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| Year : 2015
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: 17 | Issue : 76 | Page
: 172 |
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| Comment on "Concerns with amplitude variation in calibrated audiometer systems in clinical simulations" |
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David C Byrne, Christa L Themann, Mark R Stephenson
Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Cincinnati, Ohio, USA
Click here for correspondence address
and email
| Date of Web Publication | 27-Apr-2015 |
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How to cite this article:
Byrne DC, Themann CL, Stephenson MR. Comment on "Concerns with amplitude variation in calibrated audiometer systems in clinical simulations". Noise Health 2015;17:172 |
Barlow and colleagues addressed the important question of variation in pure-tone audiometric thresholds within and across audiometers in their article entitled "Concerns with Amplitude Variation in Calibrated Audiometer Systems in Clinical Simulations." [1] We agree that the reliability of thresholds obtained across test systems and individuals is a vital concern when monitoring hearing health. However, we believe that a few important details are missing, which limits the validity of this study.
The authors reported, "Each of the audiometers had recently undergone certified traceable calibration by its recommended laboratory, meaning that the tone presentation from each should theoretically be identical" [page 300]. Ideally, this would be the case; however, it is not necessarily true. An audiometer is considered "in calibration" when its output is within a certain tolerance range. The International Electrotechnical Commission (IEC) 60645-1 standard allows a deviation of ± 3.7 dB from the indicated value at test frequencies from 125 Hz through 4000 Hz, and ±6.2 dB up to and including 8000 Hz. This means, for example, that an audiometer set to generate a 50-dB HL tone at 6000 Hz could produce anywhere from 43.8 dB HL to 56.2 dB HL and be considered "in calibration." In some cases, the ±3.7 dB calibration tolerance (i.e., 7.4 dB range) at frequencies below 6000 Hz is close to the range of variation reported for the audiometers in this study. Without verifying that all four audiometers actually produced identical outputs, the measured differences cannot be assumed to be solely due to earphone placement. (Note: The allowable deviation according to American National Standards Institute (ANSI) standard S3.6 is ±3 dB at test frequencies from 125 Hz through 5000 Hz, and ± 5 dB at 6000 Hz and higher.)
The authors pointed out the potential problems with manufacturer-based calibration services, i.e., lack of laboratory accreditation in the calibration process and, therefore, the potential for calibration errors [page 300]. They also recognized the benefit gained by using a single, consistent measurement system when referring to the reliability of their manikin-based measurements. The same reasoning should have been applied to the pre-study coupler-based calibrations. Presumably, the only variation between the four audiometers in this study would be caused by headphone positioning, because the same head and torso simulator (HATS) measurement system was used to evaluate all four of the audiometers [page 302]. However, the audiometers/earphones in this study apparently were not all calibrated in the same laboratory, by the same technician using the same sound level meter with known accuracy, and yet this study relied on the assumption that the output levels across audiometers were identical. It is unknown whether the audiometers' output levels conformed exactly to the reference values or were simply somewhere within the allowable range. The audiometers' calibration statuses should have been independently verified prior to conducting the study and the output levels considered in the analysis.
Headband static force is another issue of concern. The authors acknowledged that "...headband tension needs to be sufficiently high to ensure good coupling between the headphone and ear..." [page 304] without providing any quantification of this parameter. They indicated that the headphones from Audiometer 4 were "subjectively" the loosest and from Audiometer 3 the tightest. It is unknown exactly what these particular differences were, and whether the differences encountered by the authors were representative of the respective audiometer/earphone brands. Measuring the headband static force for each set of earphones would have addressed this important variable.
The precision and reliability of pure-tone audiometric thresholds is worth investigating. This study assumed that the audiometers were all in perfect calibration; however, nothing is known about the calibration lab(s) that were used and an independent verification was not performed. At least part of the discrepancies seen between units could be attributed to the calibration tolerance limits. Without data on actual audiometer output levels and headband force differences, accurate interpretation of the study measurements is not possible.
Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.
| References | |  |
| 1. | Barlow C, Davison L, Ashmore M, Weinstein R. Amplitude variation in calibrated audiometer systems in clinical simulations. Noise Health 2014;16:299-305.  [ PUBMED]  |
Correspondence Address:
David C Byrne
Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Cincinnati, Ohio
USA
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/1463-1741.155851
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