Figures 2 and 3 show the difference in response shapes prescribed for the flat and reverse sloping loss, respectively. For these audiogram configurations, the insertion gain curves display differences in targets of 15–20 dB at 250 Hz and of 10–15 dB at 500 and 4000 Hz after normalization of gain at 1000 Hz. That is, the prescribed response shape varies substantially between procedures.
Below 1000 Hz where the hearing loss is severe in both cases, there is a distinct difference in the shapes of the targets prescribed by the two generic methods. While the DSL[i/o] target curve, aiming at loudness normalization, is relatively flat, the NAL-NL1 target curve, aiming at speech intelligibility maximization, rises steeply.
In this frequency region, the targets prescribed by Senso Diva and DigiFocus II are most similar in shape to the DSL[i/o] targets, even though none aims at pure loudness normalization. Danalogic, which does use a pure loudness-normalization rationale, prescribes targets most similar to the NAL-NL1 targets, whereas the Claro targets, also aiming at normalizing loudness, seem to produce a shape somewhere between NAL-NL1 and DSL[i/o].
Above 1000 Hz, the shapes of the two generic target curves are very similar. For the flat loss, Senso Diva and DigiFocus II follow the generic targets, whereas the Danalogic target curve is flat across the high frequencies and the Claro target curve rolls off steeply at 2000 Hz. For the reverse sloping loss, in comparison to the two generic methods, the proprietary methods all prescribe a shallower downward-sloping target curve across the frequencies above 1000 Hz. Across the entire frequency range, Senso Diva prescribes for both audiograms the flattest response of the six methods.
The shapes of the target curves for the gently sloping high-frequency loss are reported in Figure 4. For this audiogram configuration, a moderate 10-dB variation in gain is observed at both low and high frequencies after normalization of gain at 1000 Hz. In this case, both the generic methods prescribe a similar upward-sloping target curve from 500 to 2000 Hz. Outside this range, DSL[i/o] prescribes relatively more gain than NAL-NL1. Apart from Senso Diva, the proprietary methods prescribe, like NAL-NL1, an upward-sloping target curve from 250 to 2000 Hz. Above 2000 Hz, Danalogic and DigiFocus II more closely follow the DSL[i/o] targets, whereas Claro presents a roll-off like NAL-NL1, which is not typical of a loudness-normalization procedure. As for the flat and reverse sloping loss, Senso Diva prescribes a much flatter target curve than the other five methods.
After normalization of gain at 1000 Hz, spreads of more than 20 dB are seen across targets at the high frequencies for the two steeply sloping high-frequency losses (see Figures 5 and 6). For both of these, the generic targets differ above 1000 Hz. NAL-NL1 prescribes a slightly shallower slope than DSL[i/o] and its targets roll off at 3000 Hz while the DSL[i/o] targets continue to rise steeply.
Except that the Senso Diva target curve is the most shallow, there is no consistent pattern in the proprietary target shapes for the two steeply sloping losses above 1000 Hz. For example, Danalogic's targets are closest to those of DSL[i/o] for the steeply sloping loss with normal threshold up to 1000 Hz (Figure 5), but they are closest to NAL-NL1 for the steeply sloping loss with a mild low-frequency hearing loss (Figure 6).
For the Claro target, the pattern is reversed. Both of these proprietary methods aim at normalizing loudness. For the steeply sloping high-frequency loss with normal threshold up to 1000 Hz, all the generic and proprietary methods prescribe a similar flat target curve across the frequencies up to and including 1000 Hz. Figure 6, on the other hand, reveals variations of 10 dB at 250 Hz and of 15 dB at 500 Hz between targets at the frequencies below 1000 Hz for the steeply sloping high-frequency loss with a mild low-frequency hearing loss. Whereas the targets of NAL-NL1, Senso Diva, and Danalogic gently rise across the low frequencies, the DSL[i/o], Claro, and DigiFocus targets are flatter. Note that the two manufacturers (Claro and DigiFocus II) that have introduced a target curve specifically for ski-slope losses recommend very different targets across the high frequencies. In particular, Claro presents a characteristic roll-off at 3000 Hz.
