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Frequency Compression through Beltone Sound Shifter Tammara Stender, Au.D. Astrid Haastrup, M.A. Hearing loss that slopes to greater degrees in the high frequencies is very common; however, it often presents The success of this approach often depends on the sever- Of the frequency lowering approaches used today by problems with individuals’ ability to discriminate speech. In some cases, today’s hearing instruments may not ity of the hearing loss in the high frequencies; for greater the hearing instrument industry, the most commonly fit- be able to provide adequate audibility for these high frequencies. Frequency lowering strategies have been degrees of high-frequency hearing loss, traditional amplifi- ted strategy is frequency compression. Just as WDRC used in the hearing instrument industry as an alternative for clients who do not benefit from traditional amplifi- cation may have limited success in providing audibility for and other types of amplitude compression “move” low- cation in the high frequencies. However, many of these strategies also introduce significant distortion by means these high frequency sounds. intensity sounds to higher intensity levels for the individual, of this feature. Beltone Sound Shifter is a new frequency lowering approach that improves high frequency audibility, with minimal degradation of sound quality. INTRODUCTION frequency compression “moves” high-frequency sounds There are a number of reasons why a traditionally-fitted to a lower, more aidable region. The ultimate goal of both hearing instrument may not be able to effectively amplify amplitude and frequency compression is the same: audi- more severe degrees of high frequency hearing loss. The bility. To this end, Beltone introduces its new frequency most common culprits include feedback from high gain compression feature: Sound Shifter. Hearing loss that slopes to greater degrees of severity for The loss of certain phonemes produced at typical speech requirements in this frequency region, and limitations of the high frequencies is commonly encountered in hearing levels may present significant problems for the client. For the receiver. The physical properties of larger receivers, clinics. This common hearing loss configuration, however, example, the /s/ phoneme is necessary in the English lan- which are used in high power hearing instruments, result in Beltone Sound Shifter, Revealed presents its own challenges, both for the client and for the guage to distinguish plurality (e.g., “bag” versus “bags”). a lower-frequency roll-off than what is observed for lower- In addition to the shared goal of audibility, Beltone Sound hearing care professional. For the client, many consonant The sentence “Please bring the bags” will have a different power hearing instruments. Shifter employs much of the same terminology as ampli- sounds important for accurate speech discrimination lie in meaning to the client if she only hears “Please bring the the high frequency region. For example, the /s/, /th/ and bag_”. /f/ phonemes may be rendered inaudible with only a mild tude compression. Frequency compression changes the Aside from the technical issues in providing this large input to a more amenable output above a cut-off thresh- amount of high-frequency amplification, there may be a old, or knee point. The degree of frequency compression degree of high frequency hearing loss. Figure 1 shows A hearing loss such as the one shown in Figure 1 affects physiological issue to consider as well. In some cases, applied can be expressed as the frequency compression a bilateral steeply sloping high frequency hearing loss. A the audibility for environmental sounds as well as speech. providing high frequency amplification for these severe ratio. Sound Shifter compresses the frequencies above the speech banana is superimposed on the audiogram, which For this client, environmental sounds such as birdsong will hearing losses may not be beneficial. It has been sug- knee point, effectively moving them closer together. The indicates the common distribution of speech sounds in the be inaudible without some form of high frequency amplifi- gested that the corresponding high frequency areas on input frequencies are amplified in a narrower frequency frequency-intensity realm over time (aka, the long term av- cation (Figure 2). the cochlear basilar membrane may represent a “dead region that is more usable to the hearing instrument user. erage speech spectrum, or LTASS). Individual phonemes region,” characterized by non-functioning inner hair cells. indicate on the audiogram their specific frequency and Applying amplification to these “dead regions” may result Like amplitude compression, an input/output function can intensity ranges in everyday speech. Sounds below the in poorer speech understanding. be used to describe how Sound Shifter works (Figure 3). 