The earlier work on the comparison of the JRB and the Massa Artificial Ear couplers has been continued to seek the explanation of certain differences observed. The later work, for the most part, has been undertaken on couplers under active consideration for adoption by the American Standards Association.The difference in pressure in 6‐cc couplers as measured by the Western Electric 640‐AA and the Massa M‐101 microphones appears to be identified with the strong earphone coupler resonance peak rather than directly with the shape of the coupler. The effect is apparently associated with the diaphragm impedance of the microphone more closely when the microphone is in use as a measurement device than when merely present, but inactive, in the coupler, except in the case of the M‐101 microphone whose presence gives rise to an unexplained increase in pressure.For general earphone calibration work, the results obtained by means of any of the coupler systems will be satisfactory. For some physical purposes, however, the effects may lead to significant errors.