Hearing aid fitting and verification

The DSL recommended protocol for hearing instrument fitting to infants.

The DSL recommended protocol for hearing instrument fitting to children.

The DSL recommended protocol for hearing instrument fitting to adults.
 

Should the DSL algorithm generate different prescriptive targets for children and adults?

Published results of using DSL[i/o] with adults have been somewhat mixed, with some studies showing positive and acceptable results (Humes, 1999; Hornsby and Ricketts, 2003; Scollie et al., 2005), and others showing good speech recognition but higher ratings of loudness with higher level inputs and/or frequencies than those considered ideal (Lindley and Palmer, 1997; Alcántara, Moore and Marriage, 2004; Smeds, 2004).

Clinical trials that have compared DSL[i/o] with alternative fitting procedures have generally shown that less gain than prescribed by DSL is preferred by adults, either from a lower-gain prescription such as CAMFIT (Moore, Alcántara and Marriage, 2001) or from a patient-driven procedure that customizes gains to preference (Lindley and Palmer, 1997).

Currently there are differing opinions regarding the electroacoustic requirements for hearing aid performance for adults versus children. Some researchers believe that prescriptive procedures developed for adults can be used with young children (Ching, Dillon and Byrne, 2001). Others believe that infants and young children require different prescriptive procedures (e.g., Stelmachowicz, 1991; 2000; Seewald, 1995). Snik and Hombergen (1993) measured the preferred insertion gain for 40 adults and 60 children. Figure 8 displays the preferred insertion gain as a function of the pure-tone average for the adults (8A) and children (8B) in this study. The results showed that overall the mean use insertion gain was 7 dB less for the adults relative to that used by the children.

A recent study by Laurnagaray and Seewald (see Scollie et al., 2005) included 24 children who were full-time hearing aid wearers, 24 adults who were experienced hearing aid users, and 24 adults who were new hearing aid users. The hearing aids were fit to the DSL v4.1 prescription and new users were provided with a 15 to 20 day trial period. The objective of the study was to determine whether the preferred listening level (PLL) differed 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). As illustrated in Figure 9, analysis results indicated that all three of the groups differed from one another regarding their agreement between PLL and RLL. Children had a mean PLL that was approximately 2dB below the DSL target (RLL). Experienced adults had a mean PLL 9 dB below the DSL target. New adult hearing instrument users had the lowest PLLs, which were 11 dB below target on average. In summary, there was an approximate difference of 8 dB in PLL between the adults and children in this study, and adults who were new hearing aid users preferred a slightly lower listening level than adult experienced hearing aid users. This finding is similar to the 7 dB adult/child difference measured by Sink and Hombergen.

These study results indicate that the DSL[i/o] prescriptive algorithm likely overestimates preferred listening levels for adult hearing instrument users, with the greatest overestimation observed for inexperienced adults. These findings may not generalize to adults with severe-to-profound hearing loss as they have not been included in these studies. Nonetheless, the results make clear the concept that adults and children with hearing loss have distinctly different preferences for listening level. The results also agree with earlier studies of adult/child differences in listening level requirements for speech recognition performance (see above). In considering modifications to the DSL[i/o] algorithm it was decided that 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.

A study was undertaken in an effort to better understand the acceptable range for amplified conversational speech for adults (Jenstad et al, under review). The purpose of the study was two-fold; first, to define the range of optimal hearing aid settings in both high and low frequencies using subjective ratings of loudness and quality and objective measures of speech intelligibility, and secondly, to determine if the DSL[i/o] 4.1 gain-by-frequency response falls within the optimal range for adult listeners. Measures of loudness, quality and speech intelligibility were obtained for 23 adult listeners with mild to moderately-severe sensorieneural hearing loss, across a range of high and low-frequency responses. Consistent with the findings of other researchers (e.g., Dirks, Ahlstrom and Noffsinger, 1993), this study found that there was an approximately 10 dB range for these adult listeners that could be considered optimal hearing aid settings when both speech intelligibility and loudness criteria were considered together. Relative to the DSL[i/o] v4.1 prescription generated for each adult, results indicated that in the low frequencies the optimal range for hearing aid settings spanned from 2 dB above the DSL[i/o] target to 7 dB below the DSL target. In the high frequencies the optimal range of settings spanned from 3.2 dB below the DSL[i/o] target to 13.2 dB below target.

The DSL[i/o] algorithm described by Cornelisse et al., 1995, and used in the DSL Method: v4.1 attempted to define the ideal amplified output for a range of input levels. The DSL[i/o] algorithm used nonlinear scaling so that input levels corresponding to the acoustic dynamic range of the normal loudness function were mapped onto the auditory area of the loudness function associated with hearing impairment, while maintaining the normal loudness relationship per frequency (Cornelisse et al., 1995). The DSL[i/o] algorithm comprised a very broad compression phase beginning at 0 dB HL. We hypothesize that the resultant gain for low- to moderate speech input levels using this approach may contribute to higher loudness levels than preferred or necessary for adult hearing aid wearers.

