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Greater Fitting Flexibility with Three Input Levels in Aventa Jennifer Groth Abstract ReSound was the first manufacturer to offer Wide Dynamic Range Compression (WDRC) to account for loudness recruitment. The proprietary, threshold-based Audiogram+ provides a reliable starting point for fitting this WDRC system by prescribing gains for soft and loud input levels. Beginning with the ReSound Verso product family in Aventa 3.4, an extra compression kneepoint and control for moderate level inputs is provided. This gives fitters remarkable flexibility for addressing fine-tuning issues, and fitting varying degrees and types of hearing losses, as well as unprecedented precision in matching the targets of generic prescriptive formulas. Since the first ReSound hearing instruments were introduced to the market in the early 1990s, the fitting rationale has remained rooted in the same philosophy inspired by the workings of the normal ear. Based on the seminal work of Villchur1, ReSound pioneered Wide Dynamic Range Compression (WDRC), offering the first system to account for loudness recruitment by applying progressively less gain with increasing input levels. To provide a reliable starting point for use of this WDRC system, a fitting algorithm based on individual psychoacoustic measures of loudness growth was initially used. Years of clinical experience encompassing thousands of fittings with this procedure provided the foundation for development and refinement of the threshold-based Audiogram+ fitting algorithm2. Audiogram+ calculates insertion gain targets for narrowband inputs of 50 and 80 dB SPL at 11 audiometric octave and interoctave frequencies from 125 Hz through 8 kHz. The underlying compression system has always used linear compression. This means that the slope of the input-output function between the lower and upper compression kneepoints was described by a straight line. In other words, only one compression ratio has been possible between the lower and upper kneepoints. This is illustrated in Figure 1. For this system, the 50 and 80 dB input level controls are always between the compression kneepoints. Changing the gains for both of these input levels simultaneously would shift the entire input-output curve up or down, thus preserving the compression ratio. Adjusting gain for one or the other of the two levels would change the compression ratio. Figure 1. The ReSound compression system has been based on two compression kneepoints, allowing only one compression ratio in the WDRC range. For most fittings, the degree of control over gain for different sound levels provided by the ReSound system has proven adequate. However, there are cases when additional control is desirable. Beginning with the ReSound Verso in Aventa 3.4, an extra compression kneepoint at 65 dB SPL is available for fitting. This allows independent adjustment of gain for moderate level inputs, as well as two compression ratios within the range of compression, as shown in Figure 2. No changes to the Audiogram+ prescription have been made. Thus, the default gain setting for G65 when a fitting based on Audiogram+ is prescribed will be halfway between G50 and G80. By providing this extra control, numerous fitting issues can be solved. These include a better ability to address user complaints when fine-tuning, better ability to accommodate different hearing loss configurations, and better ability to match targets to generic prescriptions. Figure 2. The updated ReSound compression system includes a gain control for moderate level sounds. This allows for two compression ratios within the WDRC range. Using G65 for fine-tuning One way in which the G65 control enhances fitting flexibility is that it gives more options for addressing user complaints regarding loudness, feedback and speech audibility. By having control over soft, moderate and loud level sounds independently, it is easier to find a balance between these often opposing complaints. For example, Mr. V is fit with a Verso 62 RIE open model. At the initial fitting, the G50 curve is at the maximum stable gain limit, and there is a slight tendency for the instruments to feed back. Therefore, the fitter reduced G50 and G65 in the high frequencies in order to reduce this tendency. When Mr. V returned for a follow up, he reported that feedback was not an issue, and that the sound levels were comfortable in all kinds of listening environments. However, he often felt the need to turn up the volume somewhat when conversing with individuals and at meetings. Based on Mr. V’s experience, the fitter increased the G65 by several steps from 1kHz and above to improve the volume and clarity for moderate level speech. Technically, this reduced the compression below 65 dB, and increased it slightly above. For Mr. V, this meant that the comfort for loud sounds was not affected, feedback tendency was not reintroduced, and moderate level speech was made more audible. Figure 3. Separate adjustments