Influence of calibration method on distortion-product otoacoustic emission measurements: I. test performance

Abstract

Objective: Calibration errors in distortion-product otoacoustic emission (DPOAE) measurements because of standing waves cause unpredictable changes in stimulus and DPOAE response level. The purpose of this study was to assess the extent to which these errors affect DPOAE test performance. Standard calibration procedures use sound pressure level (SPL) to determine specified levels. Forward pressure level (FPL) is an alternate calibration method that is less susceptible to standing waves. However, FPL derivation requires prior cavity measurements, which have associated variability. In an attempt to address this variability, four FPL methods were compared with SPL: a reference calibration derived from 25 measurements before all data collection and a daily calibration measurement, both of which were made at body and room temperature.

Design: Data were collected from 52 normal-hearing and 103 hearing-impaired subjects. DPOAEs were measured for f2 frequencies ranging from 2 to 8 kHz in half-octave steps, with L2 ranging from -20 to 70 dB SPL (5-dB steps). At each f2, DPOAEs were measured in five calibration conditions: SPL, daily FPL at body temperature (daily body), daily FPL at room temperature (daily room), reference FPL at body temperature (ref body), and reference FPL at room temperature (ref room). Data were used to construct receiver operating characteristic (ROC) curves for each f2, calibration method, and L2. From these curves, areas under the ROC curve (AROC) were estimated.

Results: The results of this study are summarized by the following observations: (1) DPOAE test performance was sensitive to stimulus level, regardless of calibration method, with the best test performance observed for moderate stimulus level conditions. (2) An effect of frequency was observed for all calibration methods, with the best test performance at 6 kHz and the worst performance at 8 kHz. (3) At clinically applicable stimulus levels, little difference in test performance among calibration methods was noted across frequencies, except at 8 kHz. At 8 kHz, FPL-based calibration methods provided superior performance compared with the standard SPL calibration. (4) A difference between FPL calibration methods was observed at 8 kHz, with the best test performance occurring for daily calibrations at body temperature.

Conclusions: With the exception of 8 kHz, there was little difference in test performance across calibration methods. At 8 kHz, AROCs and specificities for fixed sensitivities indicate that FPL-based calibration methods provide superior performance compared with the standard SPL calibration for clinically relevant levels. Temperature may have an impact on FPL calculations relative to DPOAE test performance. Although the differences in AROC among calibration procedures were not statistically significant, the present results indicate that standing wave errors may impact DPOAE test performance and can be reduced by using FPL, although the largest effects were restricted to 8 kHz.

FIGURE 1

Twenty-five calculations of source impedance from the calibration cavities performed at room temperature using a single standard foam ear tip, which remained in the coupler for all measurements (first column), using a different standard foam ear tip (second column), and using the same standard foam ear tip but removing it from the coupler before each set of measurements (third column). The characteristics for only one sound-source channel are plotted. Each line in each panel represents one of the 25 measurements. The 25 cavity calibrations at body temperature, although not shown, exhibited the same pattern.

FIGURE 2

Twenty-five calculations of source pressure from the same calibration cavities used in Fig. 1. The delay values in the bottom panels represent the time difference between stimulus generation and response recordings by the soundcard. The values plotted in the bottom row demonstrate the phase values after the number of cycles occurring within the time delay were subtracted out. The 25 cavity calibrations at body temperature, although not shown, exhibited the same pattern.

FIGURE 3

DPOAE I/O functions from two normal-hearing and two hearing-impaired subjects are shown in separate columns. Each row provides data for a different f₂ frequency. The parameter within each panel is calibration method (SPL, ref room, ref body, daily room, daily body). The mean noise level (dotted lines) represents the average noise levels for all five calibration conditions. Error bars represent the standard deviations calculated from the noise level observed among the five calibration methods. Standard deviations around the mean noise levels are not provided in the panels depicting data from hearing-impaired subjects for the reasons described in the text.

FIGURE 4

Audiograms (top row) followed by DPgrams at four L₂ levels for the same subjects whose DPOAE I/O functions were shown in Fig. 3. The data obtained from all five calibration conditions are represented within each panel. The hashed lines at the bottom of each panel represent a conservative estimate of the level at which system distortion might be observed, as described in the text. .

FIGURE 5

Areas under the ROC curves as a function of L₂. Each panel represents data for a different f₂ while the parameter within each panel is calibration method. Error bars indicate 2 standard deviations from the SPL-based calibration function, using the variance estimate, as described in the body of the manuscript.

FIGURE 6

Differences between SPL A_ROC and FPL A_ROC as a function of L₂. Each panel represents results for a different f₂. The dashed line indicates no difference in A_ROC between calibration methods. Values above this line indicate conditions for which test performance following FPL calibration was better than performance following SPL calibration, while values falling below this line represent conditions in which SPL achieved better test performance.

FIGURE 7

Specificity (top panel) and sensitivity (bottom panel) as a function of frequency for two fixed sensitivities and specificities (90 and 95%) for the condition in which L₂ = 50 dB. The parameter within each panel is calibration method.