Implementation and analysis of international standard for electroacoustic performance evaluation of hearing aids

Transcription

1 Acoustical Measurements and Instrumentation: Paper ICA Implementation and analysis of international standard for electroacoustic performance evaluation of hearing aids Zargos Neves Masson (a), Eduardo Bresciani (b), Stephan Paul (c), Júlio A. Cordioli (d) (a) Federal University of Santa Catarina, Brazil, (b) Federal University of Santa Catarina, Brazil, (c) Federal University of Santa Catarina, Brazil, (d) Federal University of Santa Catarina, Brazil, Abstract Hearing aids are the most common device used by hearing impaired people. The hearing aids electroacoustic performance is of utmost importance for hearing care professionals to properly choose and adapt the device for a particular individual. Harmonized methods for determining the electroacoustic characteristics are fundamental to allow comparison between different hearing aids. Therefore, the International Electrotechnical Commission has published a series of standards giving recommendations for hearing aids electroacoustic measurement procedures. This article presents an analysis of the implementation of the standard IEC : Measurement of the performance characteristics of hearing aids. This standard is used to obtain results for manufacturer data sheets. First, a test setup validation was conducted by comparing the results with those measured by an accredited laboratory for a reference hearing aid. After that, investigations were made to analyze the influence of different measurement aspects, like hearing aid positioning, presence of a control microphone at the test space and differences between an ear simulator and a 2CC coupler. Comparisons with the reference showed good agreement between the results considering the respective uncertainties. The use of a 2CC coupler in IEC was considered the better approach from the perspective of the standard purpose. The influence of the control microphone position and presence was found to be minimum in all investigations made. Keywords: Hearing aids; Electroacoustic; Measurements.

2 Implementation and analysis of international standard for electroacoustic performance evaluation of hearing aids 1 Introduction Hearing aids (HA) are the most common device used by hearing impaired people. Generally, an audiologist will adapt a hearing aid in order to amplify sounds so that the sound pressure level (SPL) produced remain inside the range that a particular individual is still capable of hearing. The electroacoustic performance plays an important role in this process, and hearing care professionals need to know how the hearing aid performs to properly choose and adapt the device to the patients hearing loss. Harmonized methods for determining the electroacoustic characteristics are fundamental to allow comparison between different hearing aids. Therefore, the International Electrotechnical Commission has published a series of standards giving recommendation for hearing aids electroacoustic measurement procedures under the number IEC The first part of the series is entitled Measurement of the performance characteristics of hearing aids [1] and standardizes the procedures using a free-field technique to obtain results for manufacturer data sheets, which are baseline information for clinicians. The measurement procedure is defined in such a way as to be practical and reproducible, and is consequently based on fixed parameters. Recently, the IEC standard went through a revision and significant technical changes with respect to the previous edition were made. One of the changes was the substitution of the acoustic coupler used. In previous editions the Occluded Ear Simulator 1 was used [2], while the current version of the IEC standard recommends the 2CC Coupler 2 [1]. The 2CC coupler is intended for loading a hearing aid with a specified acoustic impedance, but not to model the sound pressure in a subject s ear canal. It is therefore important to note that the performance measured under test conditions may deviate substantially from the performance of the hearing aid in actual conditions of use. This paper will focus only on presenting results and analysis for the acoustic frequency response 3 procedures obtained by means of the hearing aid microphone as the inlet method provided by the standard aforementioned. First, a measurement setup validation, that used a reference results issued by an accredited laboratory, will be discussed. An investigation of different aspects that influence the measurements results, like test arrangement layout and test space, is carried out subsequently. 1 Described by IEC [3], referred only as ear simulator throughout the paper. 2 Described by IEC [4]. 3 Every mention of frequency response in this paper should be interpreted as sound pressure level frequency spectrum. Nomenclature used by IEC [1]. 2

