Variations in standard couplers used in measurement of earphones and hearing aids

In comparison to balanced armature transducers designs for ear phones and hearing aids, it is common to be have specifications that are tighter than +/−3 dB and be interested in differences between devices of less than 1 dB. Very often, measurement facilities will be scattered across the globe and thus it becomes very important to understand variations in the “standard” measurement couplers used. While couplers are standardized in IEC and ANSI and work seamlessly when looking for 5-dB variations; higher precision measurements require greater care. The user must be very aware of numerous variations possible and allowed within the standard and influenced by aspect ratios of the couplers, differences in the tuning at manufacture of the coupler, and barometric pressure at different locations. Measured data as well simulated data will be shown to help understand the range of potential errors and the frequency bands in which they occur.