Abstract

The Desired Sensation Level (DSL) Method was revised to support hearing instrument fitting for infants, young children, and adults who use modern hearing instrument technologies, including multichannel compression, expansion, and multimemory capability. The aims of this revision are to maintain aspects of the previous versions of the DSL Method that have been supported by research, while extending the method to account for adult-child differences in preference and listening requirements. The goals of this version (5.0) include avoiding loudness discomfort, selecting a frequency response that meets audibility requirements, choosing compression characteristics that appropriately match technology to the user's needs, and accommodating the overall prescription to meet individual needs for use in various listening environments. This review summarizes the status of research on the use of the DSL Method with pediatric and adult populations and presents a series of revisions that have been made during the generation of DSL v5.0. This article concludes with case examples that illustrate key differences between the DSL v4.1 and DSL v5.0 prescriptions. Work that Leads to the 2005 Algorithm As summarized by Seewald et al. in this issue (2005), the DSL Method was revised to accommodate the prescription of linear vs nonlinear hearing instruments Since that revision, DSL[i/o] has been evaluated in both adult and pediatric populations in a number of studies. In this chapter, we will summarize the current status of DSL evaluation work in children and adults and argue the need for different prescriptive targets for adults and children. We will also present research describing electroacoustic and signal processing issues that have motivated us to make modifications to the input/output structure of the DSL target functions. These © 2005 SAGE Publications. All rights reserved. Not for commercial use or unauthorized distribution. at UNIV OF WESTERN ONTARIO on February 5, 2007 http://tia.sagepub.com Downloaded from modifications will be described, and several case studies will illustrate the magnitude and type of changes to prescriptive targets in DSL v5.0. Outcomes for Children Studies using the DSL Method with the pediatric population have been done with various aims and purposes. Some studies have sought to determine whether DSL-related outcomes differ from those of alternative fittings Ongoing research will, therefore, likely always strive to determine the best methods for prescribing the signal processing characteristics of hearing instruments to optimize children's hearing Outcomes for Adults During the late 1990s, the DSL[i/o] prescription began to be used with the adult population, in addition to pediatric applications. This was likely because of a number of factors. First, the DSL v4.1 Method, along with the VIOLA procedure Second, the DSL Method became implemented in various manufacturers' hearing instrument programming or test software, or both, and was thus accessible for clinical use. Some clinics began to apply it with the adult population, often with the view of using the target as a benchmark that indicated the point at which full audibility of speech cues was likely (e.g., Published results of using DSL[i/o] with adults have been somewhat mixed. Some studies show positive and acceptable results Clinical trials that have compared DSL[i/o] with alternative fitting procedures generally have shown that adults prefer less gain than prescribed by DSL, either from a lower-gain prescription such as CAMFIT Between 1999 and 2000, we entered into a clinical collaboration with two audiology clinics that routinely used the DSL Method v4.1 with their adult populations. The audiologists at these clinics were interested in beginning a routine hearing instrument outcome test battery, in part because of the recent attention outcome measurement has received in the literature (e.g., Verification and Validation For new hearing instrument users, outcome measures were targeted for completion approximately 4 weeks after the initial hearing instrument fitting, assuming no physical fit or feedback problems. If significant problems with the fitting occurred or if other changes to the fitting were indicated, the outcome measurement was suspended until a satisfactory fitting was obtained. in 8 or after provision of replacement hearing instruments in 20 (average trial period, 53 days; range, 0 to 164 days). The frequency response and maximum output were verified by using either 2-cc-coupler simulations of ear canal levels or by direct measurement of the real-ear aided response (REAR) with the Audioscan RM500. If the hearing instrument fitting was judged to be electroacoustically adequate and the client reported that the fitting was physically comfortable, the audiologist administered a test battery of outcome measures, as follows: Results of Verification Measures Hearing instrument fittings were verified using an Audioscan RM500 probe microphone system. Responses for both speech-level measures and maximum output were evaluated to determine (1) the bandwidth over which the DSL target was met for speech-weighted inputs with an overall level of 70 dB (quantified using either the Swept signal and/or the Dynamic signal), and (2) the difference between the peak of the maximum power