3 2 IEC Standard 2.1 Measurement Setup IEC requires the measurement to be carried out in a test enclosure with low background noise level (10 db below the lowest SPL generated during tests) that allows the simulation of free-field conditions by maintaining a constant sound pressure level at a reference point using a control microphone. In the present work, measurements were carried out in a hemi anechoic chamber with working dimensions of 7 m x 7 m x 4 m with a test assembly as shown in Figure 1. The chamber s floor was covered by acoustic edges in order to minimize reflections. Overall background noise measured at the test point was below 30 db, complying with the requirements established by the standard. (a) Assembly of the test in the chamber (b) Omnidirectional HA. (c) Directional HA. Figure 1: IEC test arrangements As seen in Figure 1, the axis of the control microphone shall be orthogonal to the speaker axis and shall intersect it at the reference point of the hearing aid. The reference point is located at the center of the inlet microphone port, or midpoint of the microphone port array, marked in red in Figure 1b and 1b, respectively. A laser leveling system was used to aid positioning. 2.2 Uncertainty Analysis Sound pressure level measured in the acoustic coupler, L p,coup, is calculated by L p, coup = L V + L s + δ T + δ P + δ H + δ coup + δ cal + δ Tub + δ Ass + δ Lp,ref, (1) being L V the voltage level db re.1v acquired by the acquisition board, L s is the microphone sensitivity db re.1v/pa, δ T is the correction associated with variations in temperature, δ P is the correction associated with variations in ambient pressure, δ H is the correction associated with variations in humidity, δ coup is an input quantity to allow any errors caused by the coupler, δ cal is an input quantity to allow any errors caused by the calibration check procedure, δ Tub is an input quantity to allow any errors caused by the tube connecting hearing aid and coupler, δ Ass is an input quantity to allow any errors from the assembly, δ Ass is an input quantity to allow any errors from the sound pressure level at the reference point. All values are given in decibels. The combination of uncertainties on logarithmic scale, is realized in ISO GUM [5], and it is 3

4 considered a conservative approach [6]. Since equation 1 is a sum of logarithmic terms, combined uncertainty µ Lp, coup through is obtained µ 2 L p, coup = µ 2 L V + µ 2 L s + δ 2 T + δ 2 P + δ 2 H + δ 2 coup + δ 2 cal + δ 2 Tub + δ 2 Ass + δ 2 L p,ref. (2) The expanded uncertainty is then calculated by U Lp, coup = kµ Lp, coup, (3) where k is a coverage factor equal to 2 for a confidence level of 95,45%. Corrections due to ambient conditions were not applied, and instead it was considered in the combined uncertainty as a maximum error assuming the quantity was within a determined range. During measurements, ambient conditions were monitored to be always within the range defined by IEC Uncertainty analysis was conducted for both couplers and results are presented in Tables 1 and 2. Table 1: Uncertainty analysis table for Sound pressure level with the 2CC Coupler Source Random effects Symbol Description of Uncertainty Values [db] Distribution µ [db] µ LV Voltage measurement normal 0.16 µ Ls Microphone sensibility normal 0.06 δ T Temperature correction rectangular 0.03 δ P Ambient pressure correction rectangular 0.01 δ H Humidity correction rectangular 0.01 δ coup,2cc 2CC coupler maximum error rectangular 0.09 δ cal Pistonphone calibration maximum error rectangular 0.06 δ Tub Tubing maximum error rectangular 0.29 δ Ass Assembly maximum error normal 0.30 δ Lp,ref Sound pressure level at the reference point 0.42 normal 0.21 µ Lp, coup:2cc Combined uncertainty SPL 2CC coupler - normal 0.51 U Lp, coup:2cc Expanded uncertainty (95.45%) SPL 2CC coupler normal 1.02 The difference in the combined uncertainty between the two couplers is mainly due the lack of calibration for the ear simulator s microphone by an accredited laboratory, requiring the uncertainty to be extracted from the data sheet. Ear simulator sound pressure level uncertainty declared by the accredited laboratory was 1.5 db for frequencies up to 4kHz and 1.9 db from 4kHz up to 8kHz. The maximum permitted expanded uncertainty allowed by the standard for sound pressure level are: 2 db for frequencies up to 4kHz and 2.5 db for frequencies from 4kHz up to 8kHz. 4