1 hearing thresholds on the figure are audible to the client, However, instead of plotting input and output levels, inIf adequate gain cannot be provided by the compressor, amplification, the /s/, /th/, /f/ and /k/ phonemes are inau- or if the client does not receive benefit in speech under- dible for this individual client. standing from this degree of high-frequency amplification, an effective alternative to provide audibility may be a freFigure 2. Steeply sloping high frequency hearing loss, with typical environmental sounds superimposed to indicate areas of audibility and inaudibility for the client. quency lowering approach. Frequency lowering strategies work through the displacement of signals from higher frequencies to lower frequencies, where beneficial amplifi- put and output frequencies are used. The blue part of the High Output Frequency and sounds above the thresholds are inaudible. Without No Sound Shifter (“Off”) Compressed region Uncompressed region cation may be achieved. Although all frequency lowering Figure 1. Steeply sloping high frequency hearing loss, with the speech banana (LTASS) and English phonemes superimposed to indicate areas of audibility and inaudibility for the client. Traditional amplification using wide dynamic range com- approaches introduce some degree of distortion due to pression (WDRC) provides greater audibility for low-level the artificial movement of some frequencies to a different sounds in the necessary frequency regions, in an at- region, the benefit in audibility may result in a favorable tempt to raise the level of speech and other environmental compromise for some individuals. sounds to an audible level per the individual’s hearing loss. Frequency Compression Ratio Frequency Compression Knee Point Low Low Input Frequency High Figure 3. Input/output function describing the relationship between input and output frequencies with Beltone Sound Shifter. Frequency compression occurs only above the frequency compression knee point. Beneath the knee point, no frequency compression is performed. curve is the frequency region in which Sound Shifter is not loss may not distinguish a sound quality difference for a and blue indicating the least intense. Both hearing instru- compression knee point as close to 2 kHz as possible, active. The frequency relationships here are not changed range of frequency lowering settings.2 Sound quality for ments were programmed with amplitude compression for with the same 2:1 compression ratio as used by the “mod- by Sound Shifter. The purple part of the curve is where music is generally preferable for normal hearing and hear- a mild-to-moderate hearing loss, with similar frequency erate” setting. Sound Shifter is active, and demonstrates how the input ing impaired listeners without frequency lowering, or with compression knee points and ratios. frequency corresponds to a lower output frequency. The up to moderate frequency lowering settings. Results and input frequency at which frequency compression begins considerations from the literature were instrumental in de- Panel A shows the signal processed through Beltone 60 dB SPL (RMS) were recorded through each hearing is the knee point. The relationship between the input fre- veloping Sound Shifter and its settings, in order to provide WARP 17 amplitude compression alone, and then with instrument for each of these hearing losses. The record- quency and the output frequency in the purple part of the the desired audibility for high frequency sounds with the Sound Shifter activated. In Panel B, the same signal is ings were then analyzed through the Hearing Aid Speech curve is expressed as the frequency compression ratio. least amount of impact on the sound quality. processed first through the competitor’s wide-dynamic Quality Index (HASQI),4 which measures the “coherence” range amplitude compression, and then also through the between the input and output signals of the hearing aid. A Identical 10-second segments of pop music presented at 3 Sound Shifter was Designed with Sound Quality in Mind To provide maximal listening benefits with minimal sig- competitor’s non-proportional frequency compression. higher HASQI score predicts better sound quality, as the nal distortion, Sound Shifter is a proportional frequency The frequency compression knee points are shown as output sound will be more similar to the input sound, with Frequency lowering is a unique hearing instrument fea- compression strategy. This approach