output response and the predicted or measured loudness discomfort levels for that ear. At the final fitting, 85% of measured maximum power output responses had peaks that were no higher than +3 dB above the loudness discomfort level at the nearest frequency, indicating that the maximum output of the hearing instruments had been set to target in most of the fittings. The remaining 15% of the fittings exceeded the +3 dB limit, typically only at the peak of the maximum power output response. In all of these cases, the fitting had used predicted loudness discomfort levels. No fittings exceeded measured loudness discomfort levels, which were typically higher than the predicted values. Speech-level measures of frequency response were evaluated to determine the bandwidth over which the DSL targets were met to within ±5 dB. The fit to within the ±5 dB range was visually assessed from printed verification records generated by the Audioscan RM500, and coded to the nearest audiometric frequency. 2 The observed limits of fit to targets are displayed by the puretone average hearing level of the fitted ear in 2 Although a finer scale for indexing fit to targets would have been desirable, it would also have been difficult to achieve consistently given the small size of the verification printouts. of fittings. Excluding ears with severe-to-profound losses (pure-tone average exceeding 71 dB HL) and sloping (exceeding 35 dB between 3000 and 500 Hz) or reverse sloping losses (5 dB or less, same range), 75% of the remaining 36 ears had upper bandwidth limits at or above 3000 Hz. The remaining fittings had been adjusted away from target because of (1) user preference (4 ears), (2) feedback (3 ears), (3) occlusion effect (1 ear), or (4) gain limitations in the fitted devices (5 ears). In these cases, the gain was generally reduced either in restricted frequency regions (for occlusion or feedback) or across frequencies (for user preference). Reductions were on the order of 10 dB. These findings suggest that many adult users with lesser degrees of hearing loss were fitted with hearing instruments that closely approximated the DSL target. More severe and more sloping losses were more difficult to fit, however, and other users were underfitted according to user preference. Furthermore, fittings above the DSL target were not observed, and informal visual inspection of the verification data suggested that many fittings within the ±5 dB fitting range were in fact slightly under target in most cases. These findings may suggest that the clinicians in this study used the DSL v4.1 target as a maximum fitting level rather than fitting both above and below the target levels. Aided Loudness Measures Results Aided loudness ratings for normally hearing listeners and the hearing instrument users in this study are shown in Regardless of circuit type, 88% of the listeners rated the 70 dB SPL speech signal in the comfortable range (i.e., somewhere between comfortable but slightly soft, and comfortable but slightly loud). Of the remaining listeners, one rated it as "soft" and four as "loud but OK." The "loud but OK" rating for 70 dB SPL speech was also observed in five of the normally hearing listeners, so it is difficult to determine whether such a rating should be deemed acceptable or unacceptable on an individual basis. Overall, most of the hearing instrument users rated the 70 dB SPL speech within the comfortable range. Speech Recognition Measures Results The results obtained from the speech-in-noise test of hearing impaired listeners are shown in The raw scores for each listener were used to determine the 50%-correct point, in dB signal-tonoise-ratio (SNR), for each listener. This point is considered the speech reception threshold for sentences Self-Reported Benefit and Performance Results The listening or communication situations nominated by the patients in this study are shown in Summary and Implications The results of this study indicate that the DSL[i/o] prescription and associated clinical procedures provided good functional hearing outcomes for this group of patients. However, some patients did require gain reductions to achieve a comfortable and preferred listening level from their hearing instruments. Most fittings were between 5 dB under target and the target itself. No fittings reported an increase in gain over the DSLprescribed level. Most patients in this study had normalized loudness ratings, but a small number had excessive loudness for high-level inputs. Speech recognition scores and subjective ratings of benefit and performance tended to be high and in accordance with expectations for hearing instrument outcome. This is generally in agreement with the literature on the application of the DSL Method with the adult population. The pattern of gain adjustment, however, may indicate that a modification to the DSL Method is required if it is to be applied to adults with acquired hearing loss without routine gain reduction by the clinician. Do Adults and Children Have Different Listening Requirements? The two previous sections describe research that may seem to be in conflict. Studies of children indicate that the amplification levels prescribed by DSL may be appropriate or acceptable, but studies with adults indicate that the DSL 4.1 levels may exceed required or preferred levels, or