
oc
tave
le
vel
(dB
SP
L)
1 cm
2 cm
4 cm
mic alone
Dillon, Roe & Katsch, NAL.
Laser Doppler Velocimeter
Dillon, Roe & Katsch, NAL.
Velocity down the wind tunnel (CIC aid)
Dillon, Roe & Katsch, NAL.
Velocity out of head (CIC aid)
Dillon, Roe & Katsch, NAL.
Velocity upwards (CIC aid)
Dillon, Roe & Katsch, NAL.
Turbulent velocity (CIC aid)
Dillon, Roe & Katsch, NAL.
Turbulent velocity (ITE aid)
Dillon, Roe & Katsch, NAL.
Turbulent velocity (BTE aid)
Dillon, Roe & Katsch, NAL.
Effect on turbulence of distance from head
Dillon, Roe & Katsch, NAL.
Solution 1: Extend the microphones
Dillon, Roe & Katsch, NAL.
Wind velocity inside and outside the concha
Dillon, Roe & Katsch, NAL.
Turbulence in the concha
Tragus
Dillon, Roe & Katsch, NAL.
Sensitivity of ITC
Tragus
Dillon, Roe & Katsch, NAL.
Smoothness of ITE
Tragus
Noise measurements: Specially designed wind-tunnel.
Noise measurements: Outlet of Wind Tunnel
Noise measurements: Outlet of Wind Tunnel
x
z
y
Dillon, Roe & Katsch, NAL.
Wind velocity
5 m/sec 18 km/hr 11 m.p.h. Level 3 on 13 point Beaufort Scale Flags unfurl but droop Scattered whitecaps Gentle Breeze Exceeded 6% of time
Dillon, Roe & Katsch, NAL.
Noise at BTE position
40
50
60
70
80
90
100
25 50 100 200 400 800 1600 3150 6300
Frequency (Hz)
1/3
oct
ave
leve
l (d
B S
PL
)
-90-60-300306090
Dillon, Roe & Katsch, NAL.
Aid comparison at 0 degrees
40
50
60
70
80
90
100
25 50 100 200 400 800 1600 3150 6300
Frequency (Hz)
1/3
oct
ave
leve
l (d
B S
PL
)
BTEITEITCCIC
Dillon, Roe & Katsch, NAL.
KEMAR at 0 degrees to wind
Dillon, Roe & Katsch, NAL.
Aid comparison at 30 degrees
40
50
60
70
80
90
100
25 50 100 200 400 800 1600 3150 6300
Frequency (Hz)
1/3
octa
ve le
vel (
dB S
PL)
BTE
ITE
ITC
CIC
Dillon, Roe & Katsch, NAL.
Aid comparison at 30 degrees
40
50
60
70
80
90
100
Frequency (Hz)
1/3
oct
ave
leve
l (d
B S
PL
)
BTE
ITE
ITC
CIC
BTE no ear
Dillon, Roe & Katsch, NAL.
Solution 2:
Dillon, Roe & Katsch, NAL.
Solution 2: Remove the pinnae
Dillon, Roe & Katsch, NAL.
Solution 2: Remove the pinnae
Dillon, Roe & Katsch, NAL.
ITE noise versus azimuth
50
60
70
80
90
100
-90 -60 -30 0 30 60 90
Aziimuth (degs)
1/3
oct
ave
leve
l (d
B S
PL
)
100 Hz
200 Hz
2 kHz
Dillon, Roe & Katsch, NAL.
KEMAR at -50 degrees to wind
Dillon, Roe & Katsch, NAL.
Aid comparison at -50 degrees
40
50
60
70
80
90
100
25 50 100 200 400 800 1600 3150 6300
Frequency (Hz)
1/3
oct
ave
leve
l (d
B S
PL
)
BTE
ITE
ITC
CIC
Dillon, Roe & Katsch, NAL.
Aid comparison at -50 degrees
40
50
60
70
80
90
100
25 50 100 200 400 800 1600 3150 6300
Frequency (Hz)
1/3
oct
ave
leve
l (d
B S
PL
)
BTE
ITE
ITC
CIC
BTE no ear
Dillon, Roe & Katsch, NAL.
Solution 3: Shed the vortices gracefully
Dillon, Roe & Katsch, NAL.
ITE noise versus azimuth
50
60
70
80
90
100
-90 -60 -30 0 30 60 90
Frequency (Hz)
1/3
oct
ave
leve
l (d
B S
PL
)
100 Hz
200 Hz
2 kHz
Dillon, Roe & Katsch, NAL.
KEMAR at -90 degrees to wind
Dillon, Roe & Katsch, NAL.
Solution 4: Keep the aid towards the wind
Dillon, Roe & Katsch, NAL.
ITE noise re CIC noise
-80 -60 -40 -20 0 20 40 60 80
100
1000
10000
-10
-5
0
5
10
15
dB
Dillon, Roe & Katsch, NAL.
Factors affecting wind noise
Levels are very intense Obstacles (head, pinna, tragus) act as:
– Wind guards – Turbulence source– Turbulence shredder
Large obstacles create low-freq turbulence– head
Medium obstacles create mid-freq turbulence– pinna
Small obstacles create high-freq turbulence– tragus, inlet port
Dillon, Roe & Katsch, NAL.
Other observations
As wind speed increases:– noise levels increase– frequency spectrum extends upward
Two microphone ports produce: – correlated noise if a common source
(e.g. head or pinna)– uncorrelated noise if separate
sources (e.g. inlet port)
Dillon, Roe & Katsch, NAL.
Potential solutions
Wear one aid and orient the head Wear a scarf Don’t fit a BTE Don’t fit a fixed directional microphone Low distortion input circuitry
– up to at least 110 dB SPL
Low-cut filtering– especially over the vent-transmitted range
Smooth design Electronic signal processing from multiple
microphones
That’s all
Folks
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