Insio: A new standard in custom instruments

Transcription

1 Insio: A new standard in custom instruments Abstract: Insio custom hearing instruments introduce novel features throughout the complete fitting process in order to obtain the best possible instruments: the best physical fit, together with the best audiologic performance, while maintaining an attractive and discreet cosmetic solution. This article describes the newly introduced features, their relevance for the various process steps, as well as the underlying rationales. Matthias Froehlich, Ph.D.

2 Insio: A new standard in custom instruments 1 Custom instruments pose specific challenges to the hearing care professional as well as to the manufacturer. As individualized products, they require a careful selection of the different parameters throughout the fitting process from the counseling of the potential wearer, to the meticulous construction, and to the fitting of the instruments. The ultimate goal is to achieve the best physical fit and the best audiologic performance, while maintaining an attractive and discreet cosmetic solution. The hearing care professional and the manufacturer must work hand-inhand to make this happen. Siemens provides specific tools to integrate the various stages of the custom hearing instrument selection and fitting into one seamless process. And in addition, the fitting steps are aligned with the internal manufacturing processes to guarantee the highest quality of the final product. This paper describes the rationales behind the key elements to a successful fitting with the new Siemens Insio custom hearing instrument family. Introduction In the hearing instrument fitting process, every single step contributes to the final success: from the initial counseling and instrument selection up to the final fitting and fine tuning. Fitting custom instruments poses specific challenges for a variety of reasons. For example, some patients request to be fitted with the smallest custom instruments available, even if this increases restrictions in other aspects, such as shorter battery life, reduced stable gain and output, or less directivity. As a result, the hearing care professional has to start the fitting from an even more challenging base. But on the other hand, these same patients often have high expectations for benefit and satisfaction. These opposing factors require increased attention to every detail to ensure these expectations are met. Often, complications with custom fittings are caused by the fact that there are fewer possibilities to modify the acoustics of the custom instruments once they have been delivered. The success of the fitting frequently depends on the hearing care professional s expertise to find the highly individualized balance between a plugged sensation and the occlusion effect (vent too small), and acoustic feedback (vent too large). We know however, that both feedback and the occlusion effect can lead to hearing instrument rejection. It is very difficult to predict exactly who will suffer from the occlusion effect for a particular vent diameter, and this effect can vary by 15 db or more at different frequencies and among different patients for the same venting scheme (see [Mueller et al. 2014] for review). In a custom instrument, when the occlusion effect is perceived, there is usually no way to enlarge the vent. On the other hand, more open fittings lead to increased feedback risk, which, if it occurs regularly, will reduce hearing instrument use, or even may render the hearing instruments unacceptable to the user. The occurrence of feedback is directly related to the distance between the microphone port and the source of sound leakage. In custom instruments, this may be only a few millimeters. While the risk of feedback can be reduced in many different ways [Froehlich 2012], only few are within the control of the hearing care professional when fitting custom instruments. For example, dampers or inserts may offer the possibility to decrease the vent size to counteract feedback, but these do not work for non-cylindrical vents. This in turn means that the more modern vents with varying cross-sections along the length of the vent which allow for smaller instruments cannot be ordered if the fitter wants to keep the option of modifying the vent size. Additionally, the vent size determines the amount of low frequency amplification that effectively reaches the ear drum. The larger the vent, the more power is lost through the vent-out acoustic leakage. The acoustic characteristics of the vent-out effect can be easily described by the so-called corner frequency that indicates the beginning of the low frequent roll-off. The fitter has to consider this corner frequency as well when selecting the correct vent in order to have enough amplification head room for any current or future low-frequent gain adjustments. That is, we would not want to push to instrument to maximum gain at the initial fit. Finally, even if the hearing care professional has carefully determined the most appropriate vent size, and requests it when ordering the instruments, it may not be possible to build the particular vent for the requested shell type, given the particular ear canal anatomy. This invariably leads to frustration for both the user and the hearing care professional. All things considered, fitting custom hearing instruments is particularly challenging. As a result, the fitting and verification tools have to be optimized and adjusted to the particular characteristics of custom instruments. For this reason, on the software fitting screen, it is desirable to have a reliable representation of the hearing instrument behavior to efficiently adjust it to the needs and preferences of the patient. This can only be achieved, however, if the acoustic simulation for a particular hearing instrument setting includes all the relevant acoustic aspects.

