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03.04.2015
In principal the function of peripheral hearing is to convert sounds (air pressure differences) into electrical signals.. 91% of all subjects reported a reduction in Tinnitusthis will be written under clinical evidence.. Download a copy of the Tinnitus Reaction Questionnaire (TRQ) and see what your score is now.
Science, Technology and Medicine open access publisher.Publish, read and share novel research. Speech playback at a slower sampling rate and reduction of zero-crossing rate are some of the frequency lowering methods that were used in surveys conducted before the eighties, as reported by Hicks et al. Turner and Hurtig (1999) commented that the frequency region where the hearing loss occurs, as well as its extent, are determining factors in word discrimination.
McDermott and Dean (2000) evaluated speech recognition in individuals with sloping hearing loss whose tone thresholds at low frequencies were better than 30 dB HL, while in medium and high frequencies presented profound hearing loss. Kuk (2007) gave a brief discussion about benefits and applicability of frequency transposition. Send Home Our method Usage examples Index Contact StatisticsWe do not evaluate or guarantee the accuracy of any content in this site. Mixed hearing loss occurs when a person’s audio-gram shows both SNHL and conductive components.  That means there can be damage present both in the outer and middle ear portion as well as the inner ear.
The third identifying factor to your audiogram is the shape of your hearing loss.  Many clinicians will use a term in their report to your physician about the overall shape.
Flat loss: A flat hearing loss is one that is approximately the same at all frequencies and is essentially a straight line across the audiogram. Cookie bite loss: A cookie bite loss looks like someone took a bite out of the top end of the audiogram. It is important to know what type, degree, and shape of hearing loss you have so that you can make an informed decision when it comes time to purchase the best instrument for your needs.
Frequency lowering strategies are generally used in the hearing industry as an alternative for patients who do not benefit from traditional amplification in the high frequencies. Jennifer Groth, AuD, is Director of Audiology Communications at GN ReSound Global Audiology in Glenview, Ill. The clinical challenge for this feature resides in identifying which individuals will receive benefit from frequency lowering, and which would be fitted more successfully with traditional amplification, as no definitive candidacy criteria for frequency lowering have been established to date. Hearing loss that slopes to greater degrees of severity for the high frequencies is commonly encountered in hearing clinics.
For the patient, many consonant sounds important for accurate speech discrimination are located in the high frequency region.
The inaudibility of certain phonemes produced at typical speech levels may present significant problems for the patient. In the real-world scenario of continuous discourse, people can often decipher messages even when not all the acoustic information is present. The relative importance of different types of redundancy in speech and communication shifts, depending on both acoustic and intrinsic factors.
The need for and desirability of adequate high-frequency audibility extends beyond the realm of speech to environmental sounds. Traditional amplification using wide dynamic range compression (WDRC) will provide 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.
Aside from the technical issues in providing these significant high-frequency gains, a physiological issue has also been suggested in the literature.
If adequate gain cannot be provided by the compressor, or if the patient 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. In some frequency lowering approaches, this also results in reducing the bandwidth of the device to a lower frequency. Current research has revealed some interesting patient characteristics that would contraindicate the use of frequency lowering for certain individuals. At the other end of the age spectrum, children fitted with nonlinear frequency compression (NLFC) may perform better in some areas but poorer in others, as compared to their traditionally fitted peers.5 Poorer consonant articulation scores were revealed in this study for children fitted with frequency compression. The configuration or slope of the hearing loss may also be an important factor to consider. Finally, certain cost-benefit decisions are involved when enabling frequency lowering for a patient. In the absence of the patient considerations outlined above, and when judicious use of the feature is employed, frequency lowering may be an appropriate tool to use for individual patients who do not receive adequate benefits from traditional amplification.
