J Am Acad Audiol 2 : 156-163 (1991) Effect of Reference Microphone Location and Loudspeaker Azimuth on Probe Tube Microphone Measurements Michelle A. Ickes* David B. Hawkins t William A. Cooper t Abstract The effects of loudspeaker azimuth and reference microphone location on probe tube microphone measures were assessed . The real ear unaided response (REUR), real ear aided response (REAR), and real ear insertion response (REIR) were obtained on a KEMAR . Aided measures were obtained with both a behind-the-ear and an in-the-ear hearing aid . All three measurements were affected by changes in the loudspeaker azimuth and reference microphone location . Responses obtained with a 90 degree loudspeaker azimuth or with the reference microphone located at-the-ear revealed greater disparity than those obtained under other conditions . Most of the differences occurred at frequencies above 2000 Hz, with measurements utilizing the behind-the-ear hearing aid showing greater dispersion . These results suggest that the location of the loudspeaker and the reference microphone are important variables when utilizing probe tube microphone measurements . Key Words : Probe tube microphone measurements, hearing aids, reference microphone location, loudspeaker azimuth espite the current popularity of real ear probe tube microphone measureD ments (Cranmer, 1990), there is no standard test protocol . Relatively little effort has been made to delineate the sources of variability in the measurements . Manufacturer's recommendations often dictate the specific techniques employed by clinicians for hearing aid selection and/or validation . Two procedural issues that may differ among equipment specifications and clinicians, and which could affect test results and variability, are the loudspeaker azimuth and the reference microphone location . Killion and Revit (1987) " Dorn Veterans Administration Medical Center, Columbia, South Carolina lina t University of South Carolina, Columbia, South Caro- Reprint Requests : Michelle A. Ickes, Audiology and Speech Pathology (126), Dorn Veterans Administration Medical Center, Columbia, SC 29201 156 investigated the effects of four different loudspeaker locations on measurements of real ear insertion gain (REIG) . The locations studied were 070° (directly in front), 0°/45° (in the horizontal plane, 45 degrees offfrom center), 45°/45° (45 degrees up and 45 degrees off center), and 90°/0' (directly overhead). REIG was quite similar for 070° and 0°/45° . However, the 45745° azimuth showed increased REIG at 4000 and 5000 Hz, and the 90°/0° azimuth showed substantially higher gain above 500 Hz . The best test-retest reliability was obtained with the 45°/ 45° and 0745' loudspeaker locations . It is not clear at this point how different loudspeaker locations affect the three main probe tube measurements : real ear unaided response (REUR), real ear aided response (REAR), and REIG . Many probe tube measurement systems utilize a reference microphone to maintain a constant sound pressure level or to correct for minor deviations in the sound field. Based upon currently available instruments, the three most popular locations for this reference microphone Probe Tube Measurements/Ickes et al are over-the-ear, on-the-cheek, and at-the-ear (lateral to the external ear) . If the transfer functions from these three positions to the eardrum and/or hearing aid microphone location were different, it is possible that different values for probe tube measurements would be obtained. Variations in sound pressure distribution at different locations on the head have been examined by Madaffari (1974) and Kuhn and Burnett (1976) . Madaffari measured changes in acoustic pressure as a function of frequency due to head diffraction and baffle at 25 locations on and about the pinna with the loudspeaker location held constant at a 0 degree azimuth. Significant differences in SPL were evident when comparing one location to another, with the greatest variations occurring in the higher frequencies. Kuhn and Burnett found minor changes in SPL alongside the head for frequencies of 2000 Hz and below. Above 4000 Hz, however, larger changes in SPL were found with small variations in position about the pinna. Revit (1987) indicated that there may be as much as 3 inches between a behind-the-ear (BTE) hearing aid microphone and the reference microphone of a real ear measurement system . This difference in location can create significant differences in the SPL being received by each microphone . The effect will be seen for frequencies which have half wavelengths, that are less than the distance between the two microphones. Differences should therefore be expected at frequencies near and above 2000 Hz . Minor head movements may change the SPL arriving at each microphone location and cause inconsistencies between various real ear measurements . Due to these problems, Revit suggested that the reference microphone should be as close as possible to the hearing aid microphone. Feigin, Nelson Barlow, and Stelmachowicz (1990) examined the differences in input to the