Distribution of standing-wave errors in real-ear sound-level measurements

Abstract

Standing waves can cause measurement errors when sound-pressure level (SPL) measurements are performed in a closed ear canal, e.g., during probe-microphone system calibration for distortion-product otoacoustic emission (DPOAE) testing. Alternative calibration methods, such as forward-pressure level (FPL), minimize the influence of standing waves by calculating the forward-going sound waves separate from the reflections that cause errors. Previous research compared test performance (Burke et al., 2010) and threshold prediction (Rogers et al., 2010) using SPL and multiple FPL calibration conditions, and surprisingly found no significant improvements when using FPL relative to SPL, except at 8 kHz. The present study examined the calibration data collected by Burke et al. and Rogers et al. from 155 human subjects in order to describe the frequency location and magnitude of standing-wave pressure minima to see if these errors might explain trends in test performance. Results indicate that while individual results varied widely, pressure variability was larger around 4 kHz and smaller at 8 kHz, consistent with the dimensions of the adult ear canal. The present data suggest that standing-wave errors are not responsible for the historically poor (8 kHz) or good (4 kHz) performance of DPOAE measures at specific test frequencies.

Figure 1

Example of FPL-SPL pressure ratio (top panel) and phase (bottom panel) computed from measurements from one of the two ER-10C loudspeakers in a single subject. Because measurements were typically very similar between channels, results from only one channel will be displayed in this paper. Presentation of a single peak in magnitude with a change in phase at the corresponding frequency is typical. However, both frequency and magnitude of standing-wave minima vary across subjects.

Figure 2

Examples of FPL-SPL pressure ratios that differ from the expected typical single magnitude peak. Panel (a) is an example of two peaks in the magnitude, possibly due to ¾-wavelength resonance. Panel (b) is an example of artifact arising from errors in Thévenin-equivalent source parameter calculation.

Figure 3

Cumulative distribution of FPL-SPL differences at octave frequencies (0.5, 1, 2, 4, and 8 kHz). Variability is greater at 4 kHz than at other frequencies, where the range is approximately between –7 and –4 dB.

Figure 4

Curves representing (top to bottom) 95th, 75th, 50th, 25th, and 5th percentile of cumulative distributions of FPL-SPL magnitude differences calculated in 10-Hz steps. Inter-quartile range is shaded. Variability is the greatest for frequencies around 4 kHz.

Figure 5

Example of reflectance magnitude plotted on a decibel scale (top panel) and phase (bottom panel) for the same single-subject calibration file used in Fig. 1 The slope of the phase corresponds to time delay (τ).

Figure 6

Cumulative distribution of estimates of distance from sound source (probe-microphone) to reflection point (TM). Distances were derived by using linear regression to estimate the values of time delay (τ) across the group. The 50th percentile is indicated by a dotted line and corresponded to a distance of 23.5 mm, approximately equivalent to the ¼-wavelength value of 22 mm for 4 kHz.