In addition to the shared goal of audibility, ReSound Sound Shaper employs much of the same terminology as amplitude compression. Verification of the audibility of these compressed signals is important, since a frequency-lowered signal that is still inaudible to the patient defeats the purpose of turning the feature on. All frequency-lowering techniques introduce a compromise between audibility and sound quality. As previously stated, there are no defined guidelines about which individual patients will be the best candidates for frequency lowering. To this end, Sound Shaper is incorporated in Aventa 3 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. The effects of frequency compression on sound quality are not easily generalized to all users. To provide the greatest listening benefits with minimal signal distortion, Sound Shaper was developed as a proportional frequency compression strategy. The main benefit of a proportional frequency compression technique is that there is less distortion as compared to a nonlinear compression strategy.
The results for both devices show that the frequency of the hearing aid output is lowered, and to approximately the same degree. Sound quality for music was also evaluated for proportional and non-proportional frequency compression via a predictive model. Identical 10-second segments of pop music presented at 60 dB SPL (RMS) were recorded through each hearing instrument for each of these hearing losses. Results for all three types of hearing losses showed the same trends, so for simplicity, the averages of these results for each frequency compression setting are depicted in Figure 6.
This indicates that stronger frequency compression settings are predictive of poorer sound quality regardless of which method of frequency compression is used. Sound Shaper provides hearing care professionals with another tool to improve high-frequency audibility, especially for individuals who do not receive optimal results with traditional amplification. The device comes with 4 different musical tracks fully personalized depending on individual’s hearing level and tinnitus. Comparative graphic of average WRR values obtained in groups P and F, with compression factors K = 3, K = 2 and K = 1, now considering both ears (joined).
In general, hearing loss exceeding 60 dB HL at frequencies above 2-3 kHz causes a reduction in speech discrimination. In that article, he presents the frequency transposition algorithm developed by Widex, available on the Inteo hearing aid.
For minimum distortion at low frequencies, the warping parameter must be chosen by the ratio defined in second part of equation (1). Comparing the Figures 2-ii to 2-iv, one can clearly observe the difference between the spectrogram obtained with the non-linear and the linear compression.The lists of words were heard in order of difficulty, starting the list with K = 3 and ending with K = 1. A person will have normal or close to normal hearing in the low frequencies and the high frequencies, but a more significant hearing loss in the middle frequencies. Patient’s will have normal hearing in the middle frequencies, sloping off to considerable losses in the low frequencies and the high frequencies. However, all of these strategies introduce distortion and potentially do more harm than good.
A review of the literature reveals many studies with conflicting results and recommendations. It is for these reasons that frequency lowering strategies are typically regarded as alternatives to consider for patients who do not receive the expected benefits from traditional amplification in the high frequencies.
This hearing loss configuration, however, presents its own challenges, both for the patient and for the hearing care professional.
Steeply sloping high-frequency hearing loss, with the speech banana and English phonemes superimposed to indicate areas of audibility and inaudibility for the patient.
A speech banana is superimposed on the audiogram, which indicates the common distribution of speech sounds in the frequency-intensity realm over time. This is due to the linguistic and acoustic redundancy of speech communication; speech contains more information than is needed to decode it. One-third octave band importance functions for nonsense syllables and continuous discourse. This is illustrated by the band importance functions for different types of speech material in calculating the Speech Intelligibility Index2 (Figure 2). As shown in Figure 1, a hearing loss of this configuration and degree affects the audibility for environmental sounds, such as birdsong. Steeply sloping high-frequency hearing loss, with typical environmental sounds superimposed to indicate areas of audibility and inaudibility for the patient.
