Rumble filtering - like you really mean it

‘Rumble’ is actually not a very accurate description. The subsonic signals are not audible in themselves, but the intermodulation they cause in bass units certainly is. The worst subsonic disturbances are in the region 8-12 Hz. In bad cases the disturbances are only 20 -30 dB below maximum signal velocities. This suggested to me that a notch in the frequency response at about 10 Hz would not only greatly reduce the cart/arm resonance problem, but also steepen the initial filter rolloff so it intrudes less into the audio band.

In this article I have used elliptical high-pass filters to create one or two notches combined with a fast rolloff below about 20 Hz, and compared them with a standard 3rd-order Butterworth filter. So far as I know this is the first use of elliptical filters for subsonic filtering. The approach is described in an analogue format but should be easily transferable to DSP using 2nd-order IIR stages