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 distortionproduct otoacoustic emission (DPOAE) testing. Alternative calibration methods, such as forwardpressure 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.

Further assessment of forward pressure level for in situ calibration

“…These findings indicate that a lack of standing-wave effects on SPL calibration could not account for the general lack of improvements seen with FPL calibration. Additionally, Richmond et al (2011) demonstrated that improvements in test performance at 8 kHz could not be attributed to detrimental standing-wave effects on SPL calibration at that frequency.…”

“…However, Richmond et al (2011) re-examined the data and found the strongest evidence of standing-wave nulls at 4 kHz across subjects. These findings indicate that a lack of standing-wave effects on SPL calibration could not account for the general lack of improvements seen with FPL calibration.…”

Relation of distortion-product otoacoustic emission input-output functions to loudness

“…It is interesting to note that the rms error at 4 kHz is usually higher than the error at 1 and 2 kHz. The source of the error may be related to standing-wave errors which have been shown to occur near this frequency (e.g., Scheperle et al, 2008;Richmond et al, 2011).…”

Reliability of distortion-product otoacoustic emissions and their relation to loudness

“…Although recent data suggest that DPOAE test performance or predictions of threshold are not affected by calibration Rogers et al, 2010), concerns have been noted for potential errors associated with standing waves when using the present calibration method (Siegel, 2002(Siegel, , 2007Scheperle et al, 2008;Kirby et al, 2011). Therefore, it is possible that using either sound intensity level or forward pressure level during calibration may have reduced calibration errors Scheperle et al, 2008;Kirby et al, 2011;Richmond et al, 2011) and resulted in greater reliability.…”