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. For some individuals, it has been suggested that providing traditional high-frequency amplification for severe high-frequency hearing losses may not be beneficial if the corresponding high-frequency areas on the cochlear basilar membrane represent a “dead region.” A dead region is generally characterized by non-functioning inner hair cells. 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. For example, older patients who have a known working memory or cognitive deficit may be more susceptible to distortion created by certain hearing aid features.4 As frequency lowering is known to create distortion to some extent through artificial alteration of the high frequencies, these patients may receive greater benefit from traditional amplification. Overall, the study revealed insufficient evidence to suggest global language ability differences between the children fitted with NLFC and those fitted traditionally. A recent study indicated that, while NLFC was beneficial for high-frequency consonant recognition, it was not as beneficial for speech recognition in noise.12 Further, the methodology and care taken when fitting this feature was highlighted as a factor in the benefit it can provide. Of the frequency lowering approaches used today by the hearing industry, the most commonly fitted strategy is frequency compression.


To this end, ReSound has introduced Sound Shaper, available in Aventa 3.6 fitting software, as a tool to be used by the fitter to meet the needs of the individual patient. Frequency compression changes the input to a more amenable output above a cut-off threshold, or kneepoint. Audioscan Verifit Speechmap display showing verification measurements with Sound Shaper “Off” (blue) and Sound Shaper set to “Moderate” (green). Figure 4 illustrates one possible method of ensuring audibility for a previously inaudible stimulus. Some users will benefit greatly from the increased audibility of high-frequency sounds, while others may be bothered by the feature’s inherent distortion and perceptual sound quality.
In addition, consideration was taken to ensure that different settings provided distinctly different results, to simplify the process of setting Sound Shaper. This means that the relationships between frequency-compressed inputs are constant; in other words, the frequency compression ratio is constant for all input frequencies. Spectrograms for a swept pure-tone were measured unprocessed (upper- and lower-left panels), and with both amplitude and frequency compression activated (upper- and lower-right panels).
To illustrate the differences in the processing of a signal, a pure-tone sweep was processed through ReSound’s proportional frequency compression and another manufacturer’s non-proportional frequency compression to create spectrograms (Figure 5). The difference is that, for the non-proportional method (lower-right panel), there is more distortion in the output than with the proportional method (upper-right panel) used in Sound Shaper. In this comparison, each hearing instrument was programmed for three hearing losses: mild flat, mild sloping to moderately severe, and mild sloping to profound.
The recordings were then analyzed through the Hearing Aid Speech Quality Index (HASQI),16 which is based on coherence  between the input and output signals of the hearing aid, and yields a score on a linear scale from 0 to 100%.
Better sound quality for music is suggested by the HASQI for Sound Shaper than for a non-proportional frequency compression (FC) strategy with similar kneepoints and compression ratios. The highest HASQI scores relative to the input signal were obtained when frequency compression was deactivated (the “Off” setting). These results are also in agreement with other research regarding sound quality for music with frequency lowering.15 However, the HASQI results overall predicted better sound quality for the proportional frequency compression technique (Sound Shaper) than for the non-proportional frequency compression strategy.
Ching TYC, Day J, Zhang V, Dillon H, Van Buynder P, Seeto M, Hou S, Marnane V, Thomson J, Street L, Wong A, Burns L, Flynn C. Horizontal axis depicts the frequency range of input (original) signal and vertical axis the frequency range of output (processed) signal for the compression factors K = 2 (full line) and K = 3 (dashed line). The two knee-point frequencies of the frequency compression curve delimiting the three frequency ranges (I, II and III, see Figure 8) are shown by vertical grey lines.
All these methods involve signal distortion, more or less noticeable, usually dependent on the degree of spectral change. They used a nonlinear compression method, increasing progressively the compression ratio for high frequency sounds. The authors presented a new method of frequency transposition applied only on sounds that have significant energy at high frequencies. This was done in order to not provide clues that could facilitate the recognition of words, once all lists are composed by the same words arranged in different forms.The results were treated statistically through the Wilcoxon and Mann-Whitney non-parametric tests. This article outlines considerations when fitting a patient with frequency lowering, as well as current research suggesting when this type of feature may not be as beneficial.
There appears to be no general consensus on benefit for groups of people fitted with this feature versus those fitted without it. Individual phonemes indicate on the audiogram their specific frequency and intensity ranges in everyday speech.
