Linear Frequency Transposition_ Extending the Audibility OfHigh-Frequency Information - Hearing Review

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Linear Frequency Transposition:Extending the Audibility ofHigh-Frequency InformationPublished on October 8, 2006

People with a precipitous high frequency hearing loss often miss the highfrequency information even when they wear hearing aids. Sometimes it isbecause the high frequency gain available on the hearing aid is not sufficient toreach audibility before feedback occurs; sometimes the severity of the hearingloss in the high frequency region is so great that it is unaidable or dead fromthe complete depletion of inner hair cells.

In the former case, audibility may be achievable at the expense of a smallervent diameter on the hearing aid. This could compromise wearer comfortbecause of an increase in the occlusion effect.1 In the latter case, acousticstimulation of the unaidable region may decrease further the already depressedspeech understanding.2 The loss of audibility of high frequency sounds oftencompromises speech understanding and the appreciation of music andnatures sounds (such as bird songs).

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This article was submittedto HR by Francis Kuk, PhD,director of audiology, andresearch audiologists PetriKorhonen, MSc, HeidiPeeters, MA, and DeniseKeenan, MA, at the WidexOffice of Research inClinical Amplification(ORCA), Widex USA, Lisle, Ill;and Anders Jessen, BSEE,research engineer, andHenning Andersen, MS,manager of theDevelopment Lab, at WidexA/S, Vaerloese, Denmark.Correspondence can beaddressed to Francis Kuk,PhD: e-mail: [email protected].

What is Frequency Lowering? One of the earlier attempts to achieve audibility for high frequency sounds isthe use of frequency lowering techniques. These are simply nonlinearoperations in which high frequency sounds are moved to a lower frequencyregion. When these techniques are applied to hearing aids, the objective is toprovide audibility of the unaidable high frequency cues by changing them intoan audible lower frequency substitute. The target beneficiaries of this techniqueare people with a severe-to-profound loss in the high frequencies who cannotbenefit from conventional amplification.

Various approaches had been attempted infrequency lowering. The first attempts were donewell before non-linear and digital technology wasapplied to hearing aids. Methods such as slow-playback, time-compressed slow-playback,frequency modification with amplitudemodulation, vocoding, zero-crossing rate division,frequency shifting, and frequency transpositionwere all major approaches that have beensummarized by Braida et al.3

More recent strategies in the area includeproportional frequency compression4 andapproaches that sharpen the spectrum of thetransposed sound5 or the various transposedfeatures (eg, voiceless vs voiced). Although theseapproaches are significantly more complex than

earlier attempts and they all resulted in better aided thresholds, theiracceptance has been relatively limited.

What Are the Problems? Limitations of analog signal processing. The early attempts on frequencylowering were designed to achieve easy implementation of existing

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technologies rather than to achieve the desired signal processing results. Manywere not even practical enough to be implemented into hearing aids. Whilelowering the frequencies, these methods also altered other aspects of speechknown to be important for perception. Some of these approaches createdunnatural sounding speech, distorted gross temporal and rhythmic patterns,and extended durations (slow playback) of the speech signals. Others createdreversed spectrum (amplitude modulation based techniques) which is difficultto even recognize as speech by inexperienced listeners. In vocoder-basedsystems, both analysis and synthesis were often carried out using only alimited number of frequency bands, which resulted in unnatural speech sounds.

Unnatural sounds. Despite the recent use of digital signal processing (DSP)techniques in frequency lowering, artifacts and unnatural sounds were stillunavoidable. Some reported that the transposed sounds are unnatural,hollow or echoic, and more difficult to understand. Another commonlyreported artifact is the perception of clicks which many listeners findannoying. Such perception would most likely be exaggerated when thetransposed sounds and the original sounds do not overlap. Thus, despite itspotential for speech intelligibility improvement with extensive training, manyadults found it difficult to accept frequency lowering.

Insufficient training and limited evaluation. It would be highly desirable thatthe new acoustic cues resulting from frequency lowering resemble the originalhigh frequency sounds in some meaningful, easy-to-interpret manner.Nevertheless, these processed sounds were never heard by the hearing-impaired listeners before. As such, it is unrealistic to expect the listeners toidentify the new sounds without adequate training and experience.Unfortunately, most previous studies have not given the test subjects time toadjust to and learn to use these new acoustic cues. In studies where extensivetraining was provided, marginal improvement in speech understanding wasobserved.6

Considerations in Frequency Lowering



Minimization of artifacts and unnatural sounds. Minimal artifacts orunnaturalness will result if the frequency lowering method retains therelationships of the original frequency components in the final signal.Preferably, the relationships of the harmonic components stay the same, thespectral transitions move in the same direction as the original un-transposedsignal, and the segmental-temporal characteristics stay untouched. One shouldnot remove or sacrifice any acoustic cues that the listeners are using beforefrequency lowering. In addition, the processed speech signal should retain theextra-linguistic (prosodic) cues, such as its pitch, tempo, and loudness.Otherwise, it will make it more difficult for the listeners to accept the new soundimages initially and lengthen the training and relearning period.

