PURPOSE: To separate a sound and an air current and to evaluate whether a windscreen effect in a microphone is good or not, by separating a power spectrum of a sound in a test microphone and a power spectrum of an air current, subtracting the power spectrum of a sound, and grasping quantitatively a windscreen effect with respect to the air current in the test microphone.

CONSTITUTION: A spectrum analyzer 15 receives eletric signals from a test microphone 13 and a standard microphone 12 as data, respectively, and outputs each auto power spectrum of the test microphone 13 and the standard microphone 12, a cross power spectrum of the test microphone, and data relating to correlation of the test microphone 13 and the standard microphone 12, namely, a coherence function, in accordance with program data from a computer 16. 16 is a computer to which data from the spectrum analyzer 15 is provided, and calculation for deriving quantitatively an air current power spectrum in the test microphone 13 is executed by basing on those data.