Vowel formant transitions, which indicate place of articulation for the consonants that precede and follow them, provide acoustic cues to help identify those consonants correctly. High-frequency acoustic information is more important for correct identification of nonsense syllables (red curve) than continuous discourse (black curve), in which knowledge of the language assists in correct identification of speech.
For this patient, these environmental sounds will be inaudible without some form of high-frequency amplification (Figure 3). This study suggested that some manufacturer default settings of NLFC for a patient may still fail to provide audibility for the compressed signal, especially in the case of more severe hearing losses. Just as WDRC and other types of amplitude compression “move” low-intensity sounds to higher intensity levels for the individual, frequency compression “moves” high-frequency sounds to a lower, more aidable region.
The degree of frequency compression applied can be expressed as the frequency compression ratio. For instance, two individuals with similar hearing losses may derive the greatest benefit from different frequency compression kneepoints and ratios, or from no frequency lowering at all. This is in contrast to a non-proportional or nonlinear frequency compression strategy, in which the relationship between compressed inputs (ie, the ratio) changes as the frequency increases.
The lower-right panel shows the results for another manufacturer’s non-proportional frequency compression, while the upper-right panel shows the results for Sound Shaper.
Gains were set according to the default prescriptive targets in the respective manufacturer’s fitting software. A higher HASQI score predicts better sound quality, as the output sound will be more similar to the input sound, with less added noise and distortion relative to the original input sound. For both frequency compression techniques, the HASQI score was inversely related to the aggressiveness of the algorithm. ReSound Sound Shaper is designed as an option that enables hearing care professionals to better accommodate the needs of their individual patients.
A randomized controlled trial of nonlinear frequency compression versus conventional processing in hearing aids: Speech and language of children at three years of age. Effects of low pass filtering on the intelligibility of speech in noise for people with and without dead regions at high frequencies. Effects of low-pass filtering on the intelligibility of speech in quiet for people with and without dead regions at high frequencies.
Implications of high-frequency cochlear dead regions for fitting hearing aids to adults with mild to moderately severe hearing loss.
Determination of preferred parameters for multichannel compression using individually fitted simulated hearing aids and paired comparisons.
High-frequency amplification and sound quality in listeners with normal through moderate hearing loss.
Benefits from non-linear frequency compression hearing aids in a clinical setting: The effects of duration of experience and severity of high-frequency hearing loss.
Such schemes perceptually modify important speech characteristics such as rhythmic and time patterns, psychophysical frequency (pitch) sensation, and duration of segmental elements.
Initially, the study was conducted with six normal hearing individuals, performing consonant discrimination experiments on these normal listeners and results were compared with the control condition, using low-pass filtering.
The results were similar to those obtained in a previous experiment - which used the same speech material and procedures - performed with normal hearing individuals with a hearing loss simulation using similar low-pass filters and cutoff frequencies. To evaluate the algorithm, they conducted a study on recognition of monosyllabic words in quiet, with 17 hearing aids users with moderate to severe sloping sensorineural hearing loss. They set the dead region frequency edge considering this feature a fundamental key in the individual formatting of the frequency transposition algorithm. In this figure we can see that low frequency spectral content (below 1000 Hz) is barely compressed.For better understanding and comparison of frequency transposition and frequency compression spectral effects, in part (a) of Fig. However, research suggests that it can be highly beneficial for certain individuals, when fitted appropriately. Sounds below the hearing thresholds on the figure are audible to the patient, and sounds above the thresholds are inaudible. The physical properties of larger receivers, which are used in high-power hearing instruments, result in a high-frequency roll-off that is at a lower frequency than what is observed for lower-power hearing instruments. Sound Shaper compresses the frequencies above the kneepoint, effectively moving them closer together.
For these reasons, the best approach to applying and fitting frequency compression is often a conservative one. These spectrograms show that less distortion is generated by the Sound Shaper proportional frequency compression strategy than by the non-proportional frequency compression approach.