Overall, for an input level of 65 dB SPL, the two generic methods, which represent different and well-defined rationales, prescribe responses of very different shapes. None of the proprietary methods consistently prescribes a response shape similar to that prescribed by either DSL[i/o] or NAL-NL1. However, for frequencies below 1000 Hz, the Danalogic targets, which are based on the rationale of loudness normalization, are often much more similar to the NAL-NL1 targets. On the other hand, DigiFocus II, which has a rationale similar to that of NAL-NL1, has targets that are generally more similar to the DSL[i/o] targets below 1000 Hz.
When normalizing the targets at 1000 Hz, the various methods generally produce variations in gain at low and high frequencies between 10 and 20 dB. With a few exceptions, the differences in shapes of target curves for input levels of 50 dB and 80 dB are the same as those reported for a 65-dB input level.
Hearing loss configuration dependency/independency
Our final analysis investigated hearing loss configuration dependency. Hearing loss configuration dependency refers to the way that gain changes as the slope of the audiogram changes. A prescription is independent of hearing loss configuration if the gain at any one frequency is independent of the threshold at any other frequency. Conversely, hearing loss configuration dependency means that there is a relationship between the gain prescribed at one particular frequency and the hearing threshold at more than one frequency. In such a case, the slope of the loss together with the degree of loss influences the prescribed insertion gain at any frequency.
To test the prescriptions under investigation here for hearing loss configuration dependency, we entered into each software module four new audiograms of differing slopes that all passed through 50 dB at 1000 Hz. We examined the gain prescribed by each procedure for each hearing loss at 1000 Hz. If the prescription is independent of hearing loss configuration, then the slope of the loss would be irrelevant and the same gain would be prescribed at 1000 Hz for all audiograms. On the other hand, we would expect the gain to vary with the slope if the prescription is hearing loss configuration dependent.
Table 4 shows the results of the analysis. The three loudness-normalization procedures and the loudness-mapping procedure prescribed exactly the same gain at 1000 Hz regardless of the slope of the audiogram, indicating that they are hearing loss configuration independent. On the other hand, NAL-NL1 and DigiFocus II produced different gains for different slopes, indicating that they are hearing loss configuration dependent. It should be added that the acoustic characteristics affect the prescribed gain for the DigiFocus II. Therefore, we cannot be entirely sure if the observed differences between gains for each loss were due to hearing loss configuration dependency or to change in vent size.
DISCUSSION AND SUMMARY
The differences in the targets prescribed by the two generic prescription procedures (DSL[i/o] and NAL-NL1) have been reported before1 and can be explained from the assumptions and principles underlying the two procedures. For example, DSL[i/o] consistently prescribes more gain than NAL-NL1 at frequencies where the hearing loss is severe. It does so because DSL[i/o] is aiming at restoring loudness and ensuring audibility. However, NAL-NL1 includes a desensitization factor because it is believed that hearing-impaired listeners have reduced ability to extract useful information from speech at frequencies where the hearing loss is severe.12
On the other hand, little is known about the proprietary fitting methods and we were interested to find how much the targets from such procedures differed from those prescribed by the two generic procedures and from each other. The variation of 10 dB in prescribed overall gain for various audiogram configurations and input levels is not so critical, because this difference is easily overcome by the use of a volume control or by adjustment of the overall gain.
However, the variation in the targets for a 65-dB input is more significant. When all the targets are normalized to the same gain level at 1000 Hz, we see large differences in prescribed response shapes that produce variations in gain at lower and higher frequencies of between 10 and 30 dB (Figures 2–6).
Another study that presented insertion gain curves derived from 2-cc coupler measurements of selected generic and proprietary fitting methods implemented in commercial devices reported similar gain variations of 10 to 20 dB.13 The same study concluded that the differences have a large effect on estimated loudness, but only a small effect on estimated speech intelligibility. However, it is unclear from that study how much of the discrepancy is due to differences in prescribed overall gain (which can be compensated for) and how much to differences in response shape.
In the comparison of target curves, those for Senso Diva differed from the others in having a flatter shape and generally less overall gain. While the targets for all the other procedures are derived for a pure-tone test signal, this choice was not specifically available in Compass. We think—but have not confirmed—that the targets for Senso Diva are more likely related to a broadband complex signal, which would explain the lower overall gain. Further, whereas we obtained gain values for a range of audiometric frequencies for most of the other procedures, gain values could be extracted at only four octave frequencies for Senso Diva. That is, we obtained less detail on the shape of the targets for this device.
