Nitro micon: the Wisdom behind the Powerful Sound

Transcription

1 Nitro micon: the Wisdom behind the Powerful Sound Maja Serman, Ph.D. Ulrich Giese, Ph.D. Claudia Pischel Abstract: This paper describes challenges faced by patients with severe to profound hearing impairment and hearing care professionals trying to fit hearing instruments to such hearing losses. The fitting and signal processing strategies of the new superpower Nitro micon instrument designed to target these challenges are explained and illustrated here, with examples from the Connexx 7 fitting software.

2 Introduction In a recent survey on hearing impairment, the global prevalence of severe to profound hearing losses was estimated at 1 % for males and 0.8% for females (Stevens et al., 2013). These numbers may seem relatively small in comparison to the global prevalence of hearing loss per se which, in the same survey, was estimated at 12.2% for males and 9.8% for females. However, the relatively small percentage of the severe to profound hearing impaired patients belies the challenge of successfully compensating this type of hearing loss: fitting severe to profound hearing losses is considered to be one of the most difficult tasks in the hearing care profession. For these patients, their hearing instruments are most often the last option to preserve the link to natural sounding world before facing the challenges of undergoing cochlear implant surgery. Unquestionably, the stakes of being fitted with reliable and effective hearing instruments are high. Background Severe to profound hearing loss is a commonly used classification, although it has been defined using somewhat different audiometric thresholds (e.g. Keidser et al., 2007; Margolis and Saly, 2007). One example from Keidser et al. (2007), is that an audiometric pure tone average (PTA) for the better ear (at 0.5, 1 and 2 khz) between db HL is considered as an indication of a severe loss and a PTA above 90 db HL as an indication of a profound loss. However, PTA values on their own cannot convey the reduction in ability to understand speech in difficult listening situations for this group of hearing losses. From the physiological point of view, it is safe to assume that with this type of hearing loss both outer hair cells (OHC) and at least some of the inner hair cells (IHC) are severely damaged, resulting in: loss of audibility, loss of frequency selectivity. The difference between these two dimensions of hearing loss and their impact on the demands on hearing instrument processing is outlined in Figure 1. It is a simplified representation of the displacement of the basilar membrane due to two travelling waves (as a consequence of two tones close in frequency). Assuming critical bandwidth separation, the top panel shows that in the healthy cochlea, there is sufficient displacement of the basilar membrane to resolve the two tones, thus enabling the listener to perceive them as separate sounds. The middle panel depicts the consequences of OHC loss for the same stimuli smaller and rounder tone peaks which may result in loss of frequency selectivity and spectral smearing. The bottom panel shows that the hearing instrument can increase the amplitude of the displacement, but this does not restore the sharpness of the peaks and therefore cannot restore the original frequency selectivity of the healthy cochlea. Fig 1. A schematic representation of the impact of the OHC loss on cochlear processing. Top panel: envelope of the basilar membrane displacement in the healthy cochlea due to stimulation by two tones close in frequency. Middle panel: the consequence of the OHC loss: reduction in amplitude and 2

