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Published byLee Jennings Modified over 9 years ago
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Figures for Chapter 4 Electroacoustic Performance
Dillon (2001) Hearing Aids
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Standing-wave minimum
Figure Distance from the eardrum at which SPL in the ear canal will be a minimum. Source: Dillon (2001): Hearing Aids
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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
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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
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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
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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
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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
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Feedback Forward path (gain) Feedback path (attenuation)
Figure The feedback mechanism in hearing aids. Source: Dillon (2001): Hearing Aids
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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
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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
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Photo removed to minimize file space
Stethoclip Figure A stethoclip attached to a CIC hearing aid. Source: Dillon (2001): Hearing Aids
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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
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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
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