both. This raises an obvious question: do children and adults have different listening requirements and preferences? Studies of infant speech discrimination have shown that normally hearing infants aged 7 to 10 months old require greater stimulus levels than are required by adults to discriminate between speech sounds in quiet and in noise Children with hearing impairment have also been studied to determine the role of age-related performance. Children with hearing losses have different speech signal requirements than their agematched peers with normal hearing, even when listening to amplified signals. Studies comparing children with normal and impaired hearing have shown age-related interactions with level, bandwidth, and sensation level in the perception of fricatives or the use of semantic context in recognizing words If we can accept that children require a greater acoustic signal level than is required by adults, we may reasonably ask why this is. Some investigations indicate that children's phonemic development and other aspects of auditory processing continue to mature throughout the early school years. For example, Hnath-Chisholm et al. (1998) tested normally hearing children's perception of phonologic contrasts using a three-interval forced choice procedure. They found a strong age-related effect, with rapid improvement between ages 5 and 7 years, that plateaued by age 12. Comparisons of the Hnath-Chisholm et al. (1998) data with those reported by This theory finds support in developmental studies of speech perception and production. For example, although children perform more poorly in noise, they make effective use of contextual cues to aid their understanding of speech in noisy environments. In a recent study, normally hearing 5 year olds, 9 year olds, and adults were asked to repeat the last word of a sentence in a background of noise that was adjusted to allow for similar performance across the age groups In summary, not only do children with hearing loss seem to require higher signal levels relative to adults, these trends seem to relate well to the general literature on speech and language development. However, most of the literature has been obtained using tests of speech recognition. It is less clear how these findings relate to the listening preferences of children vs adults. This may be important: if the listening levels required by children are not preferred, or if the preferences of children and adults are similar despite differing performance requirements, the application of the adult-child differences in hearing instrument prescription would be extremely unclear. The following section reports a comparative study of the preferred listening levels of children and two groups of adults (experienced vs inexperienced hearing instrument users). The Laurnagaray and Seewald Study of Adult/Child Preferred Listening Levels This study aimed to determine whether the preferred listening level (PLL) differs between adults and children who use hearing instruments and whether adult PLLs differ between new and experienced adult users. A second purpose was to compare measured PLLs to the DSL v4.1 recommended listening level (RLL). It was hypothesized that PLLs would be greater for children and for adults with hearing instrument experience, and that both of these groups would have PLLs closer to the DSL 4.1 RLL compared with the adult new user group. Participants All participants received a standard audiometric test battery including otoscopy, tympanometry, air and bone conduction audiometry, and case history. In addition, each subject's RECD was measured by using either a foam tip (for custom hearing instrument wearers) or personal earmold (for behind-the-ear wearers [BTE]). Audiometry was conducted by using the same coupling methods to facilitate conversion of HL thresholds into ear canal sound pressure levels. Participants were evaluated during routine clinical appointments in which the preferred listening level was determined as a routine post-fitting outcome measure. Patients were included in the study if they did not exhibit conductive hearing loss, as determined by air versus bone conduction audiometry and tympanometry and if they were able to independently manipulate the volume controls of their hearing instrument(s). The study cohort comprised 72 patients in three groups: (1) 24 children who were full-time hearing instrument users (14 boys, 10 girls; 8 to 18 years old; mean age, 12.5 years), (2) 24 experienced adult hearing instrument users (11 men, 13 women; 30 to 78 years old; mean age, 62 years); (3) 24 new adult hearing instrument users (12 men, 12 women; 35 to 79 years old; mean age, 61 years). The degree of hearing loss was moderate to severe, based on pure-tone averages. The average and range of hearing losses by frequency for each subject group are shown in Prescription and Hearing Instrument Fitting Prescriptive targets were calculated according to the DSL[i/o] algorithm Hearing instruments were adjusted to meet the prescribed frequency response and outputlimiting characteristic as closely as possible, based on 2-cc measures of gain and OSPL-90. The vol- Trends In Amplification Volume 9, Number 4, 2005 ume control setting that most closely matched the targets was noted and provided to each participant and/or caregiver as a recommended use setting. Hearing instruments were adjusted to meet DSL targets for both new and previous hearing instrument users, for both adults and children. To facilitate the evaluation of the preferred listening level (and comparison to previous studies), the 2-cc gain level at 2000 Hz was measured using a Fonix FP-40 and was noted as a marker of the recommended listening level (RLL). New hearing instrument users were given a 15-to 20-day period of hearing instrument use before testing. 