3 smeared out peaks. Bottom panel: the same input signals amplified through the hearing instrument: the displacement amplitude is restored but the peaks are still rounded (adapted from Kemp, 2002; Venema, 2003; Moore, 2007). As one moves beyond a hearing loss of 60 db HL, the IHC loss becomes more prominent which may for the parts of the cochlea with very few or no IHC left, render the amplification strategy as ineffective. It is this combination of the OHC and IHC loss in severe to profound impaired patients that makes their hearing loss compensation with hearing instruments so challenging. Speech understanding in quiet and in noise The perceptual effects of cochlear frequency resolution can be observed in normal hearing listeners, through the phenomenon known as the upward spread of masking whereby a masker lower in frequency than the signal is more effective in masking the signal than a masker that is higher in frequency (Egan and Hake, 1950). The auditory filter width typically increases with increasing sensorineural hearing loss (Glasberg and Moore, 1985) which reduces frequency selectivity and thus result in an even more prominent effect of upward spread of masking in terms of masked thresholds (Klein et al., 1990). For example, the more intense lower frequency components of the speech signal mask the weaker high-frequency speech components resulting in decreased speech intelligibility (Klein et al., 1990; Stelmachowicz et al., 1985). In quiet, the availability of different, often redundant speech cues distributed across the speech spectrum may still provide some speech understanding even for the severely hearing impaired listener. In noisy situations, however, the loss of frequency selectivity due to the hearing loss has a much stronger impact on speech understanding because it reduces the ability to distinguish between spectrally important speech information and the background noise. For example, the presence of noise between the spectrum of two speech cues (such as are the first and second vowel formants) requires finer selectivity to locate the two speech cues (Assmann and Summerfield, 2004). Numerous studies indicate that hearing-impaired listeners demonstrate poorer speech perception in noise than normal hearing listeners and that the loss of frequency selectivity in subjects with hearing loss is very likely one of the causes (Stelmachowicz et al., 1980; Peters et al., 1998; Assmann and Summerfield, 2004; Athalye, 2010). Sound distortion Patients with severe to profound hearing loss also experience an increase in sound distortions. Let us look at the example of the perception of roughness (also described as dissonance and/or harshness) due to amplitude fluctuations known as beats. Beats occur when two tones that are closely spaced in frequency fall into a single auditory channel and are therefore unresolved on the basilar membrane. As mentioned above, for profoundly hearing impaired patients, the loss of OHC typically results in the broadening of the auditory filters, which may enable the interaction between the more widely-spaced tones and in smearing of the spectral details. Additionally, the tuning of the auditory filters becomes broader with increasing levels (Moore, 2003). The considerable hearing instrument gain required by this type of hearing loss may, therefore, also contribute to these perceived distortions. In short, due to the loss of frequency selectivity, the loss of loudness, the loss of cochlear compression, the presence of dead regions and possibly some other higher processing level factors, severe to profound hearing loss patients are very likely to experience an increase of unpleasant and dissonant sounds as well as suffer the loss of ability to separate competing sounds (e.g. speech from background noise). In some instances, the severe to profound sensory hearing loss is combined with a conductive component, which further increases the need for loudness compensation. Designing the hearing instrument which is able to compensate for hearing losses falling into the severe to profound range thus presents a real challenge. On the one hand, in order to compensate for a dramatic loss of audibility, a truly powerful instrument is needed; on the other hand, reduced dynamic range and an increased sensitivity to distortion and noise demand that this power is used wisely through fitting strategies and hearing instrument processing. The application range of such a hearing instrument is shown in Figure 2. 3

4 Fig 2. Customary application range of a super power instrument. Nitro micon: The Fitting Strategy Generally speaking, a fitting strategy represents the core of audiological knowledge and experience: it aims to compensate for a hearing loss while taking into account the constraints imposed by the hearing instrument. The latter is particularly true for proprietary fitting strategies such as micon fit. A fitting strategy comprises two aspects: The gain applied to the input signal: amplification as a function of frequency. The compression applied to the input signal: amplification as a function of input signal level, defined through compression knee point (CK), compression ratios (CR) and the temporal behavior of the compression system conventionally described by the attack and release time constants. The gain Anecdotal and research evidence point towards different gain preferences in new and experienced hearing instrument users (Keidser et al., 2008). The nature of severe to profound hearing loss is such that new users are rare: this type of hearing loss is either congenital or it develops over time so that these patients are usually experienced hearing instrument users. However, their individual need for gain differs according to the severity, and to some extent, the etiology of the hearing loss. Severe hearing loss may still allow for gain compensation across a relatively large frequency range, thereby increasing the chance of preserving speech understanding through gain compensation. Taking into account the effects of upward spread of masking, micon fit for severe hearing losses enhances gain in the low and middle frequency range (see Figure 3). Full on gain curve 50 db input level 65 db input level 80 db input level Fig 3. Example of a severe hearing loss (left panel) and the corresponding compressive micon fit for Nitro in the insertion gain view (right panel, target gain curves for different input levels are in red, hearing instrument simulated gain curves are dotted and black). The full on gain curve shows the maximum insertion gain for 50 db input level, for given measurement conditions. For profound hearing losses, the residual hearing which can benefit from amplification is expected mostly in low frequencies. Additionally, in the middle and high frequency regions, an increased probability of adverse gain effects may be expected (Moore and Malicka, 2013). For these reasons, micon fit for profound hearing losses provides a more broadband gain in low frequencies (see Figure 4). These two different fitting 4