Colors indicate the intensity of the signal, with red indicating the most intense and blue indicating the least intense.
This pattern is similar to what was observed by Parsa and colleagues14 in their investigation on the effect of nonlinear frequency compression on sound quality. In their paper (Hicks et al., 1981), authors presented a remarkable investigation on frequency lowering. They have observed that Hick’s frequency lowering scheme presented better performance for fricative and affricate sounds if compared with low pass filtering to an equivalent bandwidth. The authors affirmed that using normal hearing subjects on this type of experiment is advantageous because it ensures that evaluation of the algorithm is not influenced by other factors associated with sensorineural hearing loss. Their objective was to compare phoneme recognition using conventional hearing aids and those with the frequency compression algorithm embedded.
According to authors, on the first experience, some users do not like the sound quality provided by this algorithm, referring to sound harsh or unnatural.
They concluded that the benefit observed in the use of frequency transposition was reduced by the confusion generated between phonemes. Sound Shaper is a proportional frequency compression approach that can provide high frequency audibility for individuals who do not receive it through traditional amplification, and was developed with the goal of preserving sound quality and minimizing distortion to the extent that is currently possible.
This provides the input frequencies in a smaller region that is more usable to the hearing instrument user. Both hearing instruments were programmed with amplitude compression for a mild-to-moderate hearing loss, with similar frequency compression kneepoints and ratios. The “Strong” setting had a frequency compression kneepoint as close to 2 kHz as possible, with the same 2:1 compression ratio as used by the “Moderate” setting. IntroductionHearing, as a complex function, requires perfect cochlea and auditory pathways and any interference in this system may compromise its performance. Their technique involves monotonic compression of short-time spectrum, without pitch alteration, while avoiding some problems observed in other methods.
On the other hand, the performance of the low pass filtering was better for vowels, semivowels and nasal sounds.
Of the 17 participating subjects, eight had improvement in recognition of phonemes using frequency compression; eight showed no differences regarding the amplification used and one subject presented significantly lower performance with the algorithm.


They concluded that the great sound change caused by frequency transposition account for this negative reaction, requiring a long-term use so that a reorganization of the cortical tonotopic representation may occur. Thus, they hypothesized that frequency transposition, instead of improving discrimination of consonants, improves detection of fricatives.
Preliminary qualitative testBoth frequency lowering algorithms (frequency compression and frequency transposition) were not already tested with hearing impaired subjects. However, audiology professionals still face failures, usually related to speech discrimination.
In general, performance in the task of speech in quiet with conventional amplification was similar to performance with frequency compression. But we got some preliminary results with normal listeners, first considering only qualitative aspects of processed speech. Results of right and left ears are compared.As there were no statistically significant differences between the WRR obtained for the right and the left ear in both groups, as demonstrated by the p-values at bottom line of Table 3, we chose to perform the remaining analysis considering the values of both ears. This is a common situation in sloping sensorineural hearing loss, which is often associated with marked difficulties in word recognition, especially in detection and discrimination of fricative phonemes, even using hearing aids.There is a consensus that the main difficulty related to hearing loss refers to communication, with the loss in the ability of speech discrimination and recognition.
Non-linear frequency compression was used and results were compared with conventional sound amplification. The authors commented that it is possible that frequency compression has brought benefits in the discrimination of some phonemes and detriment of others, so the final score did not change.
The assessment was performed by seven subjects with hearing loss and cochlear dead regions at high frequencies. In this case, a simple low pass filtering process simulates the losses above the frequency where there is no more residual hearing. However, the increase on acoustic information available through hearing aids does not always provide the complete restoration of these abilities. DiscussionEvaluation of human speech recognition using frequency compression has been proposed by many authors in studies dating from the 70's or earlier.
Some patients present little or no benefit with amplification, particularly those with severe high frequencies hearing loss.
