Lankford, James E.
M.A. (Master of Arts)
Department of Communication Disorders
Various methods of bone conduction calibration have been proposed and the advantages and disadvantages of each reported. However, no single recent study has compared several calibration methods under controlled conditions. The purpose of the present investigation was to determine the relative accuracy and efficiency of three methods of bone conduction calibration frequently utilized as alternatives to artificial mastoid calibration. The three methods were: (1) the input voltage method, (2) the real-ear threshold method utilizing normal hearing listeners, and (3) the real-ear threshold method using hearing impaired subjects. To determine the relative accuracy of the three procedures, correction factors were calculated for each method at four conditions of audiometric output levels produced by means of a four-position variable attenuator placed between the audiometer and the bone vibrator. The audiometric output for one condition was calibrated to a level precisely equal to the Hearing Aid Industry Conference (HAIC) interim norms (Lybarger, 1966b, 1966c). The amount of deviation for each condition of attenuation from the calibrated condition was measured from an artificial mastoid. The four conditions of deviation and the equivalent correction factors were: Condition #1 — 0 dB deviation, 0 dB correction Condition #2 — +3 dB deviation, +5 dB correction Condition — -4 dB deviation, -5 dB correction Condition — -8 dB deviation, -10 dB correction. The range of percentage agreement of the correction factors obtained for each method at the four conditions with those determined from the artificial mastoid was calculated for each method. The Friedman analysis of variance by ranks and the coefficient of variation were used to determine any significant difference among the three procedures and to assess the relative variance of each method. Correction factors for the voltage method of calibration were determined by computing the difference in equivalent decibels between the observed voltages and the expected voltages published by Radioear (1970). Correction factors for the two subjective threshold methods were obtained by calculating the difference between the mean air and the mean bone conduction thresholds for the six subjects (12 ears) in each group. Air conduction thresholds were obtained on a precisely calibrated air conduction system (re: ANSI S3.6, 1969). Mastoid bone conduction thresholds were obtained at the four experimental output levels with 35 dB effective narrow band masking noise applied to the contralateral ear. The normal hearing subjects had hearing levels no worse than 10 dB hearing level (HL). The hearing impaired subjects exhibited air and bone conduction thresholds of 15 dB HL or higher. The results indicated that the three procedures were not significantly different in their accuracy. Percentage agreement within 5 dB of the reference artificial mastoid values was 85 percent for the voltage method, 80 percent for the real-ear threshold method using hearing impaired subjects, and 75 percent for the subjective threshold method using normal hearing subjects. Deviation of 15 dB from the reference values was seen for the normal hearing group at 250 Hertz (Hz). Deviation of 10 dB occurred at 500 and 1000 Hz for the hearing impaired group and at 2000 Hz for the voltage method. The coefficient of variation was .8124 for the normal hearing subjects, .5794 for the voltage method, and .3744 for the hearing impaired group. Because the threshold procedure using normal hearing subjects showed relatively poor agreement, wide deviation, and a large coefficient of variation, the voltage method or the threshold method using hearing impaired subjects appeared preferable in terms of accuracy. In terms of efficiency the input voltage method was superior. Approximately five minutes are needed for voltage calibration, compared to a minimum of four hours for either threshold method.
Evans, Mary Powers, "Bone conduction calibration : a comparative study of the voltage and real-ear threshold methods" (1976). Graduate Research Theses & Dissertations. 1663.
vii, 85 pages
Northern Illinois University
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