A Simple and Inexpensive Method for Calibration of Earphones

Abstract

Since the small laboratory or field installation often has difficulty justifying an expensive coupler system for earphone calibration, a simple method was sought which would employ more common laboratory equipment in making the necessary calibration. To this end a coupler system employing a Permoflux PDR-8 earphone driven as a microphone was devised. Outputs from a series of phones tested by the “PDR-8 coupler system” were compared with outputs of the same phones tested by means of a Western Electric 640 AA 6-cc coupler system. The observed errors between the two systems were plotted and a line of best fit drawn. This line of best fit was accepted as the plot of constant errors introduced by the PDR-8 coupler system when used to measure output of other PDR-8's, and appropriate corrections were applied to readings of the PDR-8 coupler system. Comparison data of three test phones calibrated by means of the PDR-8 coupler system and a Western Electric 640 AA 6-cc coupler system revealed Σd/n = −0.04 db and d = 1.35 db, an error considered quite acceptable for earphone test for many laboratory or field installation purposes. To apply this method one needs to secure the constant error curve for a particular phone to be used as the calibration coupler system and apply these corrections to output readings as calibration of other phones is attempted. Correction curves are specific to the particular kind of phone under test as well as to the phone used in the coupler system. (Data collected by author at U. S. Navy Electronics Laboratory, Human Factors Division, San Diego.)