3 Insio: A new standard in custom instruments 2 The Insio solution The new Insio custom instruments address the various challenges that we have outlined in a comprehensive system, where all parts work seamlessly together. All components used throughout the process are built on the same fundamental concepts. The following sections explain the various tools to make fitting Insio instruments a success. Selecting the right vent size The selection of the most appropriate vent is crucial for the ultimate success of the fitting. The risk of occlusion must be carefully balanced against the risk of feedback. Insio, the custom instrument line based on the new micon platform, offers the micon Feedback Canceller. This system is one of the most advanced feedback cancellation system available in the industry [Froehlich 2012, Powers & Beilin 2013]. While hearing care professionals familiar with the micon behind-the-ear (BTE) and receiver-in-the-canal (RIC) instruments may have learned to determine the appropriate vent size which balances user acceptance and effective amplification without feedback, when it comes to custom instruments, even experienced fitters may feel occasionally uncertain about the choice of a particular vent. To take the guess work out of vent selection, Connexx 7.1, the fitting software supporting the new Insio instruments, features a novel vent proposal which is optimized separately for each shell type, and makes the most out of the instrument properties and functionality. Vent effects, equivalent acoustic mass (EAM), and corner frequency The acoustic properties of the vent are not a simple matter. There are two main aspects to consider. First, the ventout effect describes the sound that leaks out through the vent and does not arrive at the eardrum (see Fig 1). These are usually low frequency sounds below a cut-off point we call the corner frequency. Generally, the higher the corner frequency, the greater amount the amplified sound is lost vent out [db] mm 1.6mm 2.5mm 3.0 mm Open frequency [Hz] Fig 1: Vent-out effect for various vents.

4 Insio: A new standard in custom instruments 3 The reverse situation, called the vent-in effect, describes the direct sound entering the ear through the vent (see Fig 2). Following the same physics as the vent-out effect, the lower the frequency, the more sound that can enter the ear trough the vent. And, as you would predict, the larger the vent, the more energy can enter at any fixed (low) frequency. This effect plays an important role for hearing losses with rather normal low-frequency hearing, such as mild or ski-slope hearing losses where there is little or no low-frequency amplification prescribed. We know that for these patients, providing unamplified signals in the lower frequencies through the vent improves sound quality. As soon as significant amplification is needed and provided by the hearing instrument, however, the vent-in effect can be ignored, as it no longer plays a role in the overall ear canal SPL vent in [db] mm 1.6mm 2.5mm 3.0mm frequency [Hz] Fig 2: Vent-in curves for different vent sizes. Both the vent out and vent in effects depend on the particular size of the vent. But what does size actually mean? It is not simply the diameter; otherwise, a long and a short shell or ear mold would behave in the same way acoustically. Acoustic theory uses the term acoustic mass to describe the acoustic effect of sound passing through a tube. The acoustic mass is proportional to the length of the tube, and inversely proportional to the crosssectional area : ~ (see Appendix for more details). This simple tube with uniform cross-section can be generalized to a vent of any shape. Its acoustic properties are described by the equivalent acoustic mass (EAM). The EAM takes into account the precise geometrical dimensions of the vent, but does not depend on its particular shape as such. As direct consequence, the above mentioned corner frequency is a function of the EAM. Or, conversely, the corner frequency can be calculated for each shell type (using the average vent length for each type) for any vent diameter. It can be easily recognized in the full-on-gain view of Connexx (see Table 1 and Figure 3). (Hypothetical) vent diameters Corner frequency (Hz) ITE (mm) HS/MC/CT (mm) CIC (mm)

