Reporting audiometric threshold in terms of SPL at the entrance to the ear canal makes the assumption that equal SPL Fields equal loudness regardless of the type earphone used. This is shown not to be always the case. Loudness balances between the W.E. 552 and the W.E. 705 A phone yield data composing differences among three laboratories as to what SPL at the entrance to the ear canal constitutes American “audiometric zero.” Loudness balances and threshold measurements using the W.E. 705 A and a British earphone, Standard Telephones and Cables Ltd. Type 4026 A, show the British phone generating more SPL at the entrance to the ear canal for equal loudness. This difference must be added as a correction factor to ear canal SPL using the 705 A earphone if one wishes to compare American threshold surveys with British surveys by probe tube microphony.