After, two normal hearing adults (one male and one female) listened to the processed speech signals, as well as to the control condition (low-pass filtering only). Several studies demonstrate the contribution of high frequencies on speech intelligibility.
Comparison of two frequency lowering algorithms for digital hearing aidIn this section we present a new frequency-lowering algorithm that uses frequency transposition instead of frequency compression.
Listeners are not previously informed regarding signals’ processing schemes and are asked for ranking the three speech sounds according to their subjective quality.In this preliminary test, only two different speech signals were submitted to the algorithms.
Consequently, the sloping sensorineural hearing loss is related to the difficulty in understanding speech, even with the use of hearing aids.Hearing loss is more common for high–frequency and mid–frequency sounds (1 to 3 kHz) than for low–frequency. Furthermore, the frequency transposition is applied only over fricatives and affricates, leaving the other speech sounds untouched, as previous works have shown that frequency lowering only benefits high frequency phonemes. The original and processed spectrograms of one of these speech signals (a male pronunciation of the words ‘loose management’) are shown in Fig. Frequently, there are only small losses at low frequencies (below 1 kHz) but almost absolute deafness above 1.5 or 2 kHz. 4, where we can appreciate again the visual difference between the two frequency lowering algorithms. It is important to remark that both listeners are native speakers of Brazilian Portuguese and are not used to listen to English words in their everyday tasks. For these patients, lowering the high–frequency speech spectrum to the frequencies where the losses are mild or moderate could be a good processing tool to be added in the implementation of a digital hearing aid device.
Audiometric data acquisition and processingThe first step of both frequency-lowering algorithms consists in audiometric data acquisition of the hearing impaired subject. This is important because in this way we intend to separate speech quality from speech intelligibility, although both aspects are correlated and cannot be perfectly assessed by subjective evaluation.According to our prevision, only fricative speech sounds were frequency lowered in both algorithms.
The audiometric exam is employed for measuring the degree of the hearing impairment of a given patient.
In this exam, the listener is submitted to a perception test by continuously varying the sound pressure level (SPL) of a pure sinusoidal tone in a discrete frequency scale.
But in this case, its pronunciation had high frequency energy, as we can observe in the spectrogram of the original speech signal (see Fig. For each of these frequencies, the minimum SPL in dB for which the patient is capable of perceiving the sound is registered in a graph.The audiogram is the result of the audiometric exam, which is presented by a graph with the values in dB SPL for each of the discrete frequencies. These preliminary results indicate that the frequency shifting (or transposition) method was preferred by the listeners when compared to the frequency compression method. Preliminary intelligibility testWe perform a syllable identification test over 42 normal hearing subjects, 31 male and 11 female; a simple low pass filtering process simulates the losses above the frequency where there is no more residual hearing. After processing, each utterance generates nine speech signals: low-pass filtered syllable (control), frequency compressed syllable and frequency transposed syllable. Indeed, commonly the threshold of discomfort for the hearing impaired is lower than for normal hearing subjects. Although less common, some audiograms bring both the threshold of discomfort and the threshold of hearing (Alsaka & McLean, 1996), as one can observe in Fig.
Due to the random choice of the syllables that were presented to the listeners, there were some syllables that were less listened than others. In this figure, points of audiogram corresponding to the right ear are signalled with a round mark and those corresponding to the left ear are signalled with an X mark.
In the first column we have all the possible fricatives for each (three) filter cutoff frequencies. But for the impaired subject, that never (or for a long time) had any perception of sounds with frequencies above 2 kHz, may be the difference between the processed signals was not so slight.Relatively to the results of the intelligibility test, results are difficult to analyze if we consider the set of syllables as a whole. Only patients with this type of impairment can be aided by any frequency lowering method.After that, the first frequency where there is a profound loss is determined. In the case of fricative sound [ Z ], the better results are also obtained with our frequency shifting algorithm, independently of the signal bandwidth. This destination frequency is considered as the geometrical mean between 900 Hz and the highest frequency where there is still some residual hearing. The geometrical mean was empirically chosen because it provides a good trade-off between minimum spectrum distortion and maximum residual hearing profit.