5 approaches are automatically incorporated in micon fit for Nitro instruments, as a function of the audiogram test frequencies, so as to facilitate the fitting for the hearing care professional while providing the optimum gain for the patient. Fig 4. Example of a profound hearing loss and the corresponding compressive micon fit for Nitro. The compression Although by and large experienced wearers, severe to profound hearing impaired patients differ widely in their hearing instrument usage history: some have been fitted with linear hearing instruments since early childhood, while others started wearing hearing instruments with wide dynamic range compression (WDRC). Regardless of their histories, because of the degree of their hearing loss, most of these patients are full-time hearing instrument wearers who rely heavily on their instruments, and are highly acclimatized to specific amplification characteristics. Although there is evidence that such patients can, with time, acclimatize to a different compression strategy (Convery and Keidser, 2011), they may at first be reluctant to do so. For this reason, micon fit for Nitro gives the hearing care professional the choice between two compression strategies (see Figure 5). Fig 5. Compression strategies for Nitro micon. For those patients who are already used to WDRC, micon fit for Nitro offers the More Compressive amplification strategy. This compression strategy allows for compression ratios of up to 4:1 for the whole input range above the first CK which is pre-set at 50dB SPL, thus enabling an effective compensation for the patient s reduced dynamic range. The compression effect for a static input signal can be seen in the right panels of Figures 3 and 4 by the pronounced differences between the target gain curves for different input levels (50, 65 and 80 db SPL, respectively). The More Compressive strategy is the recommended fitting approach since it enables the full use of the residual dynamic range while still keeping the peaks of the signal below the patient s uncomfortably loud region. It is also more effective in preserving both the sound quality and speech intelligibility through the advanced compression system offered with micon instruments (see below). The More Compressive strategy, therefore, is the default fitting approach in Connexx 7 fitting software. For patients who prefer linear amplification, we offer the More Linear strategy: this provides linear amplification up to the second CK set at 65 db SPL. After the second CK, only mild compression is provided (never higher than a compression ratio of 2). The effects of More Linear compression are shown in Figure 6 (for the audiograms shown in left panels of Figures 3 and 4). Here, the reduced compression effect can be observed through much smaller differences between the target gain curves for different input levels, especially between the 50 and 65 db SPL input level target curves. While it may seem counter-intuitive to offer linear amplification for such a small residual dynamic range, experience tells us that there are patients within this hearing loss group who are used to and prefer as little dynamic interventions of the hearing instrument as possible, even if this means that softer sounds will often be inaudible and higher level inputs may be limited. 5

