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Vassilakis, P.N. (2000a).  Amplitude modulation depth versus degree of amplitude fluctuation: implementation error, adjustment, and implications.  J. Acoust. Soc. Am., 108(5/2): 2597 (presented at the 140th meeting of the Acoustical Society of America, Newport Beach, CA).


AM depth values are often used in signal processing as a measure of amplitude fluctuation. In such cases, AM implementation produces an error reflecting the nonlinear relationship between the presumed and applied AF degrees. A distinction has been made between two concepts: a) "AM-depth", referring to the degree of spectral energy spread and b) "AF-degree", referring to the degree of amplitude fluctuation. The usual AM implementation equation represents consistently only "AM-depth". It is shown that, in order to apply an intended "AF-degree" an adjusted AM depth coefficient must be inserted in this equation. The adjustment affects studies where changes in the degree of amplitude fluctuation have been correlated to changes in some physical, physiological or perceptual measurement. In general, all functions derived from the results of studies based on the miscalculation of amplitude fluctuation have been distorted by a factor proportional to the identified error. If and how this distortion influenced the interpretation of the provided functions will vary according to the importance and meaning attributed to each specific functionís shape. Specific examples and general implications are discussed.