www.bestsound-technology.com binax fit: How it s simulated in Connexx, how to verify it, and how to match-to-target the easy way Richard Schultz-Amling Sivantos, 2015.
1 Introduction Since the release of the Siemens micon platform and Connexx 7.0, Siemens has introduced a completely new fitting strategy which is also utilized in the latest binax platform. The core of the fitting algorithm is a new compression scheme which is different than traditional syllabic, slow-acting, or dual compression. The background of the earlier micon fit has been explained and discussed in two white papers (e.g., Fischer et al., [1] and Menard [2]). With this new fitting strategy, it is meaningful to discuss the use of real ear measurements (in-situ measurements), to verify the fitting in a patient s ear. While there are numerous manufacturers of probe-microphone equipment, this paper will focus on the Unity 2 measurement system and Unity 4 software. An automatic fitting tool is also available since Connexx 7.1 and can be used with the Unity probe-microphone system. This is referred to as AutoFit. This special tool allows for an automatic adjustment of the hearing aid input-specific gain, which will reliably provide a good match-to-target in the individual patient s ear. This paper provides background information regarding important attributes of micon fit (which is now succeeded by the binax fit and will therefore be referred to as binax fit throughout the rest of the paper). Insights will be given into Connexx (CXX) hearing instrument simulation and what is important to know before judging a match-to-target. Then, the AutoFit measurement tool in CXX is highlighted with more details on how this feature can help you in fitting verification. Last, we discuss the recently introduced percentile analysis and how this relates to the other match-to-target measurements discussed in the previous chapters. 2 Review of binax compression With the Siemens micon instruments, a new compression scheme was introduced. The underlying principles are now included and further optimized within binax fit. The core of this scheme is the two compression knee points and the adaptive time constants. The individual knee points allow for adjusting the gain independently for all input levels ranging from soft to loud. The adaptive time constants are designed such that the compression acts slowly in case of small level changes. This is the case for example, for listening to speech in quiet. The result is a very smooth sound impression comparable to a linear setting. On the other hand, the time constants act fast if the input to the instrument has rapid fluctuations. This can occur, for example, when listening to speech in noise, when the noise has considerable fluctuations. A more detailed description of the exact behavior of the time constants can be found in the papers Fischer et al., [1] and Menard [2]. 3 Simulation vs. measurement Prior to conducting (real-ear) measurements with hearing aids, it is important to have a reference. The reference for measurements with Siemens instruments is the CXX software and the curve view window. However, there are several views to choose from. In the following sections, we discuss these different views, and how they relate to different test signals. 3.1 Background on hearing instrument simulation in Connexx To fit a hearing aid properly to a given hearing loss, we first need a target. For all formulas available in Connexx, the targets are defined as real ear insertion gain (REIG) targets for three different input levels, mostly 50, 65 and 80 db. It is however also important to know, for which stimulus a target was defined. For traditional formulas, such as the NAL formulas, the target curves are based on the LTASS (long term average speech spectrum) stimulus (more background on stimuli is given in section 3.2). For binax fit the stimulus is a differ-
ent one and is not displayed in the curve window. The reason for this will be explained throughout this chapter. To reach the gain targets, CXX must calculate the compression parameters that are programmed into the hearing instrument, namely the kneepoints, the compression ratio, and the gains in multiple channels. This calculation is very dependent on how the input signal of the original target was set. Also, there are various assumptions about the average human ear, as for example, the real ear unaided gain (REUG) used inside the first fit is an averaged one. Once the first fit calculation is done, a simulation is run to show the actual result how well the target is met by the hearing aid. In CXX, this result can be displayed in many ways. For example, one can look at the output as it would be using a 2-cc coupler with a pure tone stimulus (see Figure 1, left side) or the calculated REIG for an LTASS input (see Figure 1, right side). The displays are correct for the average human ear. The match-to-target on an individual ear may be different and should be verified by means of an in-situ measurement. First, it is important to understand, that the simulation and the target curves change with view mode in CXX. The compression parameters, however do not change, but are fixed after the first fit calculation. The hearing instrument will of course react differently to different input sounds. CXX therefore re-calculates not only the simulation curves for a different stimulus, but also the targets are recalculated to show how the targets would look like for this particular input signal and measurement setup. The match-to-target does not change with different views, but is only dependent on the number of compression channels of the hearing instrument. Figure 1. Different view modes in CXX for target