microphone of a BTE hearing aid when the reference microphone was located on the cheek versus above the ear with the loudspeaker maintained at a 0 degree azimuth. Sizeable differences were found between the two locations, especially in the frequency region of 1200 to 2000 Hz . While these differences would affect the REAR, it is unclear whether the REUR or real ear insertion response (REIR) would be affected by different reference microphone locations. Further, it remains to be seen whether different loudspeaker azimuths would interact with reference microphone location . The purpose of this study was to examine the effects of three loudspeaker azimuths (0 degrees, 45 degrees, 90 degrees) and three common reference microphone locations (over-theear, cheek, at-the-ear) on the REUR, REAR, and REIR . Aided measures were obtained for a BTE and an in-the-ear (ITE) hearing aid. METHOD M easurements were made on a Knowles Electronics Manikin for Acoustic Research (KEMAR) placed in an IAC double-walled sound treated room . AFrye 6500 probe tube microphone system was utilized for all measurements . A60 dB SPL composite weighted noise served as the stimulus. The loudspeaker was located 12 inches from the KEMAR and positioned at horizontal azimuths of 0 degrees, 45 degrees, and 90 degrees; the elevation angle was always 0 degrees. All measurements were made using the manikin's right ear fitted with the large KEMAR pinna. The reference microphone was placed at three locations around the KEMAR's pinna to simulate common probe tube microphone systems . One location was over the ear, in which the microphone was placed above the pinna on a vertical line directly above the tragus . A second location was on the cheek. A horizontal line was drawn 15 mm anterior to the tragus, with the reference microphone located 15 mm directly below this point on the cheek. A third location was at the ear or lateral to the ear. The center of the top surface of the microphone was located 25 mm lateral to the ear canal opening. Due to concern regarding the possibility of altering the probe tube location in the ear canal, all unaided measures were obtained first. Based on the length of the canal portion of the Zwislocki ear simulator and the dimensions of the pinna, for both unaided and aided measurements the probe tube was inserted to a constant depth past the tragus at a point calculated to be within several millimeters of the eardrum location . For the aided measurements with the BTE hearing aid, the probe tube was inserted through a custom drilled vent parallel to the bore of the DB-111 earmold simulator. When aided measurements were made with the ITE hearing aid, the probe tube was inserted through a parallel vent with putty placed around the outer edge to seal off the vent . 157 Journal of the American Academy of Audiology/Volume 2, Number 3, July 1991 The reference microphone was initially placed in the over-the-ear position, and measurements were made for the three loudspeaker locations. The reference microphone was next moved to the cheek and finally to the at-the-ear location . The same procedure was then followed for obtaining the REAR with the BTE hearing aid and then with the ITE hearing aid. Each measurement of the REUR and REAR was repeated three times without alteration of the location of the probe tube, loudspeaker, or reference microphone. The hearing aids were not removed in between the three repetitions of each measurement. The REIR was determined by subtracting the REUR from the REAR . The BTE instrument was a mild gain hearing aid with a front-facing microphone . The hearing aid was coupled to the ear simulator via a DB-111 earmold simulator. Earmold impression material was used to fill the concha to simulate a fully occluding shell earmold. The ITE instrument was a mild gain custom hearing aid manufactured to fit the ear of KEMAR and the Zwislocki ear simulator. A specially designed plastic ring allowed proper fit of the ITE hearing aid to the manikin's ear. The volume control wheels on both the BTE and ITE were set such that with the 60 dB SPL broadband input, the hearing aids were functioning in a linear operating range. RESULTS Real Ear Unaided Response Reference Microphone Location . Figure 1 shows the effect of reference microphone location on the REUR at the three loudspeaker azimuths .* Also shown for each azimuth are data from Shaw (1974) [numerical values were taken from Shaw and Vaillancourt (1985)] which utilized the substitution method . The best agreement with the Shaw data is seen at the 0 degree azimuth. The differences among the three reference microphone locations are also the least at Although values for the REUR and REAR are reported and plotted in dB SPL, they are actually transformed values . When the reference microphone is active on the Frye 6500 as it was in this study, the actual values are shown as "Gain," and represent the difference in dB between the SPL measured by the probe microphone and the reference microphone . Since the convention is to express the REUR and REAR as dB SPL in the ear canal, the input (60 dB SPL) was simply added to all of the gain values in orderto presentthese data as dB SPL measured in the ear canal . 