It should also be noted that the insertion gain targets for DigiFocus II, Claro, and Senso Diva were affected by the acoustic parameters and device selected. As far as possible, we equalized these factors across procedures in our investigation. We are unsure if this dependency was intended by the manufacturers, but we cannot see why it should occur.
A study of the target curves suggests that the three procedures (one generic and two proprietary) that share the rationale of normalizing frequency-specific loudness have very little in common. This may be partly because the loudness data used for each procedure are measured with different loudness tests and/or because the procedures are based on different operational principles, which in the case of the proprietary methods are virtually unknown.
With respect to hearing loss configuration dependency, the loudness-normalization and loudness-mapping methods (DSL[i/o], Claro, Danalogic, and Senso Diva) all seem to prescribe gain independent of hearing loss configuration.
In contrast, the gain prescribed by NAL-NL1 at each frequency depends on the degree of loss at several frequencies. Our investigation suggests that the prescription for DigiFocus II is also hearing loss configuration dependent. However, this observation may be a result of selecting different acoustic parameters for the different audiograms. We note that data collected at NAL in the 1980s showed that the gain-frequency response preferred by hearing-impaired listeners varied in a hearing loss configuration-dependent way.14,15
Overall, we find the outcome of this investigation thought provoking. Our data suggest that if the targets prescribed by the various fitting algorithms examined here are reached, clients with similar hearing losses can easily walk away with extremely different amplification characteristics, depending on which device and/or fitting method is chosen. In practice, however, it is likely that, due to vented earmolds, feedback, and limitations in the electroacoustic characteristic, the achieved differences in the real ear are smaller than the target differences reported here. Even so, the real-ear differences could be significant.
Personally, we think that the current paucity of information available on the various proprietary fitting methods makes it impossible for the audiologist to make an informed choice of which device and procedure will best benefit the client.
1. Byrne D, Dillon H, Katsch R, et al.: The NAL-NL1 procedure for fitting non-linear hearing aids: Characteristics and comparisons with other procedures. JAAA
2. Cornelisse L, Seewald, R, Jamieson D: The input/output formula: A theoretical approach to the fitting of personal amplification devices. J Acoust Soc Am
3. Ludvigsen C: Basic amplification rationale of a DSP hearing instrument. Hear Rev
4. Schum DJ: Adaptive speech alignment: A new fitting rationale made possible by DSP. Hear J
5. Phonak: Claro Loudness Perception Profile
. Stafa, Switzerland.
6. GN ReSound: Fitting Danalogic: The Reasoning Behind the Technology
7. Ludvigsen C: Audiological background and design rationale of Senso Diva. Widexpress
8. Jenstad LM, Seewald RC, Cornelisse LE, Shantz J: Comparison of linear gain and wide dynamic range compression hearing aid circuits: Aided speech perception measures. Ear Hear
9. Jenstad LM, Pumford J, Seewald RC, Cornelisse LE: Comparison of linear gain and wide dynamic range compression hearing aid circuits II: Aided loudness measures. Ear Hear
10. Keidser G, Grant F: Comparing loudness normalization (IHAFF) with speech intelligibility maximization (NAL-NL1) when implemented in a two-channel device. Ear Hear
11. Dillon H: Hearing Aids
. Sydney, Australia: Boomerang Press 2001: 99.
12. Ching TYC, Dillon H, Katsch R, Byrne D: Maximizing effective audibility in hearing aid fitting. Ear Hear
13. Smeds K, Leijon A: Threshold-based fitting methods for non-linear (WDRC) hearing instruments: Comparison of acoustic characteristics. Scand Audiol
14. Byrne D, Murray N: Predictability of the required frequency response characteristic of a hearing aid from the pure-tone audiogram. Ear Hear
© 2003 Lippincott Williams & Wilkins, Inc.
15. Byrne D, Parkinson A, Newall P: Hearing aid gain and frequency response requirements for the severely/profoundly hearing impaired. Ear Hear