and simulation curves. The fitting is the same. Left panel: output response of a 2-cc coupler with a pure tone input signal. Right panel: REIG for LTASS, which is the standard view for traditional formulas in CXX. For the proprietary fitting formulas, micon and binax fit, CXX does not use the LTASS to set the compression parameters for the hearing aid. The binax compression is a fast adaptive system [1, 2], and is not fully described by a static signal such a LTASS. Therefore a special signal was defined to allow the best description of the dynamic behavior within the given static context. The signal represents a spectrum which was not measured with a long averaging (125 ms) as done for LTASS but on a short term basis thus being more of a short term speech spectrum. For example, a consonant at the end of a word followed by a speech pause would result in a different spectrum averaged over a period of 125 ms used by the LTASS method than if we used a time window of 10 ms. The LTASS averaging method will show a much lower energy in the high frequencies, where most of the energy for consonants fall into, than the short term averaging method. Therefore, a short term speech spectrum is more accurately represented by pink noise, having a more even distribution of energy and thus more energy at high frequencies than the LTASS. This is why CXX uses pink noise as a standard stimulus in the curve view window for the binax fit. Another reason for the use of pink noise for the display in CXX is the level dependent gains: the kneepoints are optimized for micon and binax compression based on the short term speech spectrum. If one wants to change the level dependent gains in the fine tuning page independent of the three levels, pink noise is the best choice as the kneepoints are set approximately to pink noise. If one chooses LTASS, this independence may not be reflected anymore: the lower energy of LTASS compared to pink noise at high frequencies leads to a misinterpretation of the compression arm. For example, below the first knee point, linear gain is applied. Above the kneepoint, the instrument typically acts compressively. So with respect to the knee points, for LTASS, the device would show a linear behavior whereas pink noise shows a compressive behavior at those high frequen-
cies. The LTASS simulation is still correct, but for the level dependent gains the expected independence is not apparent. 3.2 Test signals and their spectral shape In the last section, it was mentioned already that the fitting and targets depend on the input stimulus. To make it even more clear, this section gives a more general overview about test signals. For quality control purposes, for decades it has been common to test hearing aids in couplers. While this is sufficient for many technical measurements, measuring in-situ (real ear) output is slightly more complicated, as it involves a real human ear and thus an individual coupling of the instrument s receiver to the ear. The goal of an in-situ measurement is of course to verify that the prescribed target gain is present in the ear canal of the patient. While this could be done with sinusoidal signals, these signals do not represent the most common signal a hearing aid is designed to amplify, namely speech. When referring to different speech signals, it is common to describe the signal based on the Long Term Average Speech Spectrum (LTASS). A comprehensive study of the LTASS of different speech signals was published by Byrne et al., [3] in 1994, the result of which was the development of an International speech spectrum, referred to as the ILTASS. Many test signals try to mimic the spectral and temporal behavior of real speech. Examples include: 1. LTASS noises (stationary noise spectrally shaped to match the ILTASS values from Byrne [3]) 2. IEC noise (similar to LTASS, but with higher energy in high frequencies) 3. ISTS (International speech test signal) The last signal mentioned, the ISTS, was designed by an EHIMA project [4] which started in 2006 with the intended goal to have a speech-like signal, which had the exact temporal distribution of real speech, while at the same time fulfilling the LTASS criteria defined by Byrne. It is comprised of six different languages (female voices), cut together in a manner that one is not able to understand a multi-syllabic word, but the signal still has the rhythm of real speech. While the ISTS indeed achieved the desired goal, there are some concerns when using this signal for verification, which we will discuss later. The main reason for the rising popularity of this signal was the IEC 60118-18 standard [6], finalized in 2012, (see section 5). Nowadays it is very often seen as a replacement of the stationary LTASS shaped noise, which it is not. Figure 2 shows different test signals presented with a Unity 2 (Important: set the Unity measurement method to third octave, as this is the common measurement method agreed upon in the IEC 60118 standards for HI measurements) and recorded with the tip of the probe tube microphone at the reference microphone position, thus recording the actual input signal. The stimuli were presented at 65 db SPL and the view mode is output view. If we used the gain view, we should see a flat line at 0 db for all signals as input and output are the same. The different ISTS were recorded with different measurement times and thus labeled: ISTS short: standard recording time of about 3 seconds for a single measurement in Unity 4 software in the in-situ / coupler measurement menus. ISTS long: pre play of the signal for about 45 seconds, total measuring time of about 50 seconds. The Unity LTASS shaped noise and pink noise were recorded for comparison.
