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

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?


Turbulence

f = US/L (Hz)

L

U

S = Strouhal number


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


Laser Doppler Velocimeter


Velocity down the wind tunnel (CIC aid)


Velocity out of head (CIC aid)


Velocity upwards (CIC aid)


Turbulent velocity (CIC aid)


Turbulent velocity (ITE aid)


Turbulent velocity (BTE aid)


Effect on turbulence of distance from head


Solution 1: Extend the microphones


Wind velocity inside and outside the concha


Turbulence in the concha

Tragus


Sensitivity of ITC

Tragus


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


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


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


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


KEMAR at 0 degrees to wind



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



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


Solution 2:


Solution 2: Remove the pinnae


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


KEMAR at -50 degrees to wind


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


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


Solution 3: Shed the vortices gracefully


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


KEMAR at -90 degrees to wind


Solution 4: Keep the aid towards the wind


ITE noise re CIC noise

-80 -60 -40 -20 0 20 40 60 80

100

1000

10000

-10

-5

0

5

10

15

dB


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


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)


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

For a copy of this talk, send an Email to:

[email protected]

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

Documents