by Ted Venema, PhD
Ted Venema, PhD, is an assistant professor of audiology at the University of Western Ontario.
It seems like every year hearing instrument dispensers face the challenge of absorbing and understanding yet another new fitting method. There is a plethora of methods intended for fitting an end organ (the cochlea) that most of us have never even seen. In our profession, however, no specific fitting method is universally accepted as “correct.”
First to arrive at our doorstep were fitting methods based on the half-gain rule. Intended for fitting linear hearing aids, all offer a single gain target for any particular hearing loss because linear hearing aids provide the same gain for all input levels. Fitting methods intended specifically for compression hearing aids offer more than one target, because compression hearing aids provide different gain for different input levels. All of these fitting methods have markedly different targets for any one particular hearing loss, because they all emerge from differing fitting philosophies.
In the past decade many compression-based fitting methods have emerged. The most popular of these are DSL (Seewald, 1992), Fig6, (Killion and Fikret-Pasa, 1993), IHAFF (Cox, 1995) and recently NAL-NL1 (Dillon, Katsch, Byrne, Ching, Keidser, and Brewer, 1998). NAL-NLl stands for National Acoustics Labs, Non-Linear, version 1. The purpose of this article is to describe the NAL-NL1 fitting method and show how and why the gain prescribed by this method differs from the gain prescribed by other compression-based fitting methods, especially DSL.
To become acquainted with the NAL-NL1 fitting method, it is important to understand the cornerstone philosophy behind the whole NAL family of fitting methods (NAL, NAL-R, NAL-NL1). First, all of these methods try to equalize rather than normalize loudness relationships across the speech Hz’s. According to Dillon, et al. 1998, if all speech Hz’s are amplified so that they are heard equally loud, speech intelligibility will be maximized. With normal hearing, the low-Hz vowels are heard louder than the high-Hz unvoiced consonants (Figure 1).
When other fitting methods try to preserve this normal relationship of loudness among the speech Hz’s, they prescribe more gain for the low frequencies (Dillon, et al., 1998). Think of a hearing aid for a flat 50 dB hearing loss; if intensity differences among adjacent speech frequencies are preserved so that vowels are heard louder than consonants, then vowels (lows) will be amplified so that they are heard louder by the person wearing the hearing aid. The NAL methods, on the other hand, do not try to preserve or normalize this loudness relationship among speech frequencies; instead, they strive to equalize loudness for all of the speech Hz’s.
Two salient features characterize the new NAL-NL1 fitting method for compression hearing aids: 1) the concept of equalizing rather than normalizing the loudness of adjacent speech Hz’s and 2) providing less gain for Hz’s where hearing loss is worst and more gain where hearing is best. This last point is developed further in literature describing the development of NAL-NL1, where the term “audibility” is differentiated from the term “effective audibility” (Dillon, et al. 1998).
Audibility can be described and measured in terms of sensation level, if actual hearing thresholds are known. Effective audibility refers to how much information can be extracted from speech sounds once they are audible. According to Dillon, et al. (1998), as hearing loss increases, people tend to have more effective audibility with less audibility.
The main objective of developing NAL-NL1 was to determine the gain for several input levels that would result in maximal effective audibility (Dillon, et al. 1998). To calculate NAL-NL1 gain targets, gain calculations were performed for 52 people with various audiometric configurations, for input levels from 30 to 90 dB SPL, in 10 dB increments. The new NAL-NL1 fitting method provides very similar insertion gain targets for 65 dB SPL inputs as those given by NAL-R. For gently sloping hearing loss audiograms, ranging from mild to severe in degree, NAL-NL1 target insertion gain ended up being very close to the targets provided by NAL-R. For most types of hearing loss, the mid frequencies of speech were consistently found to be similar in loudness to the lower and higher adjacent speech frequencies. The end aim was thus achieved: equal loudness of all speech frequency bands, along with maximal speech intelligibility.
Proponents of NAL-NL1 (Byrne, Dillon, Ching, Katsch, and Keidser, 2001) compared gain targets of NAL-NL1 to those for Fig6, IHAFF and DSL. The results shown in Figures 2—6 illustrate their comparisons among gain targets for five various audiometric configurations, using an input level of 65 dB SPL. Note that on some of these figures, the NAL-NL1 target does not span the entire frequency range. These Hz’s are always where the hearing loss is most pronounced. According to the philosophy behind NAL-NL1, for these particular hearing losses amplification at these Hz’s will not contribute towards effective audibility with any amount of gain.
