I have the PXI-4461 and use this audio meter in combination with the sound and vibration toolkit. When I am measuring for example THD I can define a frequency range. Now my question is; what kind of characteristics does this frequency "window" has and in what way is this influencing my signal! I must say it is working perfect, but I have to make a reference for my documentation (for example CCIR 468-3). Could someone help me???