Transcript Figures for chapter 4

Figures for Chapter 4
Electroacoustic Performance
Dillon (2001)
Hearing Aids
Ear simulator
V1
V2
V3
V4
Microphone
Dampers
Figure 4.1 Simplified internal structure of a four-branch ear
simulator.
Source: Dillon (2001): Hearing Aids
Couplers and
ear simulators
Photo removed to
minimize file space
Figure 4.2 Several couplers and their adapters,
and an ear simulators.
Source: Dillon (2001): Hearing Aids
2-cc couplers
ITE / ITC / CIC
Figure 4.3 The internal dimensions and
coupling methods for several 2-cc couplers.
Putty
HA1
Microphone
2 mm dia
Insert
earphone
25
Earmold
simulator
18 mm
18
3 mm dia
HA2
2 cc
cavity
Microphone
Source: Dillon (2001): Hearing Aids
HA2
Average canal SPL
minus 2cc SPL
Real-ear to coupler difference
20
15
10
5
0
125
250
500 1000 2000 4000 8000
Frequency (Hz)
Figure 4.4 RECD: SPL generated in the average adult real ear canal
minus SPL generated in an HA1 2-cc coupler.
Source: Dillon (2001): Hearing Aids
2-cc coupler
and
control microphone
Photo removed to
minimize file space
Figure 4.5 A hearing aid connected to a coupler,
with a control microphone positioned next to the
hearing aid microphone.
Source: Dillon (2001): Hearing Aids
Gain-frequency response
110
60
90
50
100
40
90
30
60
Coupler Gain (dB)
Coupler Output Level (dB SPL)
120
80
20
70
10
60
0
50
125
-10
250
500
1,000 2,000
Frequency (Hz)
4,000
8,000
Figure 4.6 Gain-frequency response (measured with a 60 dB SPL
input level) and OSPL90-frequency response of a BTE measured in
a 2-cc coupler with a swept pure tone. The 60 dB curve can be read
against either axis; the OSPL90 curve must be read against the left
hand axis.
Source: Dillon (2001): Hearing Aids
Input-output diagram
Output Level (dB SPL)
110
100
90
50
80
30
70
20
10
0
60
50
30
40
50
60
70
80
90 100
Input level (dB SPL)
Figure 4.7 Input-output diagram of a
compression hearing aid at 2 kHz (bold line) and
lines of constant gain (dotted lines).
Source: Dillon (2001): Hearing Aids
40
35
Equivalent Input Noise
(1/3 Octave dB SPL)
Equivalent
input
noise
Maximum
acceptable
noise
30
25
20
15
10
5
0
100
Hearing aid
noise
1000
Frequency (Hz)
10000
Figure 4.8 Equivalent 1/3-octave input noise of a typical
hearing aid as a function of frequency, and maximum
acceptable 1/3-octave noise.
Source: Dillon (2001): Hearing Aids
REAG = A - C
M
A
F
C
Figure 4.9 Location of SPLs involved in the measurement of real-ear aided
gain. F is located in the undisturbed sound field (e.g. with the head absent),
C is at the control microphone location on the surface of the head, M is at
the hearing aid microphone port, and A is within the residual ear canal close
to the eardrum.
Source: Dillon (2001): Hearing Aids
Canal SPL minus
eardrum SPL (dB)
5
o
50%, 0
0
SPL in ear canal
-5
-10
50%, 45o
-15
100%, 0o
-20
0
10
20
Distance from eardrum (mm)
Figure 4.10 Calculated pattern of SPL in the ear canal versus distance from
the eardrum at a frequency of 6 kHz. The solid curve is for total reflection from
the eardrum with no phase shift at the drum, the dashed line is for 50% power
reflected from the drum with no phase shift, and the speckled line is for 50%
reflected with a 45 degree phases shift at the drum.
Source: Dillon (2001): Hearing Aids
Distance from drum (mm)
Standing-wave
minimum
35
30
25
20
15
10
5
0
0
5
10
15
Frequency of notch (kHz)
Figure 4.11 Distance from the eardrum at which SPL in
the ear canal will be a minimum.
