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HA coupler type SPLs were independently determined by two experimenters for five repeated measurements with and without replacement of two ER-3A and two EARTONE 3A insert earphones. Measurements were made using a B however, from 3000 to 8000 Hz the DB-0138 coupler SPLs were higher than the HA-1 and HA-2 coupler SPLs for each insert earphone. This occurred because of the geometrical differences between the insert earphone to coupler connections and coupler types. The HA-1 minus DB-0138 and HA-2 minus DB-0138 coupler SPL differences, or coupler correction levels, could be explained by quarter-wave resonances and stepped-diameter tubing systems creating acoustic horn effects.
Frank et al. (Sat,) studied this question.