Probe tube systems: effects of equalization on real ear insertion and aided gain

In probe tube measures, the method used to equalize the sound field will determine the reference value used in calculating real ear insertion gain (REIG) and real ear aided gain (REAG). Several equalization methods are available with current probe tube systems. This study determined the effects of three equalization methods (substitution, modified comparison, and ipsilateral comparison) on REIG and REAG measures using open and closed earmolds for ear canals varying in circumference. Twelve subjects were selected representing a wide range of ear canal circumferences. Results revealed significant differences for REIG and REAG among the three equalization methods. These differences appear to be due to the different acoustic components included or excluded for each method. The results of this study suggest that the sound field equalization method used for probe tube measures needs to be considered when interpreting the hearing aid gain reported among various probe microphone units. Also, canal circumference may be a quick method for categorizing canal cavity size and appears to influence measured hearing aid gain.