One criterion is to lower only the frequencies that are necessary to be lowered(instead of the full range of frequencies). For example, if someone has aidablehearing up to 3,000 Hz, one should only process (or lower) sounds above 3,000Hz. This has the advantage of focusing only on sounds that are relevant.

Another criterion is to apply the right amount of processing for the individual.This is because the more aggressive the lowering (eg, higher frequencycompression ratio), the more unnatural the sound percept becomes. Aconservative or less aggressive approach will minimize the disturbance on theoriginal signals and avoid any potential interaction between the original signalsand the processed signals.

A final criterion is to preserve the temporal structure of the original signal inorder to retain any transition cues. This means the frequency lowering systemmust have the flexibility and specificity to meet individual wearers needs.

In cases where the unnaturalness is unavoidable because of the extent offrequency lowering, a strategy to minimize the exposure of artifacts is to makethe frequency lowering algorithm optional. That is, the wearer will only listen tothe processed sounds when he/she needs to. In situations where such aprogram is not needed or not beneficial, the program may be deactivated.



Proper training and evaluation. The need for training may be argued iffrequency lowering has completely altered the acoustic cues available to thewearers. Consequently, frequency lowering technique should use a two-prongapproach. First, it should preserve the existing cues while adding new ones.This requires special attention be paid to the individuals hearing needs and theflexibility with the programming to accommodate such needs. Second, itshould recommend an appropriate training program with the algorithm tofurther realize the potential of the transposition. In practice, this means that thefrequency lowering algorithm should receive high initial acceptance for dailystimuli such as natures sounds. But a structured training program that isdirected towards improving sound recognition should also be available forthose who needed the training. These criteria mean that the chosen frequencylowering algorithm must be appropriate for both speech and non-speechsounds.

Extending Audibility via Linear Frequency Transposition These considerations guided the development of the new, patent-pendingAudibility Extender (AE) algorithm in the recently introduced Widex Inteohearing aid. The AE is one form of frequency lowering technique that usesLinear Frequency Transposition to move the unaidable high frequency soundsto the aidable low frequency regions.

A feature of this algorithm is its inclusion in the Integrated Signal Processing(ISP) platform6 used in the Inteo. Briefly, ISP integrates information of thewearers, the environments, as well as the intermediate processing of eachalgorithm into the Dynamic Integrator (DI). In turn, the DI coordinates all theactivities and dispatches the appropriate commands to each algorithm so thatthe processed sounds would be as natural as possible with little or no artifacts.

How it works. First, the Inteo AE receives information of the wearers hearingloss from the Dynamic Integrator (provided from Wearers PersonalInformation) to decide which frequency region will be transposed. Thefrequency where transposition begins is called the Start Frequency. Typically,



one octave of sounds above the start frequency will be transposed. This iscalled the source octave. (Figure 1a).

Meanwhile, the Speech and Noise Tracer of the HD System Analysis moduleperforms its spectral analysis of the environment and returns the results to theDynamic Integrator. The AE picks the frequency within the source octaveregion with the highest intensity (eg, peak frequency), and locks it fortransposition. As the peak frequency changes, the transposed frequency alsochanges. In the example, 4,000 Hz has the peak intensity (Figure 1b). Onceidentified, the range of frequencies starting from 2,500 Hz will be shifteddownward to the target frequency region. In this case, 4,000 Hz (and thesounds surrounding it) will be transposed linearly by one octave to 2,000 Hz(Figure 1c).

The 4,000 Hz signal will be placed at 2,000 Hz and every frequency will beshifted down by 2,000 Hz. For example, 3,000 Hz will now be at 1,000 Hz and



4,500 Hz will be at 2,500 Hz (Figure 1d). In this way, the transposed signal islikely to be placed in a region where the hearing is aidable. To limit the maskingeffect from the transposed signal and any potential artifacts, frequencies thatare outside the one octave bandwidth of 2,000 Hz will be filtered out (Figure1e).