Let’s first look at the flat hearing loss of 60 dB (Figure 2). Here it is readily evident that NAL-NL1 prescribes the very least low-Hz gain compared to the other compression-based fitting methods. The high-Hz targets are similar for NAL-NL1, Fig6 and IHAFF. DSL prescribes the most high-Hz gain.
For a reverse slope hearing loss of 70 dB (Figure 3), IHAFF again prescribes the most low-Hz gain and NAL-NL1 also prescribes the least low-Hz gain. Note how the NAL-NL1 target for the low Hz’s abruptly stops at 500Hz. For this hearing loss, says NAL-NL1 fitting philosophy, gain at Hz’s of poorest hearing (250Hz) will not increase or maximize speech intelligibility (Ching, 2000). Interestingly, NAL-NL1 prescribes more high-Hz gain than DSL for the reverse hearing loss. This is consistent with the NAL emphasis on providing more of a flat insertion gain target, and thus, equalizing the loudness of the speech Hz’s.
For a gently sloping, moderate to moderately-severe hearing loss (Figure 4), NAL-NL1 asks for amounts of mid-Hz gain similar to the other fitting methods. For this common hearing loss configuration, NAL-NL1 prescribes less low and high-Hz gain.
For a precipitous, pronounced high-Hz hearing loss with normal low-Hz hearing up to 1000Hz (Figure 5), all the fitting methods prescribe similar gain from 1 to 2000Hz. Fig6 asks for slightly more high-Hz gain than IHAFF, and DSL asks for by far the most high-Hz gain. In comparison, NAL-NL1 does not ask for any high-Hz gain whatsoever. Byrne, et al. (2001) reminds us that these high-Hz regions, where hearing loss is severe to profound, do not contribute much to speech intelligibility. Consequently, no gain is prescribed at all for these Hz’s.
Lastly, consider the steeply sloped hearing loss that drops to a pronounced high-frequency loss without the precipitous “corner” (Figure 6). All the fitting methods shown prescribe varying amounts of gain for the low- to mid Hz’s. Once again, however, note that NAL-NL1 prescribes no high-Hz gain whatsoever.
In summary, for the flat, reverse and gently sloping audiograms shown in Figures 2, 3, and 4, the IHAFF fitting method provides the most low-Hz gain, followed by DSL, Fig6 and NAL-NL1. These results are in keeping with the NAL-NL1 point of view that, since other fitting methods attempt to normalize rather than equalize the loudness of adjacent speech frequencies, they prescribe relatively more low-Hz gain. For steeply sloping audiograms (Figures 5 and 6), DSL provides the most high-frequency gain of all the four fitting methods compared, while NAL-NL1 asks for the least amount of high-frequency gain.
NAL-NL1 and DSL targets have also been compared by proponents of DSL (Scollie and Seewald, 1999). It should be noted that the DSL method normally construes its aided targets in terms of output, not gain. Accordingly, the hearing loss and aided targets are plotted in units of dB SPL, on an “SPL-o-gram” (Figure 7). This enables hearing loss and hearing aids to “speak the same language.” The SPL-o-gram shows SPL increasing as you “go up,” and Hz increasing as you “go to the right,” just like any graph normally does. In the SPL-o-gram, the hearing loss is literally the “floor,” uncomfortable loudness levels are the “ceiling,” and aided targets are situated in between. The SPL-o-gram, visible on DSL I/O software as well as on various fitting software packages, actually makes a lot of sense and clarifies explanations for counseling. To DSL proponents, output is the sound that is actually delivered to the eardrum of the listener, while gain is merely a means to an end. Thus, for any comparison with DSL, gain targets have to be converted to output targets, or vice versa.
DSL and NAL-NL1 target output comparisons (Scollie and Seewald, 1999) are shown for the very commonly encountered gently sloping (10 dB/octave slope) audiogram configuration (Figure 8). Output target comparisons are shown for 500Hz and 4000Hz, for varying degrees of sloping hearing loss and also for inputs of 50, 80 and 100 dB SPL. Unlike gain graphs which show compression as offering less and less gain with increased input levels, output graphs show increased output with increased input levels. The top lines on each of these figures thus show the target output for the most intense input level, and the bottom lines show the target output for the softest input levels. In Figure 8, the effect of three things is being shown: fitting method (DSL or NAL-NL1), degree of hearing loss and input level. The simplest way to read and understand these graphs is to determine where the solid and dotted line functions are closest together and where they are furthest apart. Similarities between DSL and NAL-NL1 are shown whenever the lines are close together; differences are shown whenever the lines are furthest apart.
