Frequency Compression through Beltone Sound Shifter. Tammara Stender, Au.D. Astrid Haastrup, M.A.

Transcription

1 through Beltone Sound Shifter Tammara Stender, Au.D. Astrid Haastrup, M.A.

2 Hearing loss that slopes to greater degrees in the high frequencies is very common; however, it often presents problems with individuals ability to discriminate speech. In some cases, today s hearing instruments may not be able to provide adequate audibility for these high frequencies. Frequency lowering strategies have been used in the hearing instrument industry as an alternative for clients who do not benefit from traditional amplification in the high frequencies. However, many of these strategies also introduce significant distortion by means of this feature. Beltone Sound Shifter is a new frequency lowering approach that improves high frequency audibility, with minimal degradation of sound quality. INTRODUCTION Hearing loss that slopes to greater degrees of severity for the high frequencies is commonly encountered in hearing clinics. This common hearing loss configuration, however, presents its own challenges, both for the client and for the hearing care professional. For the client, many consonant sounds important for accurate speech discrimination lie in the high frequency region. For example, the /s/, /th/ and /f/ phonemes may be rendered inaudible with only a mild degree of high frequency hearing loss. Figure 1 shows a bilateral steeply sloping high frequency hearing loss. A speech banana is superimposed on the audiogram, which indicates the common distribution of speech sounds in the frequency-intensity realm over time (aka, the long term average speech spectrum, or LTASS). Individual phonemes indicate on the audiogram their specific frequency and intensity ranges in everyday speech. Sounds below the hearing thresholds on the figure are audible to the client, and sounds above the thresholds are inaudible. Without amplification, the /s/, /th/, /f/ and /k/ phonemes are inaudible for this individual client. 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. The loss of certain phonemes produced at typical speech levels may present significant problems for the client. For example, the /s/ phoneme is necessary in the English language to distinguish plurality (e.g., bag versus bags ). The sentence Please bring the bags will have a different meaning to the client if she only hears Please bring the bag_. A hearing loss such as the one shown in Figure 1 affects the audibility for environmental sounds as well as speech. For this client, environmental sounds such as birdsong will be inaudible without some form of high frequency amplification (Figure 2). Figure 2. Steeply sloping high frequency hearing loss, with typical environmental sounds superimposed to indicate areas of audibility and inaudibility for the client. Traditional amplification using wide dynamic range compression (WDRC) provides greater audibility for low-level sounds in the necessary frequency regions, in an attempt to raise the level of speech and other environmental sounds to an audible level per the individual s hearing loss. The success of this approach often depends on the severity of the hearing loss in the high frequencies; for greater degrees of high-frequency hearing loss, traditional amplification may have limited success in providing audibility for these high frequency sounds. There are a number of reasons why a traditionally-fitted hearing instrument may not be able to effectively amplify more severe degrees of high frequency hearing loss. The most common culprits include feedback from high gain requirements in this frequency region, and limitations of the receiver. The physical properties of larger receivers, which are used in high power hearing instruments, result in a lower-frequency roll-off than what is observed for lowerpower hearing instruments. Aside from the technical issues in providing this large amount of high-frequency amplification, there may be a physiological issue to consider as well. In some cases, providing high frequency amplification for these severe hearing losses may not be beneficial. It has been suggested that the corresponding high frequency areas on the cochlear basilar membrane may represent a dead region, characterized by non-functioning inner hair cells. Applying amplification to these dead regions may result in poorer speech understanding. 1 If adequate gain cannot be provided by the compressor, or if the client does not receive benefit in speech understanding from this degree of high-frequency amplification, an effective alternative to provide audibility may be a frequency lowering approach. Frequency lowering strategies work through the displacement of signals from higher frequencies to lower frequencies, where beneficial amplification may be achieved. Although all frequency lowering approaches introduce some degree of distortion due to the artificial movement of some frequencies to a different region, the benefit in audibility may result in a favorable compromise for some individuals. Of the frequency lowering approaches used today by the hearing instrument industry, the most commonly fitted strategy is frequency compression. Just as WDRC and other types of amplitude compression move lowintensity sounds to higher intensity levels for the individual, frequency compression moves high-frequency sounds to a lower, more aidable region. The ultimate goal of both amplitude and frequency compression is the same: audibility. To this end, Beltone introduces its new frequency compression feature: Sound Shifter. Beltone Sound Shifter, Revealed In addition to the shared goal of audibility, Beltone Sound Shifter employs much of the same terminology as amplitude compression. Frequency compression changes the input to a more amenable output above a cut-off threshold, or knee point. The degree of frequency compression applied can be expressed as the frequency compression ratio. Sound Shifter compresses the frequencies above the knee point, effectively moving them closer together. The input frequencies are amplified in a narrower frequency region that is more usable to the hearing instrument user. Like amplitude compression, an input/output function can be used to describe how Sound Shifter works (Figure 3). However, instead of plotting input and output levels, input and output frequencies are used. The blue part of the High Output Frequency Low Compressed region Uncompressed region Low No Sound Shifter ( Off ) Input Frequency Frequency Compression Ratio Frequency Compression Knee Point 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.

