Transcript Wind noise causes poster - National Acoustic Laboratories

The sources of wind noise in hearing aids
By Harvey Dillon, Inge Roe, and Richard Katsch, National Acoustic Laboratories
This project was kindly sponsored by GN Resound, Oticon, Phonak AG, and Widex
INTRODUCTION
AIM
Wind 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.
The 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.
BACKGROUND THEORY
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
Acoustically deadened
wind tunnel
METHOD
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.
Laser doppler
velocimeter
Coordinate
system
• 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.
z
x
y
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.
Six laser beams create interference
patterns, which allow the instantaneous
velocity of individual smoke particles to be
measured in three dimensions.
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.
1. Wind Flow Around the Head and Ear
Mean wind flow around the ear (for frontal wind at 5 m/s)
Downwind velocity (m/s)
Transverse velocity (m/s)
Vertical velocity (m/s)
Wind direction changes
rapidly inside and
just outside the concha
CIC
CIC
CIC
CIC
Out from canal entrance
Turbulence velocity (frontal wind at 5 m/s)
CIC
ITE
BTE
Turbulence decreases rapidly
with distance out from the head
(for frontal wind incidence)
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.
Tragus
ITE
Tragus
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 0° incidence, the pinna is a major
source of turbulence.
At 90° incidence, the flow separates
and does not come near either pinna,
so noise levels are relatively low.
2. Sound Pressure Levels at the Hearing Aid Microphone
Broad spectrum when
the head and pinna both
contribute to noise
100
90
80
-90
-60
-30
0
30
60
90
70
60
50
Low frequency emphasis
when wind comes
from the far side.
40
25
50
100
200
400
800
Local maximum at
-50° created by
head eddies
1600 3150 6300
Local maximum at
0° created by
tragus eddies
Frequency (Hz)
ITE quieter than ITC or
CIC over the frequency range
where the tragus
causes turbulence
100
BTE
90
ITC
80
70
ITE
BTE
ITE
ITC
CIC
60
50
CIC
40
25
50
100
200
400
100
100 Hz
90
80
70
2 kHz
60
50
-90
800 1600 3150 6300
BTE
90
ITC
80
BTE
ITE
ITC
CIC
BTE no ear
60
50
ITE
40
25
50
-30
100
200
Frequency (Hz)
400
800 1600 3150 6300
0
• The pinna creates medium-frequency turbulence,
especially at position of BTE microphone
90
100
90
BTE
80
BTE
ITE
ITC
CIC
BTE no ear
70
60
50
CIC
40
25
50
100
200
400
• 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.
800
1600 3150 6300
Frequency (Hz)
Conclusions
• The head creates low-freq turbulence, especially on
down-wind side
60
- 50 degrees (far side)
Frequency (Hz)
• 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
30
Slightly higher noise levels at
the BTE position when
the pinna is detached
Much lower noise levels at
the BTE position when
the pinna is detached
100
70
-60
Azimuth (degrees)
30 degrees
1/3 octave level (dB SPL)
1/3 octave level (dB SPL)
0 degrees
1/3 octave level (dB SPL)
ITE
1/3 octave level (dB SPL)
1/3 octave level (dB SPL)
BTE