wind noise in hearing aids harvey dillon, richard katsch, inge roe, national acoustic laboratories,...
TRANSCRIPT
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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