170 Preferred Listening Level: Procedure and Results PLLs were measured in the better-hearing ear of each participant, as determined by the four-frequency pure-tone average (i.e., thresholds at 500, 1000, 2000, 4000 Hz). The contralateral ear was plugged by leaving its hearing instrument in place, but turned off. This monaural measurement of the PLL was chosen to facilitate comparison with previously published comparisons with prescriptive targets A loudspeaker was located 1 meter away from the test subject at 0°azimuth. Phonetically balanced sentences (developed by J. M. Tato) were routed from a CD player through the audiometer to the loudspeaker at 60 dB(A). Stimulus levels were confirmed with a soundlevel meter (Lutron SL 40001) before testing, using the "slow" setting. The volume control on the hearing instrument was reduced to minimum and then each subject was asked to adjust the volume control until the talker sounded the best. Once the patient adjusted the volume control to the preferred level, the hearing instrument was removed, taking care not to disturb the volume control position, and the 2-cc coupler gain was measured and noted at 2000 Hz. Each participant completed this trial twice. The average test-retest difference was less than 1 dB for all three groups, and no individual participant varied by more than 6 dB. Experienced adults and children had test-retest differences of 4 dB or less. One-way analysis of variance (ANOVA) indicated no significant group effects for test-retest differences (F (2,69) = 1.065; p = 0.35). ANOVA on the differences between the PLL and RLL for each subject, by subject group, indicates significant differences between the three groups (F (2,69) = 258.2, p = 0.000). Tukey's honestly significant differences test indicated that the three groups differed from one another regarding their agreement between PLL and RLL. Children had PLLs that were closest to the target (or RLL) value with the mean PLL at 2 dB below the DSL v4.1 target. Experienced adults were the next closest, with a mean PLL 9 dB below target. New adult hearing instrument users had the lowest PLLs, which were 11 dB below target on average. In summary, roughly an 8-dB difference in PLL was observed between the adults and children, and PLLs in adults increase by a small but significant amount with hearing instrument use. Individual PLLs are shown against prescribed gain levels in Implications for Prescription This study found significant differences between the new and experienced adult patients' preferences. Some authors have suggested that acclimatization may play a role in determining whether the loudness associated with hearing instrument fitting is acceptable to an adult The results of this study also indicate that recommended volume control settings from the DSL[i/o] Method closely approximate children's preferred listening levels for speech inputs of 60 dB A. This finding replicates the results reported by The contrast between these findings is difficult to interpret for two reasons. First, the studies used different levels of technology, different versions of fitting methods, and different ranges of hearing levels. Second, it is not clear from these findings whether children's preferences are more influenced by prior experience or by a developmental requirement for a higher listening level. The marked difference between the adult and child preferred listening levels in this study may indicate that inherent adult-child differences are an important factor to consider in electroacoustic prescription. These results also indicate that the DSL[i/o] Method likely overestimates preferred listening levels for adult hearing instrument users, with the greatest overestimation observed for inexperienced adults. These findings may be specific to a 60-dB speech level and do not speak to the listening needs of adults with severe-to-profound hearing loss, as they were not tested in this study. Regardless, these results make clear the concept that adults and children with hearing loss have distinctly different preferences for listening level in addition to the different listening level requirements for speech recognition performance described above. This finding is perhaps consistent with the under-target trend for at least some adults in the study reported earlier in this chapter and possibly explains some differences between targets generated by more adult-focused prescriptive formulas (e.g., NAL-NL1, CAMFIT) and more pediatric-focused methods (e.g., DSL). A comprehensive prescriptive approach would need to consider that adults and children not only require, but also prefer different listening levels, perhaps by generating different prescriptions based on client age. Description of the New Algorithm Philosophy and Introduction to Version 5.0 The two principles underlying this revision