Figure 2. Spectra of different stimuli presented by the Unity system. Probe tube tip was at reference microphone position. We see here that every signal produces a slightly different spectrum at the reference microphone. While pink noise is almost flat above 250 Hz, the other signals show roughly the shape that is known as ILTASS, except the short ISTS measurement. Notice that for this input, there is considerably less energy from 750 to 4000 Hz. This is because the recording time is only three seconds, and this duration of the ISTS does not fulfill the long term average criteria of ILTASS. The LTASS noise played back by Unity is a stationary noise, spectrally shaped to the ideal ILTASS. So for this signal, one gets the ILTASS shape from the very first second, while the ISTS only gets close to the shape after a long measurement time. (This is also one of the reasons the IEC 60118-15 standard recommends a measurement time of 60 seconds. This is, however, quite an unusual long time for in-situ measurements). Consider that different input signals could cause different opinions regarding the quality of the fitting. The obvious question then becomes: which signal is the most suitable for measuring micon or binax hearing aids to verify the match-to-target? In CXX, there is no view (or stimulus for simulation) with the short measurement time of ISTS as seen in Figure 2, only the standard ILTASS. In order to make a measurement as comparable to the CXX simulation as possible, the shapes of the spectra should match. An example for how this works for ILTASS is given in Figure 3. A comparison is made between the LTASS shaped stationary noise presented by Unity 2 and what is used for the Connexx simulation. So a first answer to the above question is: one cannot go wrong with LTASS shaped noise, as shown on Figure 3, the two spectra are nearly identical.
Figure 3. Comparison between ILTASS as used for simulation in CXX, and the recording of the LTASS shaped stationary noise presented by Unity for 65 db SPL. The recording was done with the probe tube tip at the reference microphone position. To take this one step further, Figure 4 shows what happens if probe-microphone testing is conducted for a hearing instrument programmed with micon fit, so the micon compression is active. The view chosen here is insertion gain, which shows the different reaction to the stimuli, even though the stimuli is actually subtracted already from the response (gain = output input). Figure 4, left panel shows the measured REIG for different input stimuli, namely the LTASS shaped noise, pink noise and two versions of the ISTS, differentiated by their measurement time. The target is displayed as the 65 db LTASS target taken from CXX (black line). As might be predicted from Figure 2, the LTASS stimulus resulted in the best fit to the LTASS target. The reason here is that the LTASS signal is stationary, thus independent from the length of the stimuli, as long as the instrument is in a steady state and in measurement settings. The pink noise measurement does not fit to the LTASS target at all above 1 khz, which can be already predicted from the different shape of the signals itself (compare Figure 2). The higher energy of pink noise at high frequencies compared to the LTASS results in a more compressive behavior in the hearing instrument. The results for ISTS for the REIG measurement are not so easily explained. From the input shape, one would have guessed that different measurement times lead to a different result, depending on what the dynamic micon compression is actually receiving as input. While this assumption holds true, the short measurement time is in this particular example closer to the target than the long measurement time. One of the pitfalls here is that just because the measurement time is averaged over 50 seconds does not mean the same thing for the highly adaptive compression inside the hearing aid. So the output of the instrument is more or less based on what was actually presented at the microphone just about some milliseconds ago. As there are more fluctuations in amplitude towards the end of the ISTS, the long averaging did not return a proper result. It is therefore not recommended to use the dynamic ISTS for verifying a fitting against static targets, especially if you are not certain what measurement time is the correct one. Figure 4 right panel shows the same measurement with pink noise and a pink noise target. The match-totarget on this individual ear is about the same as for LTASS in the left panel. And to complete the picture, the LTASS measurement does not fit to the pink noise target as well.
Figure 4. Comparison of real ear measurement plotted as REIG of a micon fitted hearing aid for different stimuli played back with Unity and for different target views. Left panel: The LTASS target (black line) is compared to an LTASS measurement as well as two IST signals (differing in length) and pink noise measurements. Right panel: A pink noise target (black line) is the reference compared against an LTASS and pink noise measurement. The measured curves are the same as in the left panel. To summarize this section, the choice of a measurement stimulus is a crucial consideration when verifying a fitting with real measurements. Only if your reference in CXX and in the measurement hardware is aligned, can you make an accurate judgment. Also the measurement time is something important, as spectra are differently perceived by the instrument and averaging during a measurement does not reflect what the instrument itself gets as an input. This is even more applicable for gain measurements, when the input spectrum is subtracted from the output, since the input the instrument received for the measurement time frame might be different. Therefore it is important to note that because of the instationary properties of the ISTS, one cannot compare the output of the hearing instrument to a static LTASS target curve in CXX. For in-situ verification, it is not ideal to use use long measurements anyway since it is uncomfortable for the patient to listen to loud test signals for extended periods of time. A drawback of micon or binax fit may be that it is harder to verify with any measurement equipment, as the targets are only displayed in CXX. To compensate, CXX provides a verification tool, which brings us to the next chapter. 4 Background on AutoFit in CXX There have been several questions about the approach behind AutoFit in CXX since its release with CXX 7.1. Some practical guidelines were given by Kindt [6]. First of all, before one can do an AutoFit, an individual real ear unaided gain (REUG) has to be measured. The REUG taken into account in the standard fitting procedure in CXX is an average REUG, so this real ear measurement is the first step towards an individual fitting. Remember, the targets in CXX are usually given as insertion gain targets. A measurement on the ear with hearing aids is a real ear aided gain (REAG) measurement. The relation is: REIG = REAG REUG.