5 Table 2: Uncertainty analysis table for the sound pressure level with the Ear Simulator Coupler Source Random effects Symbol Description of Uncertainty Values [db] Distribution µ [db] µ LV Voltage acquisition normal 0.16 µ Ls Microphone sensibility rectangular 0.58 δ T Temperature correction rectangular 0.03 δ P Ambient Pressure correction rectangular 0.01 δ H Humidity correction rectangular 0.01 δ coup:es,up4 Ear Simulator up to 4kHz maximum error rectangular 0.23 δ coup:es,4-8 Ear Simulator 4kHz - 8kHz maximum error rectangular 0.46 δ cal Pistonphone maximum calibration error rectangular 0.29 δ Tub Tubing maximum error rectangular 0.06 δ Ass Assembly maximum error normal 0.30 δ Lp,ref Sound pressure level at the reference point normal 0.21 µ Lp, coup:es,up4 Combined Uncertainty up tp 4kHz 0.80 µ Lp, coup:es,4-8 Combined Uncertainty 4 khz - 8kHz 0.89 U Lp, coup:es,up4 Expanded Unc. (95.45%) up to 4 khz SPL ES 1.59 U Lp, coup:es,4-8 Expanded Unc. (95.45%) 4kHz - 8 khz SPL ES Measurements All measurements to obtain the frequency response were performed using pure tone signals over the range of 200 Hz Hz, using the ISO R40 preferred frequencies (approximately 1/24th octave frequencies). The signal was processed using a Hanning window, the autospectrum was calculated and the SPL in the frequency of excitation was extracted Sound pressure level adjustment Sound pressure level adjustment at the reference point is critical for carrying out the tests since it is generally assumed that the SPL at the reference point is constant. An adjustment was carried out by determining the sound source sensitivity, which can be calculated by S( f ) = L V ( f ) L p,ref ( f ), (4) since the relationship between the logarithmic voltage level L V ( f ) in db and the sound pressure level at the reference point L p,ref ( f ) in db can be assumed to be linear. Also, the calculation of sensitivity must be performed for each frequency, since the speaker s response is not flat OSPL90 The OSPL90 curve is obtained by setting the input SPL to 90 db, varying the frequency of the sound source and recording the coupler SPL versus frequency. This input SPL is more than 5

6 enough to produce the highest possible output level produced by the hearing aid at each frequency and is also known as saturation sound pressure level (SSPL). The gain control settings of the hearing aid are set to be full-on, and all adaptive functions should be turned off. From that measurement two parameters are determined: the maximum value in the curve known as Max-OSPL90, and an arithmetic mean over the OSPL at three frequencies, typically 1, 1.6 and 2.5 khz, is referred as HFA-OSPL Full-on gain response curve To measure the full-on gain response curve the input sound pressure level is set to 50 db and the gain is recorded as the difference obtained by subtracting 50 db from the acoustic coupler SPL. The full-on gain response curve (FOGRC) reflects the maximum gain that the hearing aid is capable. A 50 db input sound pressure level is generally low enough to ensure that the hearing aid does not saturate. From this response the HFA-FOG can be calculated as the arithmetic mean at the same three frequencies used in HFA-OSPL Basic frequency Response curve The basic frequency response (BFR) is measured by setting the hearing aid gain to the reference test settings (RTS). The RTS is defined as the setting of the gain control required to produce a HFA-gain within ±1.5 db of the HFA-OSPL90 minus 77 db, or, if the full-on HFA gain for an input SPL of 60 db is less than the HFA-OSPL90 minus 77 db, RTS will be the the full-on setting of the gain control. After the adjustment, input SPL is set to 60 db and the Basic Frequency Response curve is obtained stepping through all pure tone frequencies of interest and recording the response from the acoustic coupler. The frequency range is limited by f min e f max, being these the frequencies at which the BFR corresponds to the HFA output level minus 20 db. 3 Results An analog hearing aid previously tested by an European accredited laboratory using the previous version of the standard 5 was selected for the validation of the test setup and procedures. It is important to note that the only difference found between the versions that affected the results was the change of couplers. So, in order to properly compare the results and comply with the new version of the standard 6, measurements were performed with an ear simulator and a 2CC coupler and will be referred as LVA ES and LVA 2CC, respectively. Results obtained by using a commercial FONIX 8000 hearing aid analyzer were also included in the comparison, referred to as FONIX 2CC. Figure 2 presents the frequency response curves obtained, whereas Table 3 shows the parameters calculated from them. 4 HFA stands for High Frequency Average. 5 IEC :1983/amd1994 [2]. 6 IEC :2015 [1]. 6

