Why the woofer moves badly when playing certain LPs

The resonance mechanism you and others propose involves forced vibration in which some source of periodic excitation acts close enough to the resonant frequency of the tonearm/cartridge to excite resonant response. This is a plausible explanation, although it doesn't explain the noticeable discontinuity in the woofer oscillations, roughly (exactly?) once per revolution, in the video. In other words, I see two distinct periods in the video. The shorter period corresponds to the woofer oscillations, the longer period corresponds to the very regular disruption of the woofer oscillations at about 0.5 Hz.

The mechanism I'm proposing doesn't involve resonance. The warp acts like a mild ski jump that imparts a vertical acceleration to the cartridge/tonearm once per revolution --- enough to flex the cantilever but not enough for the stylus to lose contact.

If the cartridge has too high a compliance (too low a mechanical resonance) then it will behave as you describe. If the cantilever is stiffer (less compliance) the mechanical resonant frequency will be higher and thus the amplitude of the warp energy will be reduced. Or- if the mass (not the tracking pressure) near the stylus is reduced (by changing out the hardware and getting rid of the stylus guard if there is one) the effect is the same.

If this theory is correct, a rumble filter or adjusting the tonearm/cartridge resonant frequency would not correct the tracking problem (although I expect the latter would alter the frequency of the bounces).

This statement is incorrect. If the mechanical resonance is corrected the amplitude of the woofer movement will be decreased. Making the LP flatter would certainly help, but is a more expensive solution (although it probably would sound better for other reasons such as improved control of the resonance in the LP itself) than simply reducing the mass at the end of the tonearm.

From viewing the YT video, it appears to me that the arm/cartridge combo is just barely outside of the optimal range and this is borne out by some of the posts above. The thing about this is a slight amount of record warp can really affect the setup a lot more- leading to some LPs being quiet and others not. This being a smaller speaker (I was unable to find if it was ported or not) the woofer is going to flop around quite a lot with out-of-band signals.


The idea of going to lower mass mounting screws is a good one (worth a try- its cheap; maybe also remove the stylus guard), better yet a cartridge with slightly lower compliance.

^^ My speakers are about 3dB down at 18Hz. We're on the same page with that one....

A certain amount of very low frequency woofer excursion with records is perfectly normal.

Just to be clear, my signal chain has 2Hz response from the cartridge input all the way to the loudspeakers, which employ dual 15" drivers. I run them without grill cloth covers and I only see excursions on things like big bass drum whacks. It takes a pretty warped record to set them off. But I was careful to make sure that my cartridge and arm work well together.


The cartridge in the arm has something called 'effective mass'. The cartridge also has a compliance figure; that in tandem with the effective mass creates a thing called 'mechanical resonance'. These things can be calculated by the way... anyway, the mechanical resonance should fall between about 8-12Hz; if it does than minor record warp won't be bottoming out your woofers. When the mechanical resonance is not in the right window, Bad Things happen- like mistracking, even to the point of the stylus jumping out of the groove, woofers pumping, breakup during complex passages and so on.

One fairly simple explanation for the complaint is the cartridge has compliance too high for the mass of the tonearm.

@mijostyn  I highly doubt that mastering lathes are to blame- too much is at stake! I have yet to buy a new LP and run into anything like this; OTOH many of the new LPs I've bought seem to have deeper bass than many of the older LPs (I like electronic music...). I did run into one LP which was overcut, causing distortion on bass notes, but overcutting is simply a mastering error on the part of the mastering engineer.