The DSL multistage input/output algorithm (DSLm[i/o]) used in DSL v5, does not use a loudness normalization approach for several reasons. First, current loudness models do not account for the adult-child and developmental differences required for listening reported earlier in this chapter. Second, loudness normalization attempts to make all sounds audible and normally loud. It is not likely that this is an appropriate goal for low-level background noise, nor is it an attainable goal given the noise floor of most hearing instruments. In developing the DSL m[i/o] algorithm we use compression processing to meet the goals of providing audibility and comfortable loudness of important speech cues, considering the general limits of hearing instruments and the limited dynamic range of the individual hearing instrument user. As discussed above, the compression stage spans as much of the range of conversational speech across vocal effort levels as possible. As a starting place, the DSL m[i/o] input range was limited to no lower than 20 dB HL for adult listeners with acquired hearing impairment. Compared to the 0 dB HL loudness normalization strategy in DSL [i/o] this provides adults with a lower level of prescribed gain and compression ratio for the entire input-output function. As shown in Figure 10, the differences in prescriptive targets are largest for mild-to-moderate losses. A smaller correction is applied as hearing loss increases which is a desired effect because it maintains audibility of speech for more severe-to-profound hearing losses for adults and children. Further experimental evaluation of this age-related correction is required, however, it appears to be in good agreement with the adult-child differences in preferred gain reported earlier in this chapter.

In preparation for a larger study which examines the preferred listening levels of adult hearing aid wearers compared to the DSL v5 targets , a pilot study was conducted at the Andy Leeper Speech and Hearing Clinic at the University of Western Ontario. Nine adults with sensorieneural hearing loss, monaurally fitted, were included in the study. Individual measurements of real-ear-to-coupler difference (RECD) were obtained to ensure accurate assessment and hearing aid fitting. The wearer's WDRC hearing aids were set to the frequency response targets prescribed by the DSL v5.0 algorithm. Any subjective feedback from the wearer in terms of desired changes to low- or high-frequency amplification were made as requested during the fitting process. When the wearer was satisfied with the fitting, the volume control wheel of the hearing aid was set to minimum. A speech stimulus of 65 dB SPL was presented in the sound field and the individual was asked to set the volume control wheel at his or her preferred level. The real-ear aided response at this volume control wheel position was obtained using the Verifit VF-1 real-ear system in a simulated real-ear measurement mode (S-REM). The real-ear aided response (in dB SPL ear canal level) for a 70 dB SPL speech input was obtained. This was compared with the DSL v5 target for a 70 dB SPL input. It was also compared to the targets generated with the DSL v4.1 target for a 70 dB SPL input. A 70 dB SPL input was chosen because the Verifit VF-1 generated these targets for both DSL 5 and DSL 4.1 versions of the DSL Method. The average deviation of the target from the individual's PLL was calculated as a function of frequency. These are shown in Figure 11 for frequencies 250 to 4000 Hz. The DSL v4.1 targets deviated, on average, from the PLLs of these adult hearing aid wearers by 11 dB. The DSL v5.0 targets deviated, on average, from the PLLs of the same adult hearing aid wearers by 0.5 dB. We believe that the results of this pilot study indicate that the DSL v5 algorithm provides targets that more closely approximate adult hearing aid wearers preferred listening levels for conversation-level speech relative to those prescribed by the DSL v4 algorithm. Collection of a large data set with adults is currently underway.

Targets from the DSLm[i/o] algorithm have best clinical utility when displayed on an SPLogram and compared with real-ear aided response (REAR) and output limiting targets for narrowband inputs and/or the upper limits of comfort, employing probe-microphone measures of real-ear performance. DSLm[i/o] targets can also be calculated for real-ear aided gain (REAG) and real-ear insertion gain (REIG) reference. If REIG targets are calculated using the DSLm[i/o] algorithm age-appropriate or measured real-ear unaided gain (REUG) values will be used (Bagatto et al., 2005). Targets for 2cc coupler gain can be calculated automatically using the DSLm[i/o] REAR and RESR values using the following general equation:

Target coupler gain values are advantageous when probe-microphone measurements may not be possible, such as when fitting hearing instruments for infants or young children. Coupler-assisted verification procedures can be clinically useful in predicting real-ear performance using the reverse of the equation above (Moodie et al., 1994). That is, 2cc target gain + input speech + microphone location effect (MLE) + RECD = predicted real-ear aided response (in dB SPL re: ear canal).

Targets from the DSLm[i/o] algorithm are appropriate for comparison with the aided long-term average speech spectrum, measured in one-third octave bands. This type of measurement can be made for soft (50 to 55 dB SPL), conversational (60 to 70 dB SPL), or loud (75-85 dB SPL) speech signals. Speech-based verification signals are strongly recommended for use with targets derived using DSL v5. Targets can be converted for use with speech-weighted noise and pure tone verification signals. The disadvantage of the corrections used in DSL v5 for signals other than speech is that it is less accurate and only useful for input levels between 50 and 70 dB SPL (Bagatto et al., 2005; Scollie and Seewald, 2002).