of the DSL [i/o] algorithm are (1) to implement evidence-based revisions and/or additions to the approach described as the DSL [i/o] v 4.1 algorithm • avoiding loudness discomfort during hearing instrument use, • recommending a hearing instrument frequency response that ensures audibility of important acoustic cues in a conversational speech signal as much as possible, • supporting hearing instrument fitting in early hearing detection and intervention programs, • recommending compression characteristics that are appropriate for the degree and configuration of the hearing loss, the technology to be fitted, and that attempt to make a wide range of speech inputs available to the listener, • accommodating the different hearing needs of listeners with congenital versus acquired hearing loss, • accommodating the different hearing requirements of quiet and noisy listening environments. The motivation for the first principle is likely familiar to most readers, as it is the general approach-pose an algorithm, evaluate it, revise, and begin again-taken by most authors of prescriptive formulas. This cycle of development and revision has, to date, seen both the DSL and NAL families of prescriptive procedures move through approximately four major revisions along with a greater number of minor ones. In this chapter, we will describe a further collection of such evidencebased revisions in the areas of assessment data, transform data, target mapping, target modification, and targets for clinical verification of hearing instruments using either speech or non-speech test signals. This section of the chapter will focus primarily on target generation, summarizing the status of evidence that motivates each change. Where evidence is limited, we will describe the limitations and suggest directions for future evaluative research. Assessment Data Revisions As discussed by New Calibration Standards In DSL 4.1, the audiometric calibration values were compiled from a variety of sources. In DSL v5.0, audiometric calibration data are taken entirely from ANSI (American National Standards Institute, 1996), which is an ISO-harmonized standard. This should facilitate consistent agreement of the internal calculations used by DSL with those used by audiometers, probe microphone measures, and manufacturers' software packages. Two specific decisions were made, given a choice of multiple values within the ANSI at UNIV OF WESTERN ONTARIO on February 5, 2007 http://tia.sagepub.com Downloaded from 1996 S3.6 standard. First, the standard contains more than one reference equivalent threshold sound pressure level (RETSPL) for TDH-series phones to include the 39, 49, and 50 forms of this transducer. The stored data for TDH phones in DSL v5.0 correspond to the TDH-50 values. The remaining TDH calibration options agree closely with these values. Second, the RETSPL for insert phones is available for a variety of couplers: the HA1 coupler, the HA2 coupler, and an occluded ear simulator (OES; ANSI, 1996). Further, the HA2 coupler described for use in audiometric calibration has a shorter tubing length than is used for coupler measurement of BTE hearing instruments because it does not include the earmold simulator (ANSI, 1996). DSL must derive target values for BTE hearing instruments for infants and children, so this coupler definition difference had to be resolved. We have therefore taken the following steps: 3 • the insert phone RETSPL for the occluded ear simulator (OES) defines the normal hearing threshold used in the prescriptive algorithm, • the difference between the OES and the HA1 RETSPLs was calculated and used to define an average adult RECD, • this RECD is used within the HL-to-SPL transform for insert phones, whenever average adult data are used. This solution has several advantages. First, it defines a minimum audible pressure curve that is in agreement with the calibration levels of a standardized audiometric transducer. Second, it allows the RETSPL and the RECD to be defined for the same coupler. Third, it ensures that ear canal threshold values will be in agreement across audiometers, regardless of the coupler type (i.e., HA1, HA2, OES) used during audiometric calibration. Overall, this avoids calculation errors that could arise from poorly considered definitions of couplers and normative RECD values. Clearly, these modifications will cause some change to the DSL targets independently of any revisions made to the prescription algorithm itself. The impact of these modifications was investigated by calculating hearing instrument prescriptions using the DSL 4.1 algorithm, using both the DSL 4.1 and ANSI (1996) calibration values. An audiogram of 0 dB HL was used to ensure that observed changes were due to adjustments in audiometric calibration standards rather than being reduced or altered by gain and/or compression. Across frequencies, the target-to-target differences are no greater than 1.5 dB at a given frequency for both speech-level targets and predicted upper limits of comfort (see Electrophysiologic Estimates of Thresholds If audiometric thresholds are obtained using electrophysiologic rather than behavioral measures, other corrections may be required. DSL v5.0 will support data entry from electrophysiologic measures of thresholds using the frequency-specific auditory brainstem