6 Fig 6. The More Linear micon fit for Nitro. Left panel: for the severe hearing loss shown in Figure 3. Right panel: for the profound hearing loss shown in Figure 4. Observe that unlike the simulation curves shown in the right panels of Figures 3 and 4, where a clear separation between the gain for soft and average input levels is visible, this fitting strategy prescribes the same gain for soft and average inputs. Dynamic range and micon compression As mentioned in the previous section, it is important to select an appropriate amplitude compression strategy for the patients with severe to profound hearing loss: this could involve using strong compression for soft, moderate and high input levels, or in other cases, only using weaker compression for signals of average input levels or higher. The time constants of the compression algorithm are also very important for this patient group: due to the broadening of the auditory filters and consequent loss of frequency selectivity, these patients may have learned to rely more on the temporal cues of the speech signal (Turner et al., 1995; Kishon-Rabin et al., 2009). In order to preserve both spectral and temporal envelope speech cues, speech sound input would require low compression ratios and slow compression time constants. On the other hand, in order to protect the patient and stay below the uncomfortable sound levels for natural, dynamically fluctuating sounds, higher compression ratios and faster compression time constants are required. Theoretically then, given the limitations of their dynamic range, severe to profound hearing loss patients would require high compression ratios and short time constants. In practice, it has been shown that such compression settings most often result in a degradation of speech intelligibility, sound quality and consequent dissatisfaction with the hearing instrument (Gatehouse et al., 2007; Keidser et al., 2007; Souza et al., 2012). Basically, one needs an intelligent compressor which is capable of adapting compression according to the properties of the signal. This is why micon adaptive compression concept is extremely important for this patient group. For small level changes in the input signal it reacts slowly thereby preserving spectral and temporal speech cues as well as preserving the quality and naturalness of the sound. When more substantial input level changes are detected, the compression speeds up. The adaptive compression allows for a continuum of time constants as a function of level dynamics. In this way, large amounts of compression can be applied only when really needed and the time constants can be adjusted according to the input signal. The result is a balance between speech intelligibility and sound quality while ensuring full audibility and safety of the hearing impaired. A variety of studies indicate that throughout the auditory system, there are several different mechanisms influencing cochlear gain changes at different locations and with different time constants, giving normal hearing listeners the advantages of a wide dynamic range while ensuring sound quality and protection from the acoustic trauma (Guinan, 2006; Boley, 2013). This is not to imply that micon adaptive compression mimics the healthy auditory system perfectly, but rather that it strives towards the same principle: ensuring a freedom of listening in the natural sounding world without any discomfort. Standard Connexx features A number of our standard Connexx features have been developed and/or adjusted specifically for this group of patients. Let us look at them in detail. 6

7 Frequency shaping The focus of the micon fit for Nitro is on the low and middle frequencies. The rationale behind this is related to the physiology of the severe to profound hearing loss; residual hearing is predominantly in the low frequencies whereas high frequencies often remain inaudible in spite of the hearing instrument gain compensation. Research has shown that any hearing loss above approximately 60 db across the entire frequency range and approximately 70 db in the high frequency region has a high probability of cochlear dead regions (Vinay and Moore, 2007). It has also been shown that when substantial cochlear dead regions are present, gain compensation may not always prove beneficial (Ching et al., 1998; Baer et al., 2002; Munro, 2007; Moore and Malicka, 2013;). For some of these patients, providing the necessary gain may even degrade hearing (Vickers et al., 2001; Moore and Malicka, 2013). For these reasons, Nitro does not offer fine tuning in the highest frequency channels. The fine tuning of gain is divided across sixteen frequency gain channels, with the last channel including frequencies of up to 5.5 khz (see Figure 7). Frequency compression (FCO) Fig 7. Fine tuning of gain over sixteen channels in Nitro micon. For the More Compressive strategy, the candidacy and the parameters of the FCO feature are chosen as a function of the highest audible frequency, the probability of dead regions and distribution of highfrequency speech cues (Serman et al., 2013). The assumption behind this decision is that patients with this type of hearing loss are most often experienced users, fully acclimatized to the sound of their hearing instruments (both the WDRC compression and the overall loudness) and are therefore able to concentrate on changes in sounds caused by the frequency compression. These patients, therefore, are the group with the greatest probability of deriving benefit from frequency compression. If the More Linear strategy is chosen, FCO will be switched off by default. The rationale for this decision is that patients with a history of linear amplification are fully acclimatized to the linear sound of their hearing instruments and prefer as little sound interventions by the hearing instrument as possible. Studies show that the acclimatization to different gain settings and a change from linear to standard compression both require considerable time (Keidser et al., 2008; Convery and Keidser, 2011). Equally, frequency compression requires acclimatization of its own (Glista et al., 2012). When More Linear fit is chosen, the decision as to when such a patient may be ready for FCO is left to the hearing care professional. In case the hearing care professional decides to try out the FCO, individual FCO parameters for this patient will be calculated and applied automatically by the Connexx fitting software. 7

