Since the adoption of the Telephonics TDH39 pattern of earphone as the main audiometric earphone in use in Britain and elsewhere, there have been a number of superficial changes in its design. The most significant change was around 1980 when the metal-cased design was replaced by a plastic-cased version: the TDH39P. At that time it was assumed that the new earphone was equivalent to the metal-cased version since both types of earphone exhibited similar frequency responses as measured on the IEC 303 acoustic coupler. However, it has been found, through measurements taken during the course of routine audiometer calibration, that the two types of earphone appear to exhibit a significant difference in the measured SPL at 6 kHz when they are compared using an IEC 318 wide-band artificial ear.