1. Figures for Chapter 9 Prescription
Dillon (2001)
Hearing Aids

2. Dynamic range and hearing loss
Figure 9.1 Uncomfortable listening level and most comfortable level for people with sensorineural hearing loss, averaged across 500, 1k, 2k, and 4 kHz. Data shown with filled symbols are from Schwartz et al. (1988) and those with open symbols are from Pascoe (1988). The dashed line has a slope of 0.5, illustrating the relationship between MCL and the half-gain rule.
Source: Dillon (2001): Hearing Aids

3. NAL-R for a flat loss
Figure 9.2 The insertion gain response prescribed by the NAL-R formula for a flat 40 dB hearing loss.
Source: Dillon (2001): Hearing Aids

4. Desired sensation levels
Figure 9.3 Sensation level targets for the Desired Sensation Level method as a function of hearing threshold, at 1 kHz. Values are very similar at other frequencies.
Source: Dillon (2001): Hearing Aids

5. Hearing threshold (dB HL)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
Frequency (Hz)
Hearing threshold (dB HL)
Figure 9.4 Audiogram of a mild, gently sloping sensorineural hearing loss, and insertion gains prescribed by the DSL (triangles), POGO-II (squares) and NAL-RP (diamonds) procedures.
Source: Dillon (2001): Hearing Aids

6. Hearing threshold (dB HL)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
Frequency (Hz)
Hearing threshold (dB HL)
Figure 9.5 Same as Figure 9.4, but for a moderate, flat sensorineural hearing loss.
Source: Dillon (2001): Hearing Aids

7. Frequency (Hz)
Hearing threshold (dB HL)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
Figure 9.6 Same as Figure 9.4, but for a moderate, steeply sloping sensorineural hearing loss.
Source: Dillon (2001): Hearing Aids

8. 250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
Frequency (Hz)
Hearing threshold (dB HL)
Figure 9.7 Same as Figure 9.4, but for a profound, gently sloping sensorineural hearing loss.
Source: Dillon (2001): Hearing Aids

9. IHAFF prescription
Figure 9.8 An example of the three-point I-O curve, for a frequency of 2 kHz, prescribed by the VIOLA software on the basis of the IHAFF procedure.
Source: Dillon (2001): Hearing Aids

10. Fig 6 prescription
Figure 9.9 Insertion gain prescribed by the FIG6 method at any frequency as a function of hearing threshold, for each of the three input levels 40, 65, and 95 dB SPL.
Source: Dillon (2001): Hearing Aids

11. Basis of DSL THhi ULhi THn-SFt ULhi -SFt
Free field input level (dB SPL)
Ear canal output level
(dB SPL)
Figure The DSL[i/o] method, showing which input levels are mapped to which output levels, using the terminology from Cornelisse et al. (1995). UL stands for upper level of comfortable listening, and TH stands for threshold, where both are expressed in dB SPL in the ear canal. The subscripts n and hi stand for normal and hearing impaired respectively. SFt is the sound field transform from free field SPL to ear canal SPL for the unaided ear for the frequency in question, and is synonymous with REUG.
Source: Dillon (2001): Hearing Aids

12. Nonlinear prescriptions
Figure Insertion gain at input levels of 50, 65, and 80 dB SPL for each of four selection procedures for a person with a flat 40 dB hearing loss. The IHAFF prescriptions are based on the average loudness growth curves for people with various degrees of hearing loss (Cox, private communication). The DSL [i/o] prescription has been converted from real ear aided gain to insertion gain by subtracting an adult average REUG.
Source: Dillon (2001): Hearing Aids

13. Nonlinear I-O curves
Figure Input-output diagrams at 2 kHz, showing the knee-points in the curves, for the Fig 6, NAL-NL1, DSL [i/o], and IHAFF procedures, for a person with a flat 60 dB HL hearing loss and a two-channel compression hearing aid. The IHAFF procedure is based on average loudness-growth curves. The DSL[i-o] curve is drawn with the CT used in its derivation. In practice a much higher CT would be used, similar to that of the other procedures.
Source: Dillon (2001): Hearing Aids

14. Compression limiting and peak clipping
Voltage or Pressure
Time
Time
Compression Limiting
Peak Clipping
Figure A speech waveform after passing through a peak clipper and a compression limiter, where both types of limiter can pass the same peak signal level without clipping.
Source: Dillon (2001): Hearing Aids

15. OSPL90
prescription
Estimated
optimum MPO
Maximum to avoid
discomfort
Minimum to avoid
saturation
Figure The NAL SSPL selection procedure, based on values midway between the OSPL90 needed to avoid discomfort and the OSPL90 needed to avoid excessive saturation.
Source: Dillon (2001): Hearing Aids

16. (a) (b) + Volume Compression control limiters Band gains +
Figure A multi-channel hearing aid in which limiting occurs (a) independently in each channel, and, (b) on the wide-band signal after the channels have been recombined.
Source: Dillon (2001): Hearing Aids

17. Upper aidable frequency
Frequency (Hz)
250
125
500 1k 2k 4k 8k 20 40 60 80
100
120
Hearing threshold (dB HL)
Figure Two audiograms with similar losses at 2 and 3 kHz, but different upper frequency limits of aidable hearing.
Source: Dillon (2001): Hearing Aids