of gain for soft and moderate input levels helped to eliminate feedback, maintain comfort for loud sounds, and increase audibility for moderate level speech. Fitters who are accustomed to working with three input level controls will be able to apply their experience when working with the updated ReSound system. For those who are less familiar with this degree of control, the Aventa Guide provides suggestions for addressing common fitting complaints that incorporate the new 65 dB input level control. hearing losses. Although many fitting prescriptions, including Audiogram+, prescribe additional gain to overcome some of the attenuation resulting from the conductive impairment, there is no consensus on how that should be done. In such cases, the added flexibility of the moderate level control may be invaluable. For example, a user with a conductive loss might report that loudness and clarity are generally fine, but that loud sounds are uncomfortable or tend to be distorted. In this case it might be helpful to decrease G80. This should help with comfort without degrading what the user likes about the fitting. Figure 4. Aventa Guide incorporates the G65 control in providing suggestions to solve common fitting complaints. Using G65 for fitting different hearing loss configurations The ReSound WDRC fitting system was originally designed to compensate for mild-to-moderate sensorineural hearing losses. For more severe losses, this type of compression scheme presents a greater challenge to fit successfully. When fitting WDRC to individuals with severe-to-profound hearing losses, the compression characteristics will result in more gain for soft sounds and less gain for loud sounds relative to a linear scheme with the same gain for moderate level sounds. This can create problems with feedback as well as inadequate loudness. Nevertheless, those with more severe losses can potentially benefit greatly from compression if an appropriate balance in comfort and audibility can be achieved. Barker and colleagues3 found that fast-acting WDRC could be fit successfully to individuals with severe-to-profound hearing losses. However, many of the participants in their trial preferred higher compression kneepoints than typically characterize WDRC schemes. With the addition of the 65 dB input level control in Aventa, the fitter can effectively decrease compression below the moderate input level if preferred by individuals with more severe losses. This is achieved by decreasing the G50. If the G50 is decreased to the level of G65, the compression kneepoint is in fact raised to this level. In doing this, the fitter can preserve audibility for moderate level speech, maintain comfort for loud sounds, and decrease feedback risk for those with severe-to-profound losses. Fitters may also find the 65 dB input level control useful when fitting individuals with conductive or mixed Using G65 for more accurate fitting to generic prescriptions Many professionals follow fitting protocols that require fitting and verification according to scientifically validated, peer-reviewed prescriptions. Aventa facilitates this process by offering automatic adjustment to a number of such prescriptions, including NAL-NL1, NAL-NL2, DSL i/o and DSL 5. While Audiogram+ prescribes linear compression between the lower and upper compression kneepoints, generic prescriptions may differ. In the example in Figure 5, the same mild-tomoderate sloping audiogram was used to prescribe fittings based on NAL-NL2, DSL i/o and Audiogram+. The same signal type is assumed in each case. As can be seen, these three formulas each prescribe the compression in a different way. While NAL-NL2 prescribes a more linear response as input level increases, DSL i/o does the opposite. Without the separate control for G65, the hearing instrument would not be able to meet each of these prescriptive targets. References 1. Villchur E. Signal processing to improve speech intelligibility in perceptive deafness. Journal of the Acoustical Society of America. 1973; 53: 1646-1657. 2. Audiogram+: The Resound Proprietary Fitting Algorithm. ReSound white paper. 2009. 3. Barker C, Dillon H, Newall P. Fitting low ratio compression to people with severe and profound hearing losses. Ear and Hearing. 2001; 22(2): 130-141. Figure 5. Fitting rules often differ in how compression is prescribed for a particular audiogram. With the added flexibility of a separate control for moderate inputs, targets can be more accurately met for a variety of different formulas. Worldwide headquarters ReSound A/S Lautrupbjerg 7 DK-2750 Ballerup Denmark Tel.: +45 45 75 11 11 Fax: +45 45 75 11 19 www.resound.com M200770-GB-12.05-Rev.A Summary Fine-tuning to patient preferences, accounting for different types of hearing losses, and accurate fitting to generic fitting formulas are benefits of the flexibility afforded by control over gain for soft, moderate and loud sound levels. Beginning with Resound Verso, fitting professionals have access to this important fitting feature.