7 Table 3: Parameters of the reference hearing aid measured according to IEC [1]. Reference ES LVA ES LVA 2CC FONIX 2CC Max OSPL90 [db] 123,94 123,61 119,21 119,21 HFA-OSPL90 [db] 120,57 120,65 113,84 114,16 Max-FOG [db] 52,67 52,35 46,43 45,86 HFA-FOG [db] 47,79 48,64 41,49 41,50 f min [Hz] 221,8 212,8 <200 <200 f max [Hz] Analyzing the results from a metrological point of view, all LVA ES curves can be considered consistent with the reference since all lie inside the respective uncertainty limits. Small variations between the different test systems are expected since tolerances are allowed by the standard for various elements of the measurement chain. In the analysis with the 2CC coupler good agreement between LVA 2CC and FONIX 2CC results were also found. Parameters calculated from the tests, presented in Table 3, also confirm the consistency among the results. A small misalignment is observed at the first and fourth peaks ot the curves and may be caused by differences between the length of the tubes used, altering the transfer function and influencing the curve shape. It is also noteworthy that the OSPL90 curve shows smaller discrepancies when compared to the reference curve than the other curves. This is expected since this test is less prone to variations of the input SPL as the FOG measurements. That also means hearing aid positioning errors in the test space will not considerably influence the result. The 2CC coupler has a volume that approximates the adult residual ear canal when a hearing aid is worn [8]. However, it does not provide a good approximation to the ear s acoustic impedance. An ear simulator, on the other hand, mimics the ear s variation of impedance with frequency, therefore gives a better approximation of the ear s response [7]. While the resonances are almost the same for the ear simulator and the 2CC coupler, the later underestimates SPL at high frequencies. The simple geometry of the 2CC coupler makes it cheaper and less susceptible to variation in geometry compared to the ear simulator. The internal configuration of the ear simulator consists of cavities connected by small openings, which can be easily blocked. Transfer functions and correction factors can be found in the literature to transform the 2CC coupler response in a ear simulator response and vice versa [9]. The results obtained by the IEC measurement procedures are a good starting point for clinicians, but to verify the actual hearing aid acoustical characteristics, by performing measurements in each individual ear 7 adapted, is indispensable. Therefore, results obtained with the ear simulator have no clinical advantage compared to the 2CC coupler. Those reasons make the 2CC coupler more suitable to the IEC purposes. 7 These measurements generally use a probe tube microphone to measure the acoustic response inside the ear canal and are known as real-ear measurements. 7

8 SPL [db ref 20µPa] SPL [db ref 20µPa] SPL [db ref 20µPa] OSPL ,000 4,000 8, Frequency [Hz] FOGRC ,000 4,000 8, Frequency [Hz] Basic Frequency Response Curve ,000 4,000 8,000 Frequency [Hz] Reference ES LVA ES LVA 2CC FONIX 2CC Reference Uncertainty Figure 2: Frequency response results. Reference result provided by accredited laboratory using an occluded ear simulator ( ) with respective uncertainty ( ); ( ) results obtained at LVA using an occluded ear simulator; ( ) results obtained at LVA with 2CC coupler; ( ) results obtained with FONIX 8000 and 2CC coupler. 8

