Repeatability of Real-ear-to-coupler Differences Measured by an Acoustic Method for Determining Probe Tube Insertion Depth

This article reports the real-ear-to-coupler differences (RECD) of 24 children (46 ears) and the repeatability of measures for 30 ears. For measuring real-ear sound pressure level (SPL), the probe tube must be positioned close to the eardrum in order to be representative of the SPL at the eardrum. In this study, an acoustic method that used a 6 kHz standing wave minimum in the real ear to place the probe tube tip at 9 mm from the eardrum was used with custom earmoulds for measuring the real-ear portion of the RECD. The results showed a mean insertion depth from the inter-tragal notch of 24.4 mm. Across frequencies between 0.25 to 4 kHz, the test-retest difference was less than 1.0 dB, and the reliability coefficients ranged from 0.7 to 0.9. The finding on insertion depths from the inter-tragal notch confirms the appropriateness of recommendations for an insertion depth of 25 mm in measuring RECD of children of this age. Variability in measured insertion depths across individuals to achieve the same relative distance from the eardrums supports the use of the 6 kHz notch method for accurate real-ear SPL measurements. Intersubject variability is several times greater than variability associated with repeated measurements of a single subject. The repeatability of this method compares favourably with previous literature on measures using foam/immitance tips with constant probe tube insertion depth.