response (ABR). Clinicians may enter data in the normalized HL (nHL) scale or in the estimated HL (eHL) scale (see The end result of assessment data handling is the transformation of clinician-entered thresholds in dB HL (and loudness discomfort levels, if available) to a reference in ear canal SPL. In pediatric audiology, it is common to have partial audiometric data, especially at the beginning of the audiologic confirmation process. Typically, clinical practice guidelines recommend obtaining at least two threshold data points before hearing instrument fitting (The Pediatric Working Group, 1996). This is supported by DSL v5.0 by computing targets based on as few as two thresholds, with interpolation to produce targets at the frequencies in between the assessed thresholds. This provides the clinician with a complete, or nearly complete, set of target criteria for the purpose of selecting initial hearing instruments. It should be noted, however, that interpolated threshold values are not reported by the software so that confusion does not arise about which thresholds have or have not been directly measured. The impact of this strategy is that partial audiometric data can be used to generate a complete spectrum of targets across frequencies. Definition of Inputs An important starting point for this revision was the definition of speech inputs for use in defining a functional range of speech in hearing instrument prescription. Several studies have examined changes in the overall level and spectral shape of speech as vocal effort level changes Effects of Vocal Effort As vocal effort level increases from "casual" to "shouted," the overall level of speech increases from about 56 dB to 84 dB SPL re: free field, on average across men, women and children In the current era of hearing instrument prescription, however, we may wish to acknowledge a few major changes. First, hearing instruments now offer some degree of level-dependent processing for most degrees of hearing loss, allowing the hearing instrument to adjust itself for a range of speech input levels. Second, hearing instruments are now smaller and less visible than in previous years, even in the BTE category, for most levels of hearing loss. Together, these two developments may adjust our priority away from the traditional 70 dB SPL estimate of conversational level speech and toward attempting to provide support for communication with a conversational partner who is using a normal vocal effort. This change would be in better agreement with the test levels used by the Speech Intelligibility Index (ANSI, 1997). These considerations motivate several changes in the definition of "speech" in the DSL Method. 5 Because this was not intended to determine the average decibel level of the two spectra combined, power addition was specifically not done in this calculation. Several of these decisions have been revised in developing DSL v5.0 The change to the loud spectrum was made for several reasons. First, the descriptor loud is more accurate if the corresponding vocal effort level is used. Second, we felt that loud speech is more frequently encountered in real communication situations than is shouted speech. Finally, we wanted to provide a set of targets that could be used within probe-microphone verification and/or loudness rating protocols that would allow clinicians to detect and troubleshoot loudness problems. Shouted speech does not allow this, because even normally hearing listeners are apt to rate it as uncomfortably loud (see also Speech of Other Talkers versus Own Speech All of the spectra discussed above pertain to receiving speech from other talkers. For children, monitoring of their own speech is an important component of learning to produce spoken language. Acoustically, one's own speech reaches one's own ear, allowing a child to hear his or her own vocal productions, at least for children with normal hearing. For a child with hearing loss, the self-speech spectrum may not be high enough in level to allow self-monitoring, particularly at the high frequencies where speech levels are typically low and hearing thresholds are typically elevated. Specific spectra for self-speech were originally described by Removing the self-speech spectrum from the frequency-gain calculations does not diminish the importance of optimizing auditory self-monitoring in pediatric hearing instrument fitting. Recent research indicates that high frequency audibility is an important factor in the speech and language development of children with hearing loss Summary and Implications The two sections above have explained the detailed rationales for selecting certain speech spectra as points of reference within the DSL Method. More specifically, they have discussed several changes in philosophy that have been made from the 4.1 iteration of DSL to the current version 5.0. Two primary changes have been made. First, the range of input levels that receive primary prescriptive consideration now extends from 52 to 74 dB SPL rather than extending to shouted levels of speech at 82 dB SPL. Second, the conversational speech spectrum is now referenced to 60 dB SPL and is no longer shaped to reflect the spectrum of one's own speech. These reference speech targets will be applied in the next section in a revised nonlinear prescriptive algorithm.