5 Insio: A new standard in custom instruments Table 1: Relationship between corner frequency and (hypothetical) vent size (diameter) for different shell types based on the equivalent acoustic mass. 283Hz 566 Hz Fig. 3: Examples of Connexx for 1.3mm vent (left) and 2.5mm vent (right) in the Real-ear-insertion-gain (REIG) view. The corner frequency value can be recognized in the Full-on-gain (FOG) curve as the frequency where the maximum gain begins to roll off towards the low frequencies (indicated by the dotted blue lines). Vent proposal The micon vent proposal is based on the concept of the equivalent acoustic mass. For any given EAM and hearing instrument type, Connexx calculates the appropriate vent diameter. The particular hearing instrument type provides the necessary input for the vent length. As a result, the hearing care professional easily can determine the best recommendation for the most appropriate vent for the selected micon product. But how is the appropriate EAM determined? This algorithm is the heart of the micon vent proposal, and is derived from the characteristics of the selected hearing loss and prescribed gain targets. The low-frequency amplification determines the minimum vent size based on the particular corner frequency. This minimum vent allows for sufficient head room of gain also in the low frequencies for any later fine tuning. Additionally, it minimizes the risk of the occlusion effect for the given hearing loss and custom instrument type.

6 Insio: A new standard in custom instruments 5 In a second calculation, the prescribed gain in the frequency region most important for speech intelligibility determines the maximum vent size before feedback is likely to occur. This limit of course depends heavily on the exact performance of the micon Feedback Cancellation, which is also utilized by the vent selection algorithm. Having determined the limits of the minimum and maximum possible vent size, the algorithm combines all the information to provide the recommended EAM value. This value, in turn, is converted to an easier-to-interpret ventdiameter recommendation. This way, the hearing care professional already knows the recommended vent size when he or she simulates the different Insio options in Connexx prior to ordering the actual instruments. One only needs to enter the audiogram, select the Insio instrument, and the recommended vent characteristics are displayed. Of course, it is also possible to simulate the resulting acoustic properties when using different vent sizes. OptiVent Simulating the First Fit gain and output with the recommended vent prior to ordering the actual instruments is only one way to ensure the best result. Another novel feature introduced with Insio custom instruments is the OptiVent, which can be used together with the vent proposal, or as an alternative. The vent size does not only affect the acoustics and therefore the audiologic performance of the custom instrument, a larger vent may also result in a larger instrument, which may be less acceptable to the wearer. In order to overcome this potential problem without extra hassles for the hearing care professional, Insio instruments feature the new OptiVent option (see Fig 4). Fig 4: Conventional vent (left picture) and OptiVent (right picture), for the same ear impression, printed using the same scale. OptiVent can be used in two alternative ways. In the first, only the audiogram needs to be provided together with the OptiVent choice on the ordering form. The optimum vent size will then be calculated on the basis of the EAM, resulting in the smallest instrument possible providing the necessary acoustic properties. In other words, this recommended use of OptiVent incorporates the Connexx-calculated vent proposals without the necessity of any intermediate steps. This way, the full potential of micon audiology and the micon platform performance is guaranteed with the least effort for the hearing care professional. In the second case, there may be instances when the fitter wants to order a certain vent size, based on previous simulations or on his or her personal expertise and experience. He or she now selects the requested size on the order form, additionally checking the OptiVent option (e.g., 1.6mm OptiVent). The Insio instrument will then be built as small as possible with a vent that is acoustically equivalent to a 1.6mm uniform vent for the selected shell type. So rather than sticking to the old-fashioned uniform vents and building a vent of uniform 1.6mm diameter, the audiologic rationale behind the selection of this vent is implemented, resulting in the requested audiologic performance, but in most cases a smaller vent which means a smaller instrument. In other words, we no longer have to compromise between good hearing and good looks. Technically, this is realized again by using the concept of the EAM. First, the requested vent diameter is translated to the corresponding EAM. The EAM completely describes the acoustic behavior of the vent. Therefore, the precise vent