3 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. 3 Results and Identical 1-second segments of pop music presented at input frequency at which frequency compression begins considerations from the literature were instrumental in de- Panel A shows the signal processed through Beltone 6 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 Sound Shifter was Designed with Sound Quality in Mind To provide maximal listening benefits with minimal signal distortion, Sound Shifter is a proportional frequency range amplitude compression, and then also through the competitor s non-proportional frequency compression. The frequency compression knee points are shown as between the input and output signals of the hearing aid. A higher HASQI score predicts better sound quality, as the 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 9 db mal hearing and with mild to moderate hearing loss. 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- Sound Shifter Off Sound Shifter On to the non-proportional relationship of the competitor s fre- lated to the aggressiveness of the algorithm. This indi- PANEL A quency compression. More distortion products are generated by this approach. These distortion products manifest themselves on the spectrogram with a blurred quality of the response curve above the frequency compression knee point. cates that stronger frequency compression settings are predictive of poorer sound quality. When proportional and non-proportional frequency compression were compared, however, the predicted sound quality of Sound Shifter was better. PANEL B Competitor FC Off Competitor FC On Sound quality for music was also evaluated for proportional 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 setting had a frequency compression knee point as close HASQI Score,8,7,6,5,4,3,2,1 Off Sound Quality for Music Moderate Strong Setting Sound Shifter Competitor FC 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 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. to 3 khz as possible, and a compression ratio as close to 2:1 as possible. The strong setting had a frequency 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.

4 Development of Sound Shifter Settings Objective measures during the fitting appointment included real ear insertion gain (REIG) measurements for Ling sults of other studies using frequency compression, group data did not indicate optimal frequency compression set- How to Fit Sound Shifter 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 previously stated, there are no defined guidelines about which individual clients will be the best candidates for frequency 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 from no frequency lowering at all. For these reasons, the best approach to applying and fitting frequency compression is often a conservative one. Insertion Gain (IG) Effect of Different Settings on Ling /s/ Frequency (Hz) off 5_1.33 4_1.33 4_2 35_2 3_2 25_2 225_2 2_2 distribution of individual scores with and without Sound Shifter for the UWO Plurals Test. Subjects who obtained positive benefit performed better with Sound Shifter on 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 Effect of Different Settings on Ling /sh/ 5 As such, Sound Shifter in Solus Pro fitting software was designed with a limited number of settings, which were 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. To determine the most effective and differentiated com- Insertion Gain (IG) Frequency (Hz) off 5_1.33 4_1.33 4_2 35_2 3_2 25_2 225_2 2_2 Number of Subjects UWO Plurals Test Score With Sound Shifter Minus Score Without Sound Shifter Figure 8. Sound Shifter can be activated and set in the Features section. To activate Sound Shifter, choose a setting in the dropdown menu. The recommended setting based on the client s audiogram will be indicated in bold font style. When frequency compression is not expected to provide 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, pression knee points and ratios, a second internal test was conducted. The goal of this trial was to evaluate 8 fre- 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 2 of the 37 process could be simplified to a smaller number of set- a slope of 1 db or greater per octave frequency, and ing hearing loss and 2 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 4 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 Test Setting Knee Point Ratio Off n/a n/a 1 (weakest) (strongest) 2 2 Table 1. Eight frequency compression settings evaluated during the internal trial. were fitted according to which frequency compression setting enabled them to perform best on the UWO Plurals Test. 5 If equivalent scores were obtained for more than one setting, the most conservative setting (i.e., the highest compression knee point and lowest ratio) was chosen. 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- than one setting was found to provide comparable benefit for individuals, three Sound Shifter settings were derived: Mild, Moderate and Strong. Table 2 shows the corresponding frequency compression knee points and ratios for each of these settings. Sound Shifter Setting Knee Point Ratio Mild 4 Hz 1.33 Moderate 35 Hz 2 Strong 25 Hz 2 Table 2. Frequency compression knee points and ratios for each setting of Sound Shifter. has a slope of 1 db or greater per octave frequency, and the slope begins at 2 Hz (Figure 1). A Strong setting is recommended if the audiogram has a slope of 1 db or greater per octave frequency, and the slope ends at 2 Hz (Figure 11).

5 REFERENCES 1. Moore B. Dead Regions in the Cochlea: Conceptual Foundations, Diagnosis, and Clinical Applications. Ear and Hearing, 24; 25 (2), Parsa V, Scollie S, Glista D, Seelisch A. Nonlinear frequency compression: effects on sound quality ratings of speech and music. Trends in Amplification. 213; 17(1): Figure 9. Example of audiometric thresholds when a Mild Sound Shifter setting is recommended. Figure 12. When Sound Shifter is turned on, a solid vertical line and a shaded region indicate the range of frequencies that are compressed. 3. Bentler R. Hearing aids: Clarity or controversy? Audiology Online Hearing Aid Conference, 212. Course Figure 1. Example of audiometric thresholds when a Moderate Sound Shifter setting is recommended. Upon choosing a Sound Shifter setting, a solid dark vertical line and a gray area will appear in the graph. The vertical line indicates the frequency compression knee point, and the gray area denotes the frequency range that is compressed (Figure 12). Recommendations for Sound Shifter are specific to the ear s audiometric thresholds. It is possible that different Sound Shifter settings for each ear may be recommended in the case of asymmetric hearing losses. Further, Sound Shifter may be activated in one or more programs, binaurally or monaurally, per the individual s needs. 4. Kates JM, Arehart KH. The Hearing-Aid Speech Quality Index (HASQI). Journal of the Audio Engineering Society, 21; 58(5), Glista D, Scollie S. Development and evaluation of an English language measure of detection of word-final plurality markers: The University of Western Ontario Plurals Test. American Journal of Audiology, : Gatehouse S, Noble W. The speech, spatial and qualities of hearing scale. Int J of Aud, 24; 43(2): Figure 11. Example of audiometric thresholds when a Strong Sound Shifter setting is recommended. 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 Shifter is an option that enables the hearing care professional to better accommodate the needs of individual clients.

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