Again thanks for that link. I seem to remmember JBL quoting THD in their literature. I may be wrong but I beleive it is a reactionary response resulting from impedance matching as well as internal resistance in the amplifier. So by definition it must be frequency dependant. It seems whatever amplifier you used to make the measurements would have an effect on the results. Thats the part I have trouble with, how do they judge accuracy? Before I go on; let me read that post you mention.