was chosen over a the point where the slope of the response changes. (Note less added noise and distortion relative to the original input ture in that there is no general consensus in the literature non-proportional frequency compression technique, cur- there no change in slope for the response when no fre- sound. The HASQI has been found to be a valid measure about its benefits to groups of people. Some individuals rently offered by another hearing aid manufacturer. To illus- quency compression is activated). of subjective sound quality judgments by people with nor- will benefit from it while others will not. In addition, there trate the differences in the processing of a signal, a 90 dB is no clear way to identify prior to the fitting which clients SPL pure tone sweep was processed through Beltone’s The results for Beltone Sound Shifter show a clean re- will derive benefit from the frequency lowering approach. proportional frequency compression and the competitive sponse; minimal differences in energy were present in the Results for all three types of hearing losses showed the Finally, the effect of this type of processing on sound qual- non-proportional frequency compression to create spec- output above the frequency compression knee point as same trends; for simplicity, the average of these results ity is not generalizable. Individuals with milder or normal trograms (Figure 4). Each spectrogram is an input/output compared to the output below the knee point, where fre- for each frequency compression setting are depicted in hearing thresholds may distinguish a difference in sound function, showing the input frequency on the x-axis and quency compression was not active. In contrast, the re- Figure 5. The highest HASQI scores relative to the input quality with frequency lowering as compared to traditional the output frequency on the y-axis. Colors indicate the in- sults for the competitive frequency compression feature signal were obtained when frequency compression was processing, while those with greater degrees of hearing tensity of the signal, with red indicating the most intense were not as clean, as indicated by the jagged response deactivated (the “off” setting). For both frequency com- above the frequency compression knee point. This is due pression techniques, the HASQI score was inversely re- to the non-proportional relationship of the competitor’s fre- lated to the aggressiveness of the algorithm. This indi- quency compression. More distortion products are gener- cates that stronger frequency compression settings are ated by this approach. These distortion products manifest predictive of poorer sound quality. When proportional and themselves on the spectrogram with a blurred quality of non-proportional frequency compression were compared, the response curve above the frequency compression however, the predicted sound quality of Sound Shifter was knee point. better. Sound Shifter On Output Frequency (Hz) Output Frequency (Hz) PANEL A Sound Shifter Off Input Frequency (Hz) Input Frequency (Hz) mal hearing and with mild to moderate hearing loss. Sound Quality for Music 0,8 Sound quality for music was also evaluated for propor- 0,7 tional and non-proportional frequency compression. In this comparison, each hearing instrument was programmed for 3 hearing losses: mild flat, mild sloping to moderatelysevere, and mild sloping to profound. Gains were set according to the default prescriptive targets in the respective manufacturer’s fitting software. Frequency compression was set to “off,” “moderate” and “strong.” The “moderate” Input Frequency (Hz) Figure 4. Spectrograms for a swept pure tone were measured with conventional amplitude compression (left) and with both amplitude and frequency compression activated (right). Panel A shows the results obtained with Beltone Sound Shifter. Panel B shows the results Input Frequency (Hz) for a competitor’s hearing instrument. These spectrograms show the difference in distortion generated by Beltone’s proportional frequency compression strategy, compared to that generated by the competitor’s non-proportional frequency compression (FC) approach. setting had a frequency compression knee point as close to 3 kHz as possible, and a compression ratio as close to 2:1 as possible. The “strong” setting had a frequency 0,6 HASQI Score Competitor FC On Output Frequency (Hz) Output Frequency (Hz) PANEL B Competitor FC Off 0,5 0,4 0,3 0,2 0,1 0 Off Moderate Strong Frequency Compression Setting Sound Shifter Competitor FC Figure 5. Sound quality for music, as predicted by the HASQI, revealed better sound quality preservation for Sound Shifter than for a non-proportional frequency compression strategy. Development