158 REAL EAR UNAIDED RESPONSE 0 AZIMUTH 1000 10000 1000 10000 v0 100 J 90 0- N so 0 70 60 50 100 110 100 90 AZIMUTH 90 so 70 60 50 100 1000 FREQUENCY (in Hz) 10000 Figure 1. Real ear unaided response as a function of reference microphone location for the three loudspeaker azimuths . Also shown are the data from Shaw (1974) which utilized a substitution method . this location . The three reference microphone locations give similar results except above 3000 Hz for the at-the-ear location, where an increase in SPL is observed . The 90 degree azimuth results show the largest deviation from Shaw's data, as well as among the three reference microphone locations . It is clear that a reference microphone based REUR is different than a substitution method REUR, especially with the loudspeaker at the 45 degree and 90 degree azimuths . This should be expected, as the reference microphone effectively reduces some of the sound pressure build up along the head due to the head baffle, effects that will be present with the substitution method . Loudspeaker Azimuth. Figure 2 shows the effect ofloudspeaker azimuth for each reference microphone location . Differences are observed primarily above 2500 Hz for each loudspeaker azimuth for all reference microphone locations. Large peaks are present in the REUR for both the 45 degree and 90 degree azimuth location for the at-the-ear reference microphone location . The 90 degree azimuth loudspeaker orientation again gives the most deviant result for each of the three reference microphone locations. Probe Tube Measurements/Ickes et al 110 REAL EAR AIDED RESPONSE (BTE) REAL EAR UNAIDED RESPONSE 100 0 AZIMUTH OVER EAR 90 go 70 BO 50 too 110 100 90 a N so m 1000 10000 1000 - 0 AZ --- 45 AZ 90 AZ 10000 - OVER EAR -- CHEEK CHEEK 45 AZIMUTH AT EAR 90 e0 u 70 70 90 100 110 120 Tr too 110 } 100 90 9o so 90 AZIMUTH 70 so 100 10000 9o 70 501 1000 1000 FREQUENCY (in Hz) 10000 Figure 2. Real ear unaided response as a function of loudspeaker azimuth for the three reference microphone locations. 9o 100 1000 FREQUENCY (in Hz) 10000 Figure 3. Real ear aided response for a behind-the-ear (BTE) hearing aid as a function of reference microphone location for the three loudspeaker azimuths . REAL EAR AIDED RESPONSE Reference Microphone Location . Figure 3 shows the REAR as a function of reference microphone location for each of the three loudspeaker azimuths with the BTE hearing aid. The reference microphone location has the least effect with the loudspeaker at a 0 degree azimuth . Good agreement is seen at the 45 degree azimuth among the reference microphone locations across most of the frequency range tested . The at-the-ear location does, however, show a large peak above 5000 Hz . As with the REUR, the largest discrepancies as a function of reference microphone location for the REAR are seen with the loudspeaker at the 90 degree azimuth. The REAR obtained with the at-the-ear location becomes divergent from the other two response curves beginning at 2500 Hz and displays two or three large peaks in the higher frequencies. The overthe-ear and cheek reference microphone locations produced similar REARS through 4000 Hz, with the cheek location resulting in less output between 4000 and 6000 Hz . Figure 4 shows that differences among REARS with the three reference microphone locations were not as large when an ITE hearing aid was used . There was little effect ofreference microphone location at the 0 degree azimuth. 110 100 J 90 0- N go m v 70 90 so 100 1600 10600 110 100 9o 90 AZIMUTH 9o 70 901 50 100 Figure 4. Real ear aided response for an in-the-ear (ITE) hearing aid as a function of reference microphone location for the three loudspeaker azimuths . 159 Journal of the American Academy of Audiology/Volume 2, Number 3, July 1991 At 45 degrees the curves are quite close except above 4000 Hz, where the at-the-ear location showed increased output . As with the BTE, the largest differences are seen with the 90 degree azimuth loudspeaker location, with the at-theear location again showing the most deviant response. Loudspeaker Azimuth. Figure 5 shows the REAR BTE data plotted so that the effect of loudspeaker azimuth can be examined for each ofthe reference microphone locations. It is clear again that the 90 degree azimuth reveals the greatest divergence from the other two loudspeaker azimuths for each of the reference microphone locations. For the over-the-ear reference microphone location, loudspeaker azimuth begins to have an effect above 1500 Hz . With the reference microphone on the cheek, approximately a 3 to 10 dB difference is seen above 400 Hz between the 0 degree and 90 degree azimuth loudspeaker locations. The largest loudspeaker azimuth effects are seen when the reference microphone is located at-the-ear . Differences among the REARS for the three azimuths begin primarily above 1000 Hz, reaching a maximum of 23 dB at 2800 Hz, the location of a large resonant peak . The ITE REAR data plotted to show the effect of loudspeaker azimuth are shown in Figure 6. In contrast to the BTE data, the REAL EAR AIDED RESPONSE (ITE) 110 100 so 70 90 50 1000 - 10000 0 AZ --- 45 AZ CHEEK 90 AZ 100 1000 10000 1000 FREQUENCY (in Hz) 10000 110 100 90 70 80 e0 50 100 Figure 5. Real ear aided response for a behind-the-ear (BTE) hearing aid as a function of loudspeaker azimuth for the three reference microphone locations . 