Source: Dillon (2001): Hearing Aids
85
25
80
20
75
15
70
10
65
5
60
0
55
125
-5
250
500 1k
2k
4k
Frequency (Hz)
8k
Figure 4.12 Typical REAG display for a vented,
low to medium gain hearing aid, displaying the
expected low frequency plateau.
Source: Dillon (2001): Hearing Aids
Real Ear Aided Gain
(dB)
Real-Ear Aided
Resp. (dB SPL)
Real-ear aided gain
Insertion gain = A - U
M
U
F
A
F
C
Unaided
C
Aided
Figure 4.13 Location of SPLs involved in the measurement of insertion gain. F
is located in the undisturbed sound field (with the head absent), C is at the
control microphone location on the surface of the head, M is at the hearing aid
microphone port, A is at the eardrum when aided, and U is at the eardrum when
unaided.
Source: Dillon (2001): Hearing Aids
Insertion Gain (dB) Real-Ear Gain (dB)
REIG = REAG - REUG
40
30
20
REAG
REIG
10
0
100
REUG
1000
10000
1000
10000
30
20
10
0
100
Frequency (Hz)
Figure 4.14 Real ear unaided and aided gains (top half). The
Source: Dillon (2001): difference between these curves is the insertion gain, shown as the
Hearing Aids
shaded region in the top half and as the curve in the lower half.
Probe position for insertion gain
(a)
(b)
(c)
(d)
Figure 4.15 Probe positioning for measuring insertion gain: (a)
noting a landmark on the ear; (b) marking the probe; (c) measuring
the unaided response; (d) measuring the aided response.
Source: Dillon (2001): Hearing Aids
Calibrating
the
probe
Photo removed to
minimize file space
Figure 4.16 Positioning of the probe microphone
against the control microphone during calibration.
Source: Dillon (2001): Hearing Aids
Feedback
Forward path (gain)
Feedback path (attenuation)
Figure 4.17 The feedback mechanism in hearing aids.
Source: Dillon (2001): Hearing Aids
Feedback
Real Ear Aided Gain (dB)
35
30
25
20
15
10
5
0
125
250
500
1,000 2,000
Frequency (Hz)
4,000
8,000
Figure 4.18 Coupler gain of a hearing aid with the volume
control adjusted in 2 dB steps. One further increase resulted
in oscillation.
Source: Dillon (2001): Hearing Aids
Cross section of earmold
Skin around
canal
Probe-induced
feedback path
Gap created by
probe tube
Probe tube
Figure 4.19 Leakage paths created by the insertion of a
probe tube between an earmold or shell and the ear canal.
Source: Dillon (2001): Hearing Aids
Stethoclip
Photo removed to
minimize file space
Figure 4.20 A stethoclip attached to a CIC hearing aid.
Source: Dillon (2001): Hearing Aids
Feedback - ITE
Microphone tube detached
at either end
Wax pushes hearing aid away
from the canal wall
Loose fit of shell
Microphone or receiver
touching each other or
touching case
Vent too large, or vent insert fallen out,
or vent too close to microphone port,
or vent overhung by pinnae
Wax directs sound
into vent or slit leak
Receiver tube detached
at either end
Figure 4.21 Common leakage points,
leading to feedback oscillation, in ITE,
ITC, and CIC hearing aids.
Source: Dillon (2001): Hearing Aids
Feedback - BTE
Split in earhook
Tubing too
loose a fit
on earhook
Wax pushes earmold
away from the canal wall
Earhook too
loose a fit on
aid
Wax directs sound
into vent or slit leak
Microphone or
receiver
touching case
Tubing split
Earmold too loose
Vent too large, or vent
insert fallen out
Figure 4.22 Common leakage points, leading to feedback oscillation, in BTE
hearing aids.
Source: Dillon (2001): Hearing Aids