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Figures for Chapter 4

Electroacoustic Performance

Dillon (2001)

Hearing Aids



Microphone

V1

V3

V2

V4

Dampers

Figure 4.1 Simplified internal structure of a four-branch ear

simulator.Source: Dillon (2001): Hearing Aids

Ear simulator



Figure 4.2 Several couplers and their adapters,

and an ear simulators.

Source: Dillon (2001): Hearing Aids

Couplers and

ear simulators

Photo removed to

minimize file space



2 mm dia

3 mm dia

25

18

2 cc

cavity

Microphone

18 mm

Microphone

Putty

ITE / ITC / CIC

Earmold

simulator

Insert

earphone

Figure 4.3 The internal dimensions and

coupling methods for several 2-cc couplers.

HA1

HA2 HA2


2-cc couplers



0

5

10

15

20

125 250 500 1000 2000 4000 8000

equen H

A

e a

g e

a n a

S P

L

m n u

2

S P L

Figure 4.4 RECD: SPL gene ated n the a e age adu t ea ea ana

m nu SPL gene ated n an HA1 2- oup e .

Sou e: D on 2001 : Hearing Aids

Real-ear to coupler difference



Figure 4.5 A hearing aid connected to a coupler,

with a control microphone positioned next to the

hearing aid microphone.


2-cc coupler

and

control microphone

Photo removed to

minimize file space



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.

0

60

70

0

90

00

0

20

2 2 0 00 000 2 000 000 000

requency ( )

o u p l e r O u t p u t L e v e l ( d B

S P L

)

- 0

0

0

20

0

0

0

60

o u

p l e r G a i n ( d B )

60

90

Source: Dillon (200 ): Hearing Aids

Gain-frequency response



50

60

70

80

90

100

110

30 40 50 60 70 80 90 100

I t

t

t

3020

10

50

0

Figure 4.7 I t-o t t iagram of a

com r ssio h ari g ai at 2 kHz bo i a

i s of co sta t gai ott i s .

o rc : Di o 2001): Hearing Aids

Input-output diagram



0

5

10

15

20

2530

35

40

100 1000 10000

Frequency (Hz)

E q u i v a l e n t I n p u t N o i s

e

( 1 / 3 O c

t a v e d B

S P L

)Maximum

acceptablenoise

Hearing aidnoise

Figure 4.8 Equivalent 1/3-octave input noise of a typical

hearing aid as a f unction of f requency, and maximum

acceptable 1/3-octave noise.


Equivalent

input

noise



A

C

M

F

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.


REAG = A - C



-20

-15

-10-5

0

5

0 10 20

Distance from eardrum (mm)

C a n a l S P L m i n u s

e a r d r u m S P

L ( d B )

100%, 0o

50%, 0o

50%, 45o

Figure 4.10 Calculated pattern of SPL in the ear canal versus distance fromthe eardrum at a frequency of 6 kHz. The solid cur ve 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.


SPL in ear canal



0

5

10

15

20

25

30

35

0 5 10 15Frequency of notch (kHz)

D i t a n c e

f r o m d

r u m ( m m )

Figure 4.11 Di tance from the eardrum at which SPL in

the ear canal will be a minimum.


Standing-wave

minimum



55

60

65

70

75

80

85

125 250 500 1k 2k 4k 8kr equency ( )

e a l -

r

d e d

R e s p

. ( d B S P L )

-5

0

5

10

15

20

25

R e a l E

a r

d e d G a i n

( d B )

Figure 4.12 Typical REAG display for vented,

low to medium g in hear ing id, displaying the

expected low fr equency plateau.

Sour ce: Dillon (2001): Hearing Aids

Real-ear aided gain



U

C

F

Unaided

A

C

M

F

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.


Insertion gain = A - U



0

10

20

30

100 1000 10000

Frequency (Hz)

I n s e

r t i o n

G a i n

( d B )

Figure 4.14 Real ear unaided and aided gains (top half ). The

diff erence between these cur ves is the insertion gain, shown as the

shaded region in the top half and as the cur ve in the lower half.

0

10

20

30

40

100 1000 10000 R e a l

a r G a i n

( d B )

R G

R UG

R IG

Source: Dillon (2001):

Hearing Aids

REIG = REAG - REUG



(a) (b)

(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.


(c)

Probe position for insertion gain



Figure 4.16 Positioning of the probe microphone

against the control microphone during calibration.


Calibrating

the

probePhoto removed to

minimize file space



Forward path (gain)

Feedback path (attenuation)

Figure 4.17 The feedback mechanism in hearing aids.


Feedback



0

5

10

15

20

25

30

35

125 250 500 1,000 2,000 4,000 8,000Fr ( )

l

r

i

G

i

(

)

Figure 4.18 Co pl r g i of h r i g i with th vol m

o tr ol j st i 2 dB st ps. O f r th r i r s r s lt d

i os ill tio .

So r : Dillo (2001): Hearing Aids

Feedback



Cross section of earmold

Probe tube

Gap created by

probe tube

Skin around

canal

Figure 4.19 Leakage paths created by the insertion of a

probe tube between an earmold or shell and the ear canal.


Probe-induced

feedback path



Figure 4.20 A stethoclip attached to a CIC hearing aid.


StethoclipPhoto removed to

minimize file space



Loose fit of shell

Wax directs sound

into vent or slit leak

Receiver tube detached

at either end

Microphone or receiver

touching each other or touching case

Microphone tube detached

at either end

Vent too large, or vent insert fallen out,

or vent too close to microphone port,

or vent overhung by pinnae

Figure 4.21 Common leakage points,

leading to feedback oscillation, in ITE,

ITC, and CIC hearing aids.

Wax pushes hearing aid away

from the canal wall


Feedback - ITE



Figure 4.22 Common leakage points, leading to feedback oscillation, in BTE

hearing aids.

Wax directs sound

into vent or slit leak

Tubing tooloose a fit

on earhook

Tubing split

Split in earhook

Wax pushes earmoldaway from the canal wall

Earhook too

loose a fit on

aid

Microphone or receiver

touching case

Earmold too looseVent too large, or vent

insert fallen out


Feedback - BTE

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