But considering invidual phonemes, we can conclude that if we incorporate a simple automatic phoneme classifier in the system, it is possible to choose the better frequency lowering algorithm to be applied for each specific phone, given the maximum frequency where there is some residual hearing. In order to obtain more accuracy in the losses thresholds, audiogram points are linearly interpolated.As will be explained further, tests were done first in normal hearing people, simulating hearing losses by means of low-pass filtering.
This is not difficult to do, considering the advances observed in the performance of automatic phoneme recognition algorithms over the last years (Scanlon et al., 2007).
Finally, it is important to remark that both algorithms have demonstrated to be fast enough to enable their usage in digital hearing aid devices.3.
Speech data acquisition and processingSpeech signals are sampled at 16 kHz and Hamming windowed with 25 ms windows. Frequency compression and its effects on human speech recognitionIn this section we present the development and evaluation of the same frequency compression algorithm described in section 2, but with some modifications.
A 1024-point FFT is used for representing the short–time speech spectrum in the frequency domain.If in the previous audiometric data analysis a ski–slope kind of loss was detected and the frequency transposition criterion was matched, a destination frequency has already been determined. Then, we have to find out (in a frame–by–frame basis) if the short–time speech spectrum presents significant information at high frequencies that justify the frequency transposition operation. The criterion used for transposing or not the short–time spectrum of each speech frame depends on a threshold. All participants signed the free and informed consent form.The study included 18 normal listeners of both genders, with ages between 21 and 42 years. When the signal has high energy in high frequencies the algorithm transposes high frequency information to lower frequencies. Speech material used in this study consisted of monosyllabic words applied through TDH 39 headphones at 60 dB NA intensity, in silence, monotic task, both ears. The origin frequency is the frequency 100 Hz below the beginning of the 500 Hz bandwidth window with maximum energy. The part of the spectrum that will be transposed corresponds to the range of all frequencies above the origin frequency.
This empirical criterion guarantees that the unavoidable distortion due to the frequency lowering operation will be profitable.
A new organization of the words list was played in another CD in three different sequences of the same words, to reduce the listener learning effect.To determine the pure tone and speech tests thresholds, we used the Aurical™ audiometer from Madsen Electronics™, coupled to a personal computer. The speech procedures were applied in a sound proof booth using a portable compact disc player, model 4147 from Toshiba™, coupled to the Aurical™ audiometer and TDH 39 headphones, besides the CD containing speech samples.The words in the lists had the speech spectrum modified by frequency compression.
We performed all speech processing using Matlab™, at the Engineering and Modeling Center of the ABC Federal University (UFABC.
These curves were implemented directly in the frequency domain, in a frame-by-frame basis, using the equation shown in the lower right corner of the figure, where variable a controls the degree of nonlinearity of the curves (a = 0 turns the curve into a straight line). Horizontal axis depicts the frequency range of input (original) signal and vertical axis the frequency range of output (processed) signal for the compression factors K = 2 (full line) and K = 3 (dashed line).The total lack of compression corresponds to K = 1 and a = 0. When a = 0 and K = 2 for example, the compression is linear (a = 0) in the ratio of 2:1 (K = 2). This means, in this example, that output frequencies (of processed signal) correspond exactly to half the values of input frequencies (of original signal).
That is, if the original signal has a frequency component at 2000 Hz, this will correspond to 1000 Hz in the processed signal.On the algorithm originally proposed (6), the curves were approximately linear and with no compression in the range from 0 to 1 kHz. In this study, the approximate range of linearity (and no compression) was extended up to 1.5 kHz, aiming to reduce as most as possible the perceptual distortion of main pitch harmonics and formants of the original speech signal.



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