8 The micon platform allows for processing of the input signal up to 12 khz. As already mentioned, for this group of hearing losses, it is important to provide as much gain as possible in the low frequencies, and for this reason, the receiver is tuned to the lower frequency output. Consequently, when FCO is switched off, Nitro provides output up to approximately 6 khz (see Figure 8, left panel). When the FCO is switched on, a wider input signal range (of up to 11.5 khz) can be used and compressed to lower frequencies. In this way, the full frequency range of the input signal potentially becomes audible for the hearing instrument users who are candidates for FCO (see Figure 8, right panel). Fig 8. The More Compressive Nitro micon fit for a profound hearing loss (shown in Figure 4). Left Panel: FCO switched OFF. Right Panel: default FCO ON parameters for the same audiogram. The light shaded area represents the destination of the frequency compressed range. The dark shaded area contains no signal after frequency compression. SoundBalance Because of the importance of low frequencies for the patients with severe and profound hearing loss, Nitro allows the patient to directly modify gain in the low frequencies individually via the SoundBalance control (see Figure 9). This option is available with the rocker switch on the instrument or with different types of remote controls. The maximum gain range can be set to +/- 16 db. The SoundBalance gain is effective in the frequency range of up to 1400 Hz with the strongest effect at around 400 Hz. Basic Tuning Fig 9. Connexx user interface for activating SoundBalance. The Basic Tuning menu addresses the most frequent programming changes encountered by the hearing care professional. It offers several options to make hearing instrument adjustments faster and more efficiently. For example, due to their severely reduced dynamic range, severe to profound hearing loss patients often require a precise individual adjustment of the compression parameters. To facilitate this need, the Basic Tuning options for Nitro allow for a precise yet simple less and more compression adjustment (see Figure 10). 8

9 Fig 10. Basic Tuning options for severe to profound hearing loss in Connexx fitting software. Standard adjustments available include master gain, loudness and sound quality (grayed out). The new Basic Tuning option for Nitro comprises two compression buttons (on the left is the more Linear and on the right the more compressive button). The resulting compression effects can be observed in Figure 11, through changes in differences between the hearing instrument simulation gain curves for different input levels. Pressing the compression button on the left of the Basic Tuning menu will decrease the CR and thus address the problem of soft sounds being too loud and loud sounds being too soft (see Figure 11, middle panel). The right button increases the CR, thus addressing the problem of loud sounds being too loud and soft sounds being too soft (see Figure 11, right panel). Pressing the buttons 2-3 times will increase the strength of the desired effect (less or more compression). Fig 11. Left panel: Nitro default fit (target gain curves are in red, hearing instrument simulation curves are dotted and black). Middle panel: changes in the default fit after pressing the more Linear button. Right panel: changes in the default fit after pressing the more Compressive button. Microphone modes Recent studies show that severe to profound hearing impaired listeners can indeed benefit from directional microphones in difficult listening situations (Ricketts and Hornsby, 2006; Gnewikow et al., 2009). In fact, directional microphone technology is the most efficient way available in hearing instrument processing for separating the speech signal from background noise and thus compensating for the reduced frequency selectivity in hearing impaired patients. All standard directional microphone systems allow for less gain in the low frequencies than provided with omnidirectional microphones (Chalupper and Wu, 2011). For the patients in the severe to profound hearing loss group, the reduction of gain, especially in the low frequencies, may be detrimental for speech intelligibility. Furthermore, it may substantially decrease their general comfort and acceptance of the amplified signal (Gnewikow et al., 2009). The standard way hearing instruments compensate for this disadvantage is by adding gain in the low frequencies. This approach, however, is constrained on one side by the maximum available stable gain of the hearing instrument and on the other by the fact that this amplifies low frequency microphone 9

