Q6.5: Regarding the following statement: “Digitally high-pass filter the acceleration recordings in the frequency domain using a 3rd order Butterworth filter with a corner frequency of 0.8 Hz.” Why is the Butterworth filter applied to the Fourier transform of the signal and not the signal itself? Shouldn’t a Fourier filter be applied to a Fourier transform instead of a Butterworth filter? Could the thought process for doing this sort of filtering be explained?

A: The Butterworth filter should be applied to the signal itself in the time domain, not the amplitude spectrum in the frequency domain. The wording is incorrect in Section 6.7 and will be amended. It is recommended, however, to look at the response of the signal in the frequency domain to understand why a Butterworth […]