Figure 5. The AutoFit dialog is available under the AutoFit tab, accessible on the bottom right beside the First Fit / Recalculate Fit button. In the AutoFit workflow, the individual REUG is already taken into account in the first fit, which is programmed into the hearing aid directly before the AutoFit measurement starts. The display in the AutoFit tab is REIG, so the targets are the same as in the REIG view in the main CXX view, if the stimulus is set to LTASS. (Remember, the standard view in CXX for micon and binax fit is pink noise as explained in section 3.1). The initial match-totarget is also not affected by the measured REUG. However, there may be more differences between the individual ear behavior and the simulation in CXX. This is due to the fact that CXX has no knowledge of the precise length of an earmold or the exact fitting of a dome in the ear. Therefore, the residual deviations are addressed by the actual AutoFit measurement, which determines the remaining gain deviations between the REIG target and the individual REIG achieved after first fit (Figure 6, left panel). This deviation is compensated by the recalculation of the channel gains. Once this is done (Figure 6, right panel), one should see a match-to-target in the AutoFit window that is equally as good as the simulation for the average ear. Of course, this is again dependent on the performance level and the corresponding number of channels in the hearing aid. A broader channel may not smooth ripples in the frequency response of the individual ear, resulting in a less precise match-to-target for this frequency region. While measuring with external software, you always have to remember to turn off all adaptive noise reduction parameters. AutoFit on the other hand, automatically programs the hearing aid to test settings. It also reverts the hearing aid back to the original settings afterwards. Figure 6. Left panel: Initial measurement of AutoFit before adjustment. The match-to-target is not yet precise. Right panel: After AutoFit, the measured insertion gain matches to the given target.
After the adjustment, a second measurement is completed, which runs immediately after the initial one. In the AutoFit window, the measurement curves are actually shown, which is why in the beginning a mismatch may be seen followed by a match-to-target after the adjustment. Once the AutoFit dialog is closed and the AutoFit result is accepted, the regular view is shown in the main window. Here it now looks as if the precise match-to-target is completely gone. The reason is that in this view, again the simulation of the new device setting is shown, not the measurement. So if there was for example more gain needed for a frequency region of 1 to 2 khz, the simulation curve is now above the targets. In fact, it is unlikely that an exact match would be shown at this time, as most real ears and coupling methods are not average. Figure 7. The AutoFit results got accepted, and now the simulation curves show the result of the new compression parameters. For lower frequencies, the gain is higher than originally prescribed, which might be due to an underestimated vent effect. 4.1 Why is AutoFit conducted with LTASS? After what we have discussed about targets and simulation stimuli, one might wonder why there is no difference between an AutoFit for binax fit and for NAL-NL2, as the REIG targets are defined for different stimuli. This is due to a number of reasons. 1. The stimulus underlying binax fit is not available in any measurement equipment. 2. LTASS shaped stationary noise is a stimulus that is used by a vast number of REM devices. 3. CXX can easily recalculate the binax targets to LTASS for display purposes. The underlying compression scheme is not changed through the recalculation. Similarly, the kneepoints are not changed after AutoFit. Of course, using the LTASS signal does not show the real dynamic behavior of the binax compression, this can only be shown with real speech or ISTS. But the purpose of AutoFit is to achieve an individual match-to-target to a static simulation.
