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

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

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

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

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

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

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

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

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

SPL in ear canal 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

Standing-wave minimum Figure 4.11 Distance from the eardrum at which SPL in the ear canal will be a minimum. Source: Dillon (2001): Hearing Aids

Real-ear aided gain 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

Insertion gain = A - U U M A F F Aided Unaided C C 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

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

Probe position for insertion gain (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

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

Feedback Forward path (gain) Feedback path (attenuation) Figure 4.17 The feedback mechanism in hearing aids. Source: Dillon (2001): Hearing Aids

Feedback 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

Probe-induced feedback path Cross section of earmold Skin around canal 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

Photo removed to minimize file space Stethoclip Figure 4.20 A stethoclip attached to a CIC hearing aid. Source: Dillon (2001): Hearing Aids

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 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 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 4.22 Common leakage points, leading to feedback oscillation, in BTE hearing aids. Source: Dillon (2001): Hearing Aids