10 noise, particularly in soft or moderately loud situations, which may again decrease the patient s satisfaction with the instruments. One of the advantages of the new Nitro hardware design is that it provides an automatic microphone mode with up to 76 db of coupler gain. Combined with the adaptive directional beamformer (Fischer et al., 2013) the Nitro automatic mode thus offers severe to profound hearing impaired patients the benefits of changing between the omnidirectional and directional mode without experiencing a loss of gain or an increase in microphone noise. Additionally, in order to improve the front-back localization in the omnidirectional mode within the automatic program, micon platform offers the TruEar feature. It is known that a healthy auditory system uses differences in spectral shaping of the sound caused by the torso, the head and most importantly the pinna for vertical plane localization (Langendijk and Bronkhorst, 2002; Van den Bogaert et al., 2011). The TrueEar feature mimics this effect by spectrally shaping the hearing instrument gain for sounds coming from the rear (Chalupper et al., 2009), resulting in a more natural spatial impression of the omnidirectional mode within the automatic program. For more details about the micon high-frequency resolution directivity and directional speech enhancement, see Fischer et al. (2013). The steady sound of Nitro micon Our experience with severe to profound hearing impaired patients is that they often prefer steadier sound (fewer and smaller sound changes and less signal processing) than less hearing impaired patients. This implies that they often prefer less automatic adjustments from the hearing instrument. Several smaller adjustments were therefore introduced for this hearing loss group. For example, the default setting for noise reduction in Nitro renders it active only when a speech in noise situation is detected (see Figure 12). The same rationale also explains why Sound Equalizer, the feature by which frequency shaping is modified depending on the detected acoustic situation, is switched off by default for Nitro. Fig 12. Speech in Noise Management for Nitro micon is switched on only when speech in noise is detected. Additionally, the threshold of switching from omnidirectional to directional microphone in the automatic mode is set to high for this group of patients. Study Results The most recent study with Nitro micon was conducted across the USA, Australia and Europe in 15 hearing care centers. Altogether, a total of 38 patients with severe to profound hearing loss were fitted with Nitro micon and evaluated after two weeks of home trials. The average age of the patients was 62 years with the average hearing loss as shown in Figure 13 (Left panel). 10

11 Fig 13. Home trial study with Nitro micon instrument. Left panel: averaged left and right ear audiogram of the subjects taking part in the study. Right panel: overall satisfaction with Nitro after two weeks of home trials. Bearing in mind that this group of patients were experienced hearing instrument users and were thoroughly acclimatized to the loudness of their own hearing instruments, the overall satisfaction with Nitro instruments was impressive: after short fine tuning and two weeks of wearing the hearing instruments, of the 36 patients who completed the study, 36% judged the overall satisfaction with satisfied and 47% with very satisfied marks (Right panel in Figure 13). The hearing care professionals from the 15 hearing care centers were also asked for an evaluation of Nitro in terms of their own impression and recommendation to their patients. The marks given to the instrument are shown in Figure 14. Given the intricacies and problems related to this type of hearing losses mentioned in the introduction, it is encouraging that a great number of these hearing care professionals were satisfied with Nitro and would recommend it to their clients. Fig 14. Satisfaction with Nitro micon (left panel) and its potential recommendation (right panel), evaluated in 15 hearing care centers across three continents. Conclusions Fitting hearing instruments to the severe to profound hearing impaired patient group is one of the most demanding challenges in the field: these patients are completely dependent on their hearing instruments and consequently, both through the physiology of the hearing loss and the long wearing times, are extremely sensitive to their hearing instrument performance. Often, a lot of patience is required both on the side of the hearing care professional performing the fine tuning and the patient acclimatizing to the new instruments. Our study results indicate that with a combination of advanced hearing instrument technology and wise fitting decisions, Nitro can offer a very successful solution for a large number of patients with severe to profound hearing loss. 11

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