5 Short introduction to percentile analysis and its difference to current fitting target verification on real ears Since 2012, there is a new recommendation for evaluating hearing instruments, namely the IEC 60118-15 standard [6], which is about a percentile analysis of the amplified speech signals. This new analysis tool has been incorporated in the Unity 4 and 5 Software and can be found in various other measurement tools under the name speech mapping. This section gives a rough overview of this new standard and especially, where it is different to the measurement methods from previous chapters. The main difference is that the analysis via IEC 60118-15 is recommended for 2cc couplers, while the focus of this paper was mainly on the in-situ verification on a real ear. Another difference is the measurement time and signal: instead of doing a short (stationary) measurement, a one minute analysis (discarding the first 15 seconds) of the instrument output is done, and the input signal is always the IST signal played back at 65 db. The analysis is then shown as 30%, 65% and 99% percentile curves. These curves, however, cannot easily be transferred to the static curves that are now shown in CXX as there are no percentile targets defined for any available fitting formula to this day. The meaning of the percentile curves is indeed different. While the static targets for any formula show how much gain should be applied for a given input stimulus inside the hearing instrument, percentile curves show how the input levels are statistically distributed after the compression and any signal processing at the output of the hearing instrument. For stationary measurements, it is highly recommended to turn off all adaptive parameters in the instrument, whereas percentile analysis considers the impact of signal processing as well. The underlying assumption is of course, that the IST signal is not degraded by a noise reduction algorithm anyhow. The 65 th percentile curve for example means that 65% of all measured output levels lie below this curve and the remaining 35% of the levels are above. The 99 th percentile can be interpreted as an indicator of the maximum output levels. The 65 th percentile curve is not to be compared to the 65 db IG gain target, as was shown in section 3.2: the spectral shape of the ISTS is not the same as the stationary LTASS shaped noise and the analysis times of the percentile measurement (which is set to 125 ms by the standard) takes into account a different level distribution than a compression algorithm like the highly adaptive binax compression. The percentile analysis is a useful new tool and can, for example, be used to verify if the subscribed gain really compensates the given hearing loss. This can be done by looking at the hearing threshold and the lowest percentile: if the lowest percentile curve is above the threshold, the patient should be able to fully understand speech in quiet; the highest percentile should not cross the UCL. You could also make an easy comparison between the input and output to see how the dynamic range is decreased: after compression, the area between the highest and lowest shown percentile becomes smaller. What the IEC 60118-15 measurements cannot do is replace an in-situ measurement to verify if the REIG targets shown in the fitting software are really met in the patient s ear. The main reason as has been explained already, is because ISTS is not a stationary signal, whereas current targets are static, and the measurement time has a very high impact on the outcome of the measurement. A nice summary of this limitation is given by the standard itself: Measurement methods that take into account the acoustic coupling of a hearing aid to the individual ear and the acoustic influence of the individual anatomical variations of an end-user on the acoustical performance of the hearing aid, known as real-ear measurements, are outside the scope of this particular standard. More tips and explanations of what you can do with the percentile analysis can be found for example on the website of the EUHA [7].
6 Summary This paper describes the connection between the simulation of a hearing instrument in CXX and real ear probe-microphone measurements conducted either with an external stand-alone measurement software such as Unity, or the CXX feature AutoFit. It was shown that different stimuli used for measurements lead to different output curves, and that they have to match the simulation in CXX to be comparable. As the underlying simulation in CXX is static, a highly fluctuating signal, such as the ISTS is to be used carefully for any kind of hearing instruments measurements, especially if short measurement times are selected. It is recommended to use a LTASS shaped noise provided in most measurement systems and in the CXX software to obtain optimal results. It is also described why binax fit defaults to pink noise in the standard curve view window, and why this allows better manipulation of the level dependent gains. The AutoFit feature in CXX is reviewed and explained. With this tool, any target formula available in CXX can be evaluated with probe-microphone measurements and even automatically compensate for any deviations that remain between the individual fitting and the average assumptions that are made by the fitting targets. Lastly, a comparison between the new IEC 60118-15 standard and the previously described measurement conditions are given. References [1] Fischer, et al. micon fit: Striking the balance between sound quality and audibility, Siemens White Paper, 2012. [2] Menard, et al. micon Compression and Connexx 7, Siemens White Paper, 2014. [3] Byrne, et al., An international comparison of long-term average speech spectra, JASA, vol. 96, no. 4, pp. 2018 2120, October 1994. [4] Inga Holube, Stefan Fredelake, Marcel Vlaming, Birger Kollmeier, Development and analysis of an International Speech Test Signal (ISTS), International Journal of Audiology, vol. 49,pp. 891-903, 2010. [5] Kindt, Marjolijn. How to Use AutoFit in Connexx 7.3. Siemens Publication, 2014. [6] IEC 60118-15, Electroacoustics - Hearing aids - Part 15: Methods for characterising signal processing in hearing aids with a speech-like signal, 2012. [7] http://www.euha.org/information/percentiles-study-group/ Legal Manufacturer Sivantos GmbH Henri-Dunant-Strasse 100 91058 Erlangen Germany www.bestsound-technology.com Sivantos GmbH is a Trademark Licensee of Siemens AG.