of Sound Shifter Settings Objective measures during the fitting appointment includ- sults of other studies using frequency compression, group How to Fit Sound Shifter ed real ear insertion gain (REIG) measurements for Ling data did not indicate optimal frequency compression set- Sound Shifter settings can be found in the “Features” sec- All frequency lowering techniques introduce a compromise sounds /s/ and /sh/. Forty ears were evaluated for each of tings nor average benefit. Also as expected, certain indi- tion of Solus Pro (Figure 8). As need for frequency com- between audibility and sound quality. Some users will ben- the 8 frequency compression settings plus the “off” condi- viduals did obtain improved outcome measure results and pression is not easily generalizable to particular hearing efit greatly from the increased audibility of high-frequency tion. Results revealed 3 distinct grouping of REIG meas- expressed preference for frequency compression process- losses or client characteristics, the feature is set to a de- sounds, while others may be bothered by the feature’s in- urements among the conditions (Figure 6). ing, although variability was observed. Figure 7 shows the fault “off” setting in each program. herent distortion and perceptual sound quality. As previ- distribution of individual scores with and without Sound Effect of Different Settings on Ling /s/ ously stated, there are no defined guidelines about which lowering. Further, there is no “cookbook approach” to fitting the technology. For instance, two individuals with similar hearing losses may derive the greatest benefit from different frequency compression knee points and ratios, or Insertion Gain (IG) individual clients will be the best candidates for frequency from no frequency lowering at all. For these reasons, the best approach to applying and fitting frequency compres- Shifter for the UWO Plurals Test. Subjects who obtained 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 positive benefit performed better with Sound Shifter on off 5000_1.33 4000_1.33 4000_2 3500_2 3000_2 2500_2 2250_2 2000_2 100 sion is often a conservative one. 1000 Frequency (Hz) 10000 this test than with conventional amplification. Two of the 16 subjects achieved better scores with conventional amplification than frequency compression, which is indicated by the negative benefit. Test subjects who preferred Sound Shifter processing over conventional amplification indicated that the sound quality was crisper and clearer. Benefit Obtained with Sound Shifter 5 Effect of Different Settings on Ling /sh/ found through internal testing to provide the greatest audibility benefit while also minimizing deleterious effects on the sound quality. Results of initial testing indicated that the compression frequency knee point had a greater effect on the feature than the compression ratio. Insertion Gain (IG) designed with a limited number of settings, which were Figure 8. Sound Shifter can be activated and set in the “Features” section. 4 off 5000_1.33 4000_1.33 4000_2 3500_2 3000_2 2500_2 Number of Subjects As such, Sound Shifter in Solus Pro fitting software was 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 3 To activate Sound Shifter, choose a setting in the dropdown menu. The recommended setting based on the 2 client’s audiogram will be indicated in “bold” font style. 1 When frequency compression is not expected to provide 2250_2 2000_2 100 1000 Frequency (Hz) 10000 0 -2 -1 0 1 2 3 4 5 6 7 UWO Plurals Test Score With Sound Shifter Minus Score Without Sound Shifter To determine the most effective and differentiated com- optimal results for the client (e.g., flat hearing losses, noise notches, reverse slope hearing losses), the recommended setting will be “Off.” The criteria for recommending “Mild”, Figure 6. REIG measurements among frequency compression settings for the /s/ and /sh/ phonemes. Figure 7. Most individuals obtained improved scores on the UWO Plurals Test when Sound Shifter was activated. “Moderate” and “Strong” settings are as follows: quency compression settings in addition to the “Off” set- For the field trial component of this testing, frequency The overall conclusion from these tests was that the fitting • A “Mild” setting is recommended if the audiogram has ting (Table 1). Seventeen test subjects with steeply slop- compression settings were determined for 20 of the 37 process could be simplified to a smaller number of set- a slope of 10 dB or greater per octave frequency, and ing hearing loss and 20 with severe-to-profound hearing participants based on a real ear measurement protocol to tings. Since REIG measurements revealed three grouped the slope begins at 