For sloping audiograms, it appears that the target differences between NAL-NL1 and DSL are more evident at 500Hz than at 4000Hz, with DSL prescribing more low-Hz output than NAL-NL1. This observation is consistent with that provided by the proponents of NAL-NL1 (Figure 4). Unlike that comparison, however, the proponents of DSL saw only minimal output target differences at 4000Hz. In fact, at high input levels, NAL-NL1 appears to prescribe more output than DSL (Figure 8)! The differences in targets are most pronounced at soft 50 dB input levels, and the differences themselves increase with more hearing loss.
Lastly, the author of this article programmed a digital hearing aid to compare the output targets of NAL-NL1 and DSL. When it was programmed to best meet the output targets of DSL, its output was measured on a probe tube real ear system. The same hearing aid was then programmed to best meet the gain targets of NAL-NL1 and its output was measured again.
For the flat hearing loss of 60 dB, NAL-NL1 prescribed approximately 10 dB less output than DSL for Hz’s below 1000Hz and above 4000Hz. On the other hand, their output targets for the same hearing loss were very similar between 1000 and 4000Hz. These results showed considerably less difference between the two methods than indicated by the proponents of NAL-NL1.
For the reverse hearing loss, NAL-NL1 again prescribed less low-Hz output than did DSL. Once again, the differences were not as extreme as those shown by the proponents of NAL-NL1. Interestingly, NAL-NL1 actually prescribed more high-Hz output than DSL for the reverse hearing loss shown here. These results are consistent with the findings by the NAL-NL1 proponents.
For the sloping hearing loss, NAL-NL1 and DSL prescribed very similar low and mid-Hz output. NAL-NL1 prescribed less (10—15 dB) high-Hz output than DSL. These results differed markedly from those shown by NAL-NL1 proponents in Figure 4, where larger differences were shown, especially for the low Hz’s.
For the precipitous hearing loss, NAL-NL1 and DSL prescribed almost identical low and mid-Hz output. The differences appeared again for the high Hz’s, where NAL-NL1 prescribed less output than DSL. Once again, however, the differences here were not nearly as great as those shown by the proponents of NAL-NL1, in Figure 5.
In general, findings from NAL-NL1 proponents and those from the author show that for flat and reverse hearing losses, NAL-NL1 prescribes less low-Hz gain/output than most other compression fitting methods, especially DSL. For sloping and precipitous hearing losses, all three comparison sources (NAL-NL1, DSL and the author) agree that NAL-NL1 also prescribes less high-Hz gain/output than other methods, especially DSL. The differences illustrated by the proponents of NAL-NL1, however, appear much greater than those shown by other investigators.
Based on the past record of NAL-R, the new NAL-NL1 should be an extremely popular and widely accepted fitting method for compression hearing aids. As mentioned earlier, there is a psychological appeal to actually “hitting” gain targets, and this is more easily accomplished with NAL-NL1 than with some other fitting methods. Clinical reality, however, dictates that we need to confront issues with facts. For example, there is a widely held but erroneous assumption that compared to NAL-NL1, DSL is always “hungry” for high-Hz gain. The truth of the matter is that for most types of hearing loss the greatest difference between these two fitting methods concerns low-Hz gain or output. NAL-NL1 prescribes less low-Hz gain than most other fitting methods, especially DSL. Only for sloping and precipitous hearing losses does DSL prescribe more high-Hz gain than NAL-NL1.
Lastly, consider that the new NAL-NL1 software (Dillon, Byrne, Brewer, Katsch, Ching, and Keidser, 1998) incorporates the SPL-o-gram, much as it is found on DSL software (Seewald, Cornelisse, Ramji, Sinclair, Moodie, and Jamieson, 1997). For DSL, which typically advocates this type of display, imitation is the finest form of flattery. THP
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Adapted with permission from Byrne, Dillon, Ching, Katsch and Keidser (2001).
Adapted with permission from Byrne, et al. (2001).
Adapted with permission from Byrne, et al. (2001).