Psophometric voltage

Note 1: "Psophometric voltage" should not be confused with "psophometric emf," i.e. , the emf in a generator or line with 600Ω internal resistance. For practical purposes, the psophometric emf is twice the corresponding psophometric voltage.

Note 2: Psophometric voltage readings, V , in millivolts, are commonly converted to dBm(psoph) by dBm(psoph) = 20 log₁₀V- 57.78.

Source: from Federal Standard 1037C