4000 Hz or higher (Figure 9). loss participated in the trial. optimize audible bandwidth. The remaining 17 participants responses across the eight settings evaluated, and more • A “Moderate” setting is recommended if the audiogram were fitted according to which frequency compression than one setting was found to provide comparable benefit has a slope of 10 dB or greater per octave frequency, setting enabled them to perform best on the UWO Plu- for individuals, three Sound Shifter settings were derived: and the slope begins at 2000 Hz (Figure 10). rals Test. If equivalent scores were obtained for more than “Mild,” “Moderate” and “Strong.” Table 2 shows the cor- • A “Strong” setting is recommended if the audiogram one setting, the most conservative setting (i.e., the highest responding frequency compression knee points and ratios has a slope of 10 dB or greater per octave frequency, compression knee point and lowest ratio) was chosen. for each of these settings. and the slope ends at 2000 Hz (Figure 11). pression knee points and ratios, a second internal test was conducted. The goal of this trial was to evaluate 8 fre- Test Setting Frequency Compression Knee Point Frequency Compression Ratio ”Off” 1 (weakest) 2 3 4 5 6 7 8 (strongest) n/a 5000 4000 4000 3500 3000 2500 2250 2000 n/a 1.33 1.33 2 2 2 2 2 2 Table 1. Eight frequency compression settings evaluated during the internal trial. 5 Post-field trial measures included clarity ratings, sound quality ratings for male and female speech and music, speech testing, and the Speech, Spatial and Qualities of Hearing Scale (SSQ)6. As was expected, based on the re- Sound Shifter Frequency Compression Setting Knee Point Mild Moderate Strong 4000 Hz 3500 Hz 2500 Hz Frequency Compression Ratio 1.33 2 2 Table 2. Frequency compression knee points and ratios for each setting of Sound Shifter. REFERENCES 1. Moore B. Dead Regions in the Cochlea: Conceptual Foundations, Diagnosis, and Clinical Applications. Ear and Hearing, 2004; 25 (2), 98-116. 2. Parsa V, Scollie S, Glista D, Seelisch A. Nonlinear frequency compression: effects on sound quality ratings of speech and music. Trends in Amplification. 2013; 17(1): 54-68. Figure 9. Example of audiometric thresholds when a “Mild” Sound Shifter setting is recommended. 3. Bentler R. Hearing aids: Clarity or controversy? Audiology Online Hearing Aid Conference, 2012. Course Figure 12. When Sound Shifter is turned on, a solid vertical line and a shaded region indicate the range of frequencies that are compressed. 21332. Upon choosing a Sound Shifter setting, a solid dark verti- 4. Kates JM, Arehart KH. The Hearing-Aid Speech Quality cal line and a gray area will appear in the graph. The verti- Index (HASQI). Journal of the Audio Engineering Soci- cal line indicates the frequency compression knee point, ety, 2010; 58(5), 363-381. and the gray area denotes the frequency range that is compressed (Figure 12). 5. Glista D, Scollie S. Development and evaluation of an English language measure of detection of word-final Recommendations for Sound Shifter are specific to the plurality markers: The University of Western Ontario ear’s audiometric thresholds. It is possible that different Plurals Test. American Journal of Audiology, 2012. 21: Sound Shifter settings for each ear may be recommended 76-81. in the case of asymmetric hearing losses. Further, Sound Figure 10. Example of audiometric thresholds when a “Moderate” Sound Shifter setting is recommended. Shifter may be activated in one or more programs, binaurally or monaurally, per the individual’s needs. Summary Sound Shifter provides yet another tool for hearing care professionals to improve high frequency audibility, especially for individuals who do not receive optimal audibility through traditional amplification. For certain individuals, Sound Shifter can improve audibility for high frequency speech sounds, such as /s/ and /th/, while maintaining sound quality to a greater degree than can be offered by a non-proportional frequency compression strategy. Research revealed that a limited number of settings were appropriate to fit Sound Shifter for a wide range of individuals who may benefit from this feature. Thus, Beltone Sound Figure 11. Example of audiometric thresholds when a “Strong” Sound Shifter setting is recommended. Shifter is an option that enables the hearing care professional to better accommodate the needs of individual clients. 6. Gatehouse S, Noble W. The speech, spatial and qualities of hearing scale. Int J of Aud, 2004; 43(2):85-99. M200993-GB-13.09-Rev.A Worldwide Headquarters Beltone A/S Lautrupbjerg 7 DK-2750 Ballerup Denmark Tel.: + 45 45 75 11 11 Fax: + 45 45 75 11 19 www.beltone-hearing.com