The level of the transposed signal will be automatically set by the AE so it isabove the in-situ threshold (sensogram) of the wearer. A separate manual gainadjustment of the transposed signal is also available. The linearly transposedsignal is mixed with the original signal below the start frequency (2,500 Hz inthis case) as the final output. (Figure 1f).

Other considerations. To ensure ease of use of the AE program, simple defaultrules are implemented that consider the degree and slope of the audiogram indeciding the default start frequency (for transposition). The optimum startfrequency is critical in ensuring acceptance and successful use of the AE.

By definition, the start frequency is the frequency where the hearing loss isunaidable. Thus, instead of amplifying that frequency, the AE transposes itwithout amplification. Consequently, too low a start frequency (below theoptimum) will result in some of the aidable frequencies not being amplified.This removes some of the acoustic cues that are usable by the wearers toresult in distortion and unnaturalness of the signal. Too high a start frequencywill leave some of the unaidable high frequencies inaudible. In both cases, it willunnecessarily decrease the initial acceptance of the AE and prolong the time to



fully realize its potential. To meet the individual hearing needs of the wearersand to increase the flexibility of the AE program, options to manually adjust thestart frequency from 630 Hz to 6,000 Hz at 1/3 octave intervals (as well asindividualized fitting guidelines) are available.

Another advantage of the AE is that it is an optional program. This means onecan set this program as the master default program for use in all listeningsituations; alternatively, it can be used only in situations where the wearerdesires. The former may be a pediatric fitting where the child uses the AE all thetime so he or she can hear all the high frequency sounds in many environmentsfor speech and language purposes. The latter may be an adult who is satisfiedwith the default settings of the hearing aids in most situations, but desires theAE program for listening to birds, music, or other sounds. In this way, individualpreferences and usage habits are considered.

How is the Audibility Extender Different? The Audibility Extender is different from other frequency lowering schemes inseveral aspects:

1. It transposes only the high frequency sounds (above the start frequency)regardless of their voicing characteristics (eg, voiced or voiceless). Thus, it isequally effective on periodic and aperiodic sounds. Systems that are active onlyfor voiceless signals may miss high frequency periodic signals including musicand bird songs.

2. It is active during all segments of speech and not at specific linguisticsegments, (eg, voiced versus voiceless).

3. Typically only one octave (although two octaves may be allowed) of highfrequency sounds above the start frequency is transposed to a lower octave.Frequencies higher and lower than the transposed region are filtered. Thislimits the amount of masking and avoids the need for compression.



4. For simple stimuli, it preserves the transition cues and the harmonicrelationship between the transposed signal and the original signal. Thispreserves as much of the original signal as possible.

5. The transposed signal is mixed with the original signal to give a richer, morenatural sound perception. Systems that do not overlap the transposed soundswould risk exaggerating any unnaturalness of the transposed sounds.

6. By transposing frequencies linearly, the temporal structure of the signal ispreserved. Thus, it can be easily recognized as the original source signal but ata lower frequency.

Efficacy of the Audibility Extender: Interim Field Report While clinical studies are being conducted to better understand the efficacy ofthe AE algorithm, we have completed some preliminary studies that examinedthe initial subjective preference for the AE using different stimuli.

Subjects. A total of 16 individuals with hearing impairment, primarily with highfrequency sensorineural hearing loss, were tested to examine their preferencefor the AE for bird songs, music, and discourse speech stimuli. Of thesesubjects, 5 individuals had a precipitously sloping hearing loss with normalhearing below 1000 Hz, and 11 had a sloping high frequency hearing loss ofmoderate to severe degree.

Hearing devices. All the subjects with a precipitous hearing loss and 6 subjectswith a sloping high frequency hearing loss wore the open-fit Inteo lan duringthe study. The rest of the subjects wore the Inteo IN-9 and IN-X (ITC) with theappropriate vent diameter (1-3 mm diameter). The fitting of the Inteo hearingaids including the AE algorithm, followed the default recommendations (eg, noindividual fine-tuning).

Stimuli and testing. Three sets of stimuli were used to evaluate the subjectivepreference for the AE. A set of 12 bird songs (different species, with mostly



high frequency content up to 6,000 Hz), 12 musical passages (including singleinstruments, ensemble, and songs with lyrics), and 12 short discoursepassages read by a female announcer were used. Each stimulus was about 5-10 s in duration. Subjects listened to each stimulus in the AE-On and AE-Offconditions (the same frequency response and feature settings were usedbetween AE-On and Off) and the subjects indicated the setting (eg, AE-On orAE-Off) they preferred for the particular stimulus.

