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

2. Ear simulator V1 V2 V4 V3 Dampers Microphone
Figure 4.1 Simplified internal structure of a four-branch ear simulator.
Source: Dillon (2001): Hearing Aids

3. 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

4. 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
HA2
2 cc
cavity
Microphone
Source: Dillon (2001): Hearing Aids

5. Real-ear to coupler difference
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

6. Photo removed to minimize file space
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

7. Gain-frequency response
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

8. Input-output diagram
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

9. Equivalent input 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.
Source: Dillon (2001): Hearing Aids

10. REAG = A - C M A F C
Figure 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

11. SPL in ear canal
Figure 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

12. Standing-wave minimum
Figure Distance from the eardrum at which SPL in the ear canal will be a minimum.
Source: Dillon (2001): Hearing Aids

13. Real-ear aided gain
Figure Typical REAG display for a vented, low to medium gain hearing aid, displaying the expected low frequency plateau.
Source: Dillon (2001): Hearing Aids

14. Insertion gain = A - U U M A F F Aided Unaided C C
Figure 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

15. Source: Dillon (2001): Hearing Aids
REIG = REAG - REUG
Figure 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.
Source: Dillon (2001): Hearing Aids

16. Probe position for insertion gain
(c)
(d)
Figure 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

17. Photo removed to minimize file space
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

18. Feedback Forward path (gain) Feedback path (attenuation)
Figure The feedback mechanism in hearing aids.
Source: Dillon (2001): Hearing Aids

19. Feedback
Figure 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

20. Probe-induced feedback path Cross section of earmold Skin around canal
Gap created by probe tube
Probe tube
Figure Leakage paths created by the insertion of a probe tube between an earmold or shell and the ear canal.
Source: Dillon (2001): Hearing Aids

21. Photo removed to minimize file space
Stethoclip
Figure A stethoclip attached to a CIC hearing aid.
Source: Dillon (2001): Hearing Aids

22. Feedback - ITE
Wax pushes hearing aid away from the canal wall
Microphone tube detached
at either end
Wax directs sound
into vent or slit leak
Loose fit of shell
Receiver tube detached
at either end
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
Figure Common leakage points, leading to feedback oscillation, in ITE, ITC, and CIC hearing aids.
Source: Dillon (2001): Hearing Aids

23. Feedback - BTE
Split in earhook
Wax pushes earmold away from the canal wall
Tubing too
loose a fit
on earhook
Microphone or receiver touching case
Earhook too
loose a fit on aid
Wax directs sound
into vent or slit leak
Tubing split
Vent too large, or vent
insert fallen out
Earmold too loose
Figure Common leakage points, leading to feedback oscillation, in BTE hearing aids.
Source: Dillon (2001): Hearing Aids