Figures for Chapter 4 Electroacoustic Performance Dillon (2001) Hearing Aids.

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.


Couplers and ear simulators

Photo removed to minimize file space

2 mm dia

3 mm dia

25

18

2 cccavity

Microphone

18 mm

Microphone

Putty

ITE / ITC / CIC

Earmoldsimulator

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

Frequency (Hz)

Ave

rage

can

al S

PL

min

us 2

cc

SP

L

Figure 4.4 RECD: SPL generated in the average adult real ear canal minus SPL generated in an HA1 2-cc coupler.


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.

50

60

70

80

90

100

110

120

125 250 500 1,000 2,000 4,000 8,000Frequency (Hz)

Cou

pler

Out

put

Leve

l (dB

SP

L)

-10

0

10

20

30

40

50

60

Cou

pler

Gai

n (d

B)

60

90


Gain-frequency response

50

60

70

80

90

100

110

30 40 50 60 70 80 90 100

Input level (dB SPL)

Ou

tpu

t L

evel

(dB

SP

L)

3020

10

50

0

Figure 4.7 Input-output diagram of a compression hearing aid at 2 kHz (bold line) and lines of constant gain (dotted lines).


Input-output diagram

0

5

10

15

20

25

30

35

40

100 1000 10000

Frequency (Hz)

Equ

ival

ent I

nput

Noi

se

(1/3

Oct

ave

dB S

PL)

Maximumacceptable

noise

Hearing aid noise

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.


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)

Can

al S

PL

min

us

eard

rum

SP

L (d

B)

100%, 0o

50%, 0o

50%, 45o

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.


SPL in ear canal

0

5

10

15

20

25

30

35

0 5 10 15Frequency of notch (kHz)

Dis

tanc

e fr

om d

rum

(m

m)

Figure 4.11 Distance 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 8kFrequency (Hz)

Re

al-E

ar

Aid

ed

R

esp

. (d

B S

PL

)

-5

0

5

10

15

20

25

Rea

l Ear

Aid

ed G

ain

(dB

)

Figure 4.12 Typical REAG display for a vented, low to medium gain hearing aid, displaying the expected low frequency plateau.


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)

Inse

rtio

n G

ain

(dB

)

Figure 4.14 Real ear unaided and aided gains (top half). The difference between these curves is the insertion gain, shown as the shaded region in the top half and as the curve in the lower half.

0

10

20

30

40

100 1000 10000Rea

l-Ear

Gai

n (d

B)

REAG

REUG

REIG


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 8,000Frequency (Hz)

Rea

l Ear

Aid

ed G

ain

(dB

)

Figure 4.18 Coupler gain of a hearing aid with the volume control adjusted in 2 dB steps. One further increase resulted in oscillation.


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-inducedfeedback path

Figure 4.20 A stethoclip attached to a CIC hearing aid.


Stethoclip Photo removed to minimize file space

Loose fit of shell

Wax directs soundinto vent or slit leak

Receiver tube detachedat either end

Microphone or receivertouching each other or

touching case

Microphone tube detachedat 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 soundinto vent or slit leak

Tubing too loose a fiton earhook

Tubing split

Split in earhookWax pushes earmold

away from the canal wall

Earhook tooloose a fit on

aid

Microphone or receiver

touching case

Earmold too looseVent too large, or vent insert fallen out


Feedback - BTE

Figures for Chapter 4 Electroacoustic Performance Dillon (2001) Hearing Aids.

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Transcript of Figures for Chapter 4 Electroacoustic Performance Dillon (2001) Hearing Aids.