EFFECTS OF PROBE INSERTION DEPTH ON REAL EAR MEASUREMENTS

The purpose of this article is to provide an overall summary of the ro le probe insertion depth has on real ear measurements, and to compare the real ear sound pressure level measured by a probe microphone syste m, using three methods for positioning the probe in an ear canal. The probe insertion techniques that were compared included: (1) an acousti c method that incorporates use of the quarter-wave antiresonance prope rty of the ear to determine acoustically the location of the probe tub e relative to the eardrum in an individual ear; (2) a constant inserti on depth method (25 mm from the intratragal notch); and (3) the earmol d +5 mm method, which places the probe 5 mm beyond the tip of the indi vidual's earmold in the canal, thereby avoiding problems associated wi th the transition region, where sound exits from the bore of the earmo ld into the larger ear canal. Measurements were obtained for each meth od at 32 frequencies in the unoccluded ears of 17 subjects. Results in dicated that the sound pressure levels measured by the acoustic method were significantly larger than those measured by the other two method s. This result was most evident in subjects with long ear canals (> 25 mm) and at high test frequencies (4.0 to 6.3 kHz). For subjects with short or average length ear canals, the three methods provided essenti ally equivalent results.