9 In order to evaluate relative position sensibility between the HA and the control microphone, three measurements were carried out for three different distances between them: 5 mm, 12 mm and 20 mm. Furthermore, an analysis of how the control microphone presence affected the result was realized by taking the control microphone out of the test after adjusting the sound pressure level at the reference point. After that, a different configuration was evaluated. A free field microphone was used to perform the adjustment of sound pressure level at the reference point, and then was substituted by the HA in the same position. This configuration is referenced as FFM in the graph. The results from these measurements are presented in Figure 3 for the BFR. Basic Frequency Response Curve SPL [db ref 20µPa] ,000 4,000 8,000 Frequency [Hz] 5mm 5mm - Without Control Mic 12mm 12mm Without Control Mic 20mm FFM Figure 3: Basic Frequency Response Investigation varying parameters. From the results presented in Figure 3 it is possible to notice a small deviation between the different control microphone position results. Therefore, it is expected that small errors in the microphone positioning would not have significant impact on the results. Similarly, taking the control microphone out of position after adjusting the input sound pressure level does not alters the acoustic field around the reference point in a way that influences the final result. It is important to note that adjusting the reference input sound pressure level with a free field microphone, and then replacing it with the hearing aid, resulted in slightly different results if compared to the other curves. So, for the FFM case, the acoustic field is significantly altered by the geometry differences and positioning uncertainty ends up being greater than in the other test arrangements. 4 Conclusions The authors evaluated different test set-ups for the eletroacoustic evaluation of hearing aids according to different revisions of the IEC standard, and validated the approaches using reference results issued by an accredited laboratory and a commercial test-box system used for this purpose. Good agreement comparing the results from different measurement 9

10 systems was found. It was observed that the results obtained by the authors are within the uncertainty bounds given by the accredited laboratory. Although the ear simulator gives a better approximation of the SPL at the ear canal, the 2CC coupler delivers a better cost benefit for the IEC measurement purposes. Investigations regarding the relative positioning of the control microphone and hearing aid, showed that this parameters did not represent substantial influence in the results of the hearing aid tested. This is noteworthy because it credits confidence to single microphone systems, that are used both for reference SPL adjustment and hearing aid testing. Moreover, the results showed that taking the control microphone out of position after adjusting the sound pressure level at the reference point, does not affect the result. However further investigation with directional hearing aids is necessary. The test using a free field microphone to adjust the sound pressure level at the reference point without the HA in place presented a larger discrepancy due to a lack of correspondence between the geometries of the hearing aid geometry and the microphone used to adjust the sound pressure level at the reference point. Acknowledgements The Authors would like to Acknowledge the National Council for Scientific and Technological Development (CNPq), the National Council for the Improvement of Higher Education (CAPES) for the financial support under this project and DELTA laboratory for the hearing aid and results provided. References [1] IEC, International Standard IEC :2015 Electroacoustics - Hearing aids - Measurement of the performance characteristics of hearing aids, [2] IEC, International Standard IEC :1983/amd1:1994 Electroacoustics - Hearing aids. Part 0: Measurement of electroacoustical characteristics, [3] IEC, International Standard IEC :2010 Electroacoustics - Simulators of human head and ear - Part 4: Occluded-ear simulator for the measurement of earphones coupled to the ear by means of ear inserts, [4] IEC, International Standard IEC :2006 Electroacoustics - Simulators of human head and ear - Part 5: 2 cm3 coupler for the measurement of hearing aids and earphones coupled to the ear by means of ear inserts, [5] JCGM, Evaluation of measurement data Guide to the expression of uncertainty in measurement, JCGM, [6] Pedroso, M. A.; Aspectos metrológicos da calibração de audiômetros. Master s thesis, Universidade Federal de Santa Catarina, [7] Dillon, H.; Hearing Aids, Boomerang Press, Sydney (Australia), second edition, [8] Romanow, FF.; Methods of measuring the performance of hearing aid, Journal Acoustic Society of America, volume 13(1), pp , [9] Bentler, R. A.; Pavlovic, C. V., Transfer functions and correction factors used in hearing aid evaluation and research. Ear and Hearing, 10(1), 1989, pp