As a frame of reference, at about 30 Hz, the amplitude of a vibration only needs to be 0.002" (0.05mm) to reach the Meister Threshold of 'Extremely Uncomfortable'. If those amplitudes are knocked in 1/2 to 0.001", then they drop out of the 'uncomfortable' zone and if they're cut in 1/2 again, they would be classified only as 'noticeable'. We're talking about very small changes (in an absolute sense) making a huge difference in perception.