7 Insio: A new standard in custom instruments 6 dimensions can now be optimized in the modeling software to obtain the specified EAM according to the particular shell geometry. This way, the size of the final instrument will be as small as possible without compromising the performance of the instrument according to the explicit request of the hearing care professional. While conventional uniform vents will still be available with the Insio instruments, in general, it is highly recommended to order OptiVent using one of the two described options for maximum performance and minimum size of the final instruments. Fitting Insio instruments But the ease doesn t stop with the ordering the next step to success is the fitting process. Once the new Insio instruments arrive, the First Fit algorithm can be applied based on the individual s audiogram. As with the BTE family of instruments, Connexx provides the full range of tools to easily fit, verify, and fine tune the prescribed gain for the Insio custom products. For Insio instruments, the micon First Fit formula has been optimized for maximum Soundability. This is achieved by careful modeling the particular acoustics applicable for the selected instrument type. A particular focus has been placed on the microphone location effect (MLE). This curve contributes significantly to the overall acoustic performance. (See Mueller and Hall for more information on MLE.) Consider that as the microphone is placed deeper in the concha, the characteristics of the sound striking the microphone are altered, and these differences must be accounted for in the fitting software. In addition to the MLE, all other acoustically relevant factors, such as RECD, critical gain, etc., are also carefully measured for each instrument type and considered during the simulation. As we discussed previously, one key parameter for an adequate modeling of the acoustic performance of the instrument are the vent effects, which are determined by the EAM. For Insio instruments ordered with OptiVent, the correct EAM is stored in the instrument, and read out by Connexx for the correct acoustic modeling. For Insio instruments ordered with a conventional vent, the vent size is also stored in the instrument and automatically utilized by Connexx. In case of vent inserts, the hearing care professional may change the vent size in Connexx to adjust the acoustic model underlying the simulation, and store the selected vent size in the fitting session. This way, the correct vent size is always reflected in the simulation, providing the solid foundation necessary for a good fitting and fine tuning. The fitting of the Insio instruments itself follows the same rationales and workflow as the BTE instruments [Fischer et al 2012]. This way, fitting all micon instruments is an easy and consistent process.

8 Insio: A new standard in custom instruments 7 Conclusion The Insio custom instrument product line combines proven micon functionality with dedicated innovations for custom instruments. In particular, the new OptiVent options, either specifying the equivalent conventional vent diameter, or leaving optimization entirely to the manufacturer s expertise, provide a simple means to optimize the instrument in four dimensions simultaneously: minimum occlusion effect, sufficient head room in the low frequencies, minimum feedback risk, and smallest size possible for the selected shell type. The underlying principle of the equivalent acoustic mass powered algorithm provides a seamless progression throughout the complete ordering, manufacturing, and fitting process.

9 Insio: A new standard in custom instruments 8 Appendix The acoustic mass is often also referred to as acoustic inertance. It describes the acoustic properties of a volume of air inside an enclosure that is open at both ends, such as a tube or tube segment. The enclosed air vibrates when driven by a time varying pressure meaning incoming sound at one of its ends. The mass of the air enclosed in the volume is simply given by =, with being the density of air, and the length of the tube with the uniform cross-sectional area. However, the acoustic mass is described by a different equation: =. So while both the mass and the acoustic mass are directly proportional to the length of the tube, the acoustic mass is inversely proportional to the area = ( being the diameter). Thus, the acoustic mass is influenced to a higher degree (power of 2) by the vent size (diameter) than by the length of the vent. For further reading see e.g. [Wolfe 2013].

10 Insio: A new standard in custom instruments 9 Citations Fischer R-L, Pape S, Giese U, Dressler O (2012): micon fit: Striking the balance between sound quality and audibility. Siemens Global/publications Froehlich M (2012): micon Feedback Cancellation. Siemens Global/publications Mueller HG, Bentler RM, Ricketts TA (2014). Modern Hearing Aids San Diego: Plural Publishing. Mueller HG, Hall JW (1998). Audiologists Desk Reference Volume II. 1998: San Diego: Singular Publishing Group, Inc. Powers T & Beilin J (2013): True Advances in Hearing Aid Technology: What Are They and Where s the Proof? Hearing Review, Jan 2013 Wolfe J (2013). Physclips Vol II.: Sound and Waves University of New South Wales, Sydney.