In contrast to [ 48 ] where modes 2 and 3 were vibrating at the same time and the acceleration data is double-time integrated to estimate cable amplitudes whereby the noise in the in- and out-of-plane signals is suppressed, the acceleration data here is bandpass filtered by a Butterworth filter of order 6 with lower cut-off frequency at 6 Hz and upper cut-off frequency at 17 Hz in order to suppress measurement noise but not attenuate the predominant monoharmonic vibration signal at approximately 12 Hz.
Source: wiktionary