A baseline vibration signature should be established as a reference, but no historical record of periodic analyses need be maintained. Vibration amplitudes at various frequencies are compared.

A baseline vibration signature should be established as a reference, and a historical record of periodic analyses should be maintained. Vibration amplitudes at various frequencies are compared.

A historical record of periodic analyses should be maintained, but no baseline vibration signature need be established as a reference. Vibration amplitudes at various frequencies are compared.

A baseline vibration signature should be established as a reference, and a historical record of periodic analyses should be maintained. Vibration amplitudes are compared with no need to examine frequencies.