The stimuli were presented in random order and at a comfortable listening levelchosen by the subject in the AE-Off condition. The number of stimuli within astimulus set that the subject preferred with the AE-On was recorded andexpressed as a percentage displayed in the following figures.

Preference for bird songs. Figure 2 shows the individual preference for AEusing bird songs as stimuli. Each bar represents the percentage of time the AEwas preferred by a specific subject. For example, a preference of 100% (Subject#18) indicated that the subject preferred the AE-On for all 12 bird songs,whereas a preference of 50% (subject #15) indicated that the subject preferredthe AE-On for 6 of the 12 bird songs (the other 6 for the AE-Off). One can seethat subject preferences varied dramatically. Subject 9 preferred the AE-On foronly one bird song, whereas subject 18 preferred the AE-On for all the birdsongs. On average, AE-On was preferred for over 60% of the bird songs.



Figure 3 shows the individual preferences for AE using music as stimuli. Similarto using bird songs as stimuli, one sees a range of preferences for the AE fromless than 10% to over 90% of the time. As a group, the preference for AE-Onwas about 50% of the stimuli. These preferences are slightly lower than whenbird songs were used as stimuli.

Preference for speech. Figure 4 shows the individual preference whenconversational speech passages were used as stimuli. There was an evenwider range of preference for the AE, with Subject #7 showing no preference forAE-On (eg, all AE-Off) and Subject #2 preferring only the AE-On condition. As agroup, AE-On was preferred for 33% of the stimuli.

Conclusions There are several observations when one examines the preference data acrosssubjects and stimuli. First, subjects with a sloping high frequency hearing losssubjectively prefer the AE when listening to birds, music, and speech. Second,the preference for the AE-On varied with the complexity of the stimuli. Birdsongs are simpler in spectral content than music and speech, and thepreference for the AE was the highest for birds (over 60%), less for music (55%),and least for running speech (33%). This suggests that the simpler the stimuli,the higher the preference for the AE.

Every subject preferred the AE-On for at least one stimulus. However, subjectpreference for AE in one stimulus category does not predict preference inanother stimulus category. For example, Subject #18 preferred the AE-On 100%of the time when listening to bird songs, but less than 10% of the time when it



comes to music and speech stimuli. Whereas Subject #2 preferred AE-On 100%of the time when listening to running speech, he only preferred it 33% of thetime when listening to bird songs.

It needs to be emphasized that the above performance was noted when thesubjects were initially fitted with the default settings without additional fine-tuning to the wearers hearing needs. Furthermore, no experience with thetransposed sounds was provided prior to the study. With additional experienceand fine-tuning, one would have considered the individual hearing needs insetting the optimal transposition parameters. This could further improve thepreference for the AE. This is being evaluated and will be reported later.

With appropriate training and fine-tuning, Linear Frequency Transposition mayimprove the recognition of high frequency words for those who are limited bytheir high frequency hearing loss. This could be especially beneficial forchildren during critical speech and language development periods. Anotherpotential application is in open-fittings where this algorithm increases theaudibility of high frequency sounds while open-fit provides excellent listeningcomfort.

References 1. Kuk F, Ludvigsen C. Amplcusion Management 101: Understanding variables.The Hearing Review. 2002;9(8):22-32.

2. Moore B. Dead regions in the cochlea: conceptual foundations, diagnosis,and clinical applications. Ear Hear. 2004;25(2):98-116.

3. Braida L, Durlach I, Lippman P, Hicks B, Rabinowitz W, Reed C. Hearing AidsA Review of Past Research of Linear Amplification, Amplitude Compressionand Frequency Lowering. In: ASHA Monographs; No 19. Rockville, MD:ASHA;1978.

4. Turner C, Hurtig R. Proportional frequency compression of speech for



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listeners with sensorineural hearing loss. J Acoust Soc Am. 1999;106(2): 877-886.

5. Aguilera-Muoz C, Peggy B, Rutledge C, Gago A. Frequency loweringprocessing for listeners with significant hearing loss. In: Proceedings of theSixth IEEE International Conference on Electronics, Circuits and Systems (Cat.No.99EX357), Part 2(2); 1999:741-744.

6. Kromen M, Troelsen T, Pawlowski A, Fomsgaard L, Suurballe M, HenningsenL. InteoA Prime Example of Integrated Signal Processing. In: Integrated SignalProcessingA New Standard in Enhancing Hearing Aid Performance. LongIsland City, NY: Widex Hearing Aid Co; 2006.

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Linear Frequency Transposition_ Extending the Audibility OfHigh-Frequency Information - Hearing Review