160 OVER EAR 100 1000 10000 1000 10000 1000 ' " " 10000 120 110 J 100 a N 90 m so 70 8o 100 120 110 100 9o 70 90 80 1 100 FREQUENCY (in Hz) Figure 6. Real ear aided response for an in-the-ear (ITE) hearing aid as a function of loudspeaker azimuth for the three reference microphone locations. differences are relatively minor in all conditions, with the exception of the 90 degree azimuth cheek position where a 6 to 10 dB reduction is seen between 2500 and 5000 Hz . Real Ear Insertion Response OVER EAR 90 REAL EAR AIDED RESPONSE (BTE) Reference Microphone Location. Figure 7 shows the effect of reference microphone location on the REIR for the three loudspeaker azimuths for a BTE hearing aid. The three reference microphone locations yield virtually identical REIRs through 3000 Hz for the 0 and 45 degree loudspeaker azimuths and through 2500 Hz for the 90 degree azimuth. In the higher frequencies the agreement is best between the over-the-ear and cheek locations. Similar results were obtained with the ITE hearing aid. Figure 8 shows the ITE REIRs as a function of reference microphone location . The results are quite similar to those shown above with the BTE . Excellent agreement is seen in the low and middle frequencies, with substantial differences occurring primarily with the at-the-ear location above 3000 to 4000 Hz . Loudspeaker Azimuth. Figure 9 shows how the REIR changes with loudspeaker azimuth for each of the reference microphone loca- Probe Tube Measurements/Ickes et al 50T 40 30 REAL EAR INSERTION RESPONSE (BTE) 0 AZIMUTH REAL EAR INSERTION RESPONSE (BTE) +0 30 OVER EAR -_ 20 10 20 10 0 0 100 so 1000 .10000 -10 100 1000 10000 10000 100 1000 10000 50 .~ 40 m v 30 c ~. 20 ? 1o a CD o -10 100 -, 1000 . 50 90 AZIMUTH AT EAR 40 30 :ate 20 10 -f0 100 1000 FREQUENCY (in Hz) 0 -10 100 10000 Figure 7. Real ear insertion response for a behind-theear (BTE) hearing aid as a function of reference microphone location for the three loudspeaker locations. 1000 FREQUENCY (in Hz) Figure 9. Real ear insertion response for a behind-theear (BTE) hearing aid as a function of loudspeaker location for the three reference microphone locations. REAL EAR INSERTION RESPONSE (ITE) REAL EAR INSERTION RESPONSE (ITE) 0 AZIMUTH OVER EAR 100 1000 - Q U 10000 50 40 30 90 AZ CHEEK 0. -10 100 1000 10000 50 40 90 AZIMUTH 30 20 20 10 10 0 -10 100 10000 0 AZ --- 45 AZ m T c 1000 10000 AT EAR 0 1000 FREQUENCY (in Hz) 10000 Figure S. Real ear insertion response for an in-the-ear (ITE) hearing aid as a function of reference microphone location for the three loudspeaker locations. -10 100 Figure 10 . Real ear insertion response for an in-the-ear (ITE) hearing aid as a function of loudspeaker location for the three reference microphone locations. Journal of the American Academy of Audiology/Volume 2, Number 3, July 1991 tions for the BTE hearing aid. For all three reference microphone locations, the 90 degree azimuth loudspeaker results in more insertion gain between 400 and 4000 Hz . The agreement is reasonably good between the 0 and 45 degree azimuth REIRs for all reference microphone locations. The ITE loudspeaker azimuth data are shown in Figure 10 . The REIR is similar for the three azimuths through 3000 Hz for the overthe-ear and cheek reference microphone locations and through 2000 Hz for the at-the-ear location . In the higher frequencies less REIG is observed with the 45 and 90 degree azimuths, especially with the at-the-ear reference microphone location . Test-Retest Reliability. Three repetitions were made for each condition. Since the probe tube, loudspeaker azimuth, and reference microphone were not altered between the repetitions and the hearing aid was not removed, it was expected that the test-retest reliability would be excellent. The standard deviations were less than 1 .0 dB and averaged approximately 0.3 to 0.7 dB for each condition, indicating excellent test-retest reliability . DISCUSSION be results of this study demonstrate that T the REUR, REAR, and REIR are affected by location of the loudspeaker and the reference microphone . These findings are not surprising given the known variations in sound pressure level around the head and pinna and loudspeaker azimuth effects. The data of Shaw (1974) clearly demonstrated the effect of loudspeaker azimuth on the REUR using the substitution method . There is an increase in SPL at the eardrum across the entire frequency range when the signal arrives from a 45 or 90 degree azimuth rather than 0 degrees . When a reference microphone is placed on the head and the SPL is kept constant at that location, the azimuth effect is altered, but differences among the loudspeaker locations are still present . The REUR can be altered with changes in the reference microphone location as a result of differing transfer functions from the microphone to the measurement point in the ear canal. The azimuth differences for the REUR are confined to frequencies above 2500 Hz for all three reference microphone locations (see Fig. 2) . The difference between the substitution method REUR and the reference microphone REUR 162 increases substantially at the 45 and 90 degree azimuths . If the substitution method REUR is viewed as the more valid representation because of the presence of all head and body diffraction effects, then closest agreement to these data will be obtained with a 0 degree azimuth loudspeaker when a reference microphone is employed . Even with a 0 degree azimuth, however, there can still be substantial differences (up to 8 dB at some frequencies) between the substitution and reference microphone REURs . The findings of this study support the conclusion of Feigin et al(1990), that the location of the reference microphone can alter the SPL entering the hearing aid microphone and thus affect the REAR . The differences observed in this study were larger for BTE than ITE hearing aids and were confined mostly to frequencies above 2500 Hz . If one excludes all the 90 degree azimuth data and the at-the-ear reference microphone data for 45 degrees, then the differences among the remaining REAR data are relatively small. The results of this study further show that the REIR can also be affected by reference microphone location and loudspeaker azimuth. Since the REIR is the difference curve between the REUR and REAR, it might be expected that the REIR would be unaffected by changes in these two variables. That is, ifchanges occurred at different azimuths or were caused by reference microphone locations, these changes would be the same for the REUR and REAR, and therefore the REIR would not be affected . This does not seem to be the case . An explanation could be that as the loudspeaker azimuth or reference microphone location is altered, changes occur in the differences between the transfer functions from the reference microphone and the hearing aid microphone to the eardrum location . These differences would lead to changes in the REIR . These changes all occur in the higher frequencies and were larger for the BTE than ITE hearing aid. If the REIR is being measured with an ITE hearing aid at a 0 or 45 degree azimuth, only minor differences can be expected to occur below 4000 Hz for the three reference microphone locations. Other differences might be expected if various styles of ITE hearing aids (e .g ., half shell or in-the-canal) were employed, because of changes in the location of the hearing aid microphone . The results ofthis study have some implications for the procedure of correcting prescription values for deviations of an individual's Probe Tube Measurements/Ickes et al REUR from the "average" REUR (Mueller, 1989). These data show that the "average" REUR depends on the loudspeaker azimuth and the location of the reference microphone . Therefore, the REUR chosen to represent the average person should be obtained under the same conditions as would be employed for the clinical measurements . A reasonable procedure would be to place a KEMAR in the test environment and obtain a reference REUR using the loudspeaker location and reference microphone position that will be used with the client . The client's REUR would then be corrected by differences from the KEMAR REUR measured under the same test conditions . If a KEMAR is not available, REURs obtained from a group of subjects could be averaged and serve as the reference. CONCLUSIONS T his study has demonstrated that the loudspeaker azimuth and reference microphone location affect REUR, REAR, and REIR . Most of the effects are limited to frequencies above 2000 to 3000 Hz and tend to be more pronounced with BTE hearing aids . A loudspeaker azimuth of 90 degrees is not recommended, as the results can be quite deviant and show large peaks and valleys. These deviations appear most prominently with an at-the-ear reference microphone location . Only minor differences, again confined to the higher frequencies, occur in the REIR if a 0 or 45 degree azimuth is used with any of the three reference microphone locations. Since it appears that many factors affect the variability of probe tube microphone measurements, especially in the high frequencies, it may not be realistic for the clinician to view results above 3000 Hz with the same confidence as those obtained at lower frequencies. Acknowledgment . The authors would like to acknowledge the assistance of Lucille Beck and Ed Burnett in providing in-the-ear hearing aids manufactured for a KEMAR and adapters allowing for appropriate coupling of the hearing aids to the ear simulator. Portions of this paper were presented at the American Speech-Language-Hearing Association Convention in Seattle, Washington in November, 1990. REFERENCES Cranmer K. (1990) . Hearing instrument dispensing1990 . 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