The sources of wind noise in hearing aidsBy Harvey Dillon, Inge Roe, and Richard Katsch, National Acoustic Laboratories

This project was kindly sponsored by GN Resound, Oticon, Phonak AG, and Widex

1. Wind Flow Around the Head and Ear

ITECIC BTE

Turbulence decreases rapidlywith distance out from the head

(for frontal wind incidence)

Turbulence velocity (frontal wind at 5 m/s)

Mean wind flow around the ear (for frontal wind at 5 m/s)

Wind direction changes rapidly inside and

just outside the concha

Out from canal entrance

Transverse velocity (m/s)Downwind velocity (m/s) Vertical velocity (m/s)

CIC CICCICCIC

When wind flows past an obstacle, the stream lines separate and recombine. This process creates turbulence. Turbulence comprises rapid alteration of wind flow direction and rapid alteration of wind pressure. These pressure fluctuations can directly activate a microphone diaphragm or can cause a sound wave to propagate outwards from the turbulence.

Turbulence has strong components at the frequency:

f = US/L

where U is the mean wind velocity

S is the Strouhal number ( = 0.2 for a cylinder)

L is the width of the obstacle

Large obstacles, like the head, should therefore produce low frequency noise. Smaller obstacles, like the pinna should produce mid-frequency noise. Very small obstacles, like the tragus or a microphone inlet port, should produce high frequency noise. Turbulence is strongest at the point at which eddies shed from the downwind side of the obstacle.

BACKGROUND THEORY

AIMThe aims of the study were to:1. Quantify the amount of noise caused by wind.2. Identify the sources of the turbulence causing the noise.

Acoustically deadened wind tunnel

A wind tunnel with a working area of 750 mm x 750 mm contains air-flow straighteners and sound absorbers. It allows wind speeds up to 5 m/s to be obtained.

Laser doppler velocimeter

Six laser beams create interference patterns, which allow the instantaneous velocity of individual smoke particles to be measured in three dimensions.

Coordinate system

The x coordinate corresponds to distance along the wind tunnel, the y coordinate is laterally out from the head, and the z coordinate is motion vertically upwards.

x

z

y

METHOD

• Flow was measured by measuring the velocity of smoke particles at multiple sites around the ear using a laser doppler velocimeter.

• SPL was measured using a hearing aid microphone positioned over the ear or inserted inside earmolds shaped like ITE, ITC or CIC hearing aids.

INTRODUCTIONWind causes noise at the output of hearing aids, and this is a major problem to hearing aid wearers. While the cause is known to be associated with turbulence at the input to the hearing aid microphone, little is known about the nature or source(s) of this turbulence.

2. Sound Pressure Levels at the Hearing Aid Microphone

At 0° incidence, the pinna is a major source of turbulence.

At -50° incidence, the head causes large, low-frequency eddies which detach near the pinna causing high

noise levels. The pinna reduces turbulence as they detach.

At 90° incidence, the flow separates and does not come near either pinna,

so noise levels are relatively low.

• Wind noise has a broad spectrum whose shape depends on wind direction, and the spectrum extends down to very low frequencies

• Obstacles (head, pinna, tragus) can act as:

Wind guards

Turbulence source

Turbulence shedder

• The head creates low-freq turbulence, especially on down-wind side

• The pinna creates medium-frequency turbulence, especially at position of BTE microphone

• Small obstacles (e.g. tragus and microphone inlet port) create high-frequency turbulence.

• BTEs are noisiest because the microphone is in the wake of the pinna.

• CICs are least noisy for most angles because the microphone is shielded by the concha.

• ITEs are least noisy for some angles and frequencies because the aid and the pinna form a smooth surface.

• Levels are very intense, even for a light breeze

• Noise levels easily saturate hearing aids.

Conclusions

Tragus Tragus

ITE

ITC and CIC aids are shielded from turbulence created by the head, but are in the path of turbulence created by the tragus, pinna and concha. The profile is smoother for ITEs.

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BTE

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BTE no ear

BTE

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- 50 degrees (far side)

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Azimuth (degrees)

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100 Hz

ITE

Much lower noise levels at the BTE position when the pinna is detached

Low frequency emphasis when wind comes from the far side.

Broad spectrum whenthe head and pinna both

contribute to noise

Local maximum at -50° created by

head eddies

Local maximum at0° created by tragus eddies

ITE quieter than ITC or CIC over the frequency range

where the tragus causes turbulence

Slightly higher noise levels at the BTE position when the pinna is detached