Why the woofer moves badly when playing certain LPs

What’s your speakers ?
Did you ever check phase, polarity etc ? Are you sure everything connected right from the cartridge to the speakers ?

As you can see even +/- 3cu is important, but it's not that bad with OP's tonearm and his cartridge

Audio Technica AT150Sa
Weight 8.0g,
Static Compliance 40 x 10-6 cm/dyne,
Dynamic Compliance 10 x 10-6 cm/dyne (100Hz)

Pro-Ject the classic turntable
Effective Arm Mass: 13.5 g

Resonant frequency of arm/cartridge: 7.5Hz

Look like arm/cartridge mismatch is the problem !

@imhififan


If AT’s compliance is 10cu @ 100Hz you have to convert it first to 10Hz and it will be about 17cu @ 10Hz (Mid Compliance).

13.5 (tonearm mass) + 8g (cartridge weight) = 21.5g and you can add hardware too, probably 23g total.

The you can cross the lines (17cu and 23g) and it will be around 8Hz which is OK according to Ortofon:

Resonance frequency can be calculated by using the formula

f = 1000 : (2 x π x √ (M x C)) where:
f - Cartridge resonance frequency in Hz
π - 3.14159265359...
C - Cartridge compliance lateral in µm/mN
M - Total tonearm system mass which is a sum of Mass of cartridge, Mass of headshell and screws and Effective mass of tone arm (all values in gram).

• Resonance frequency within 7-12Hz is optimal for the system.
• Resonance frequency slightly outside optimal interval 6,5-7Hz and 12-14Hz can be considered as a possible that probably can be used without problems.
• Resonance frequency outside the interval 6,5-7Hz and 12-14Hz is questionable, and the system might not work properly.

The OP could use the lightest possible mounting screws to reduce the mass slightly. Or to remove stylus protector on AT if it's possible (to reduce the weight of the cartridge). 

Or to find a slightly lower compliance cartridge for that tonearm, for example a cartridge with 15cu @ 10Hz like Pickering XSV/3000SP will be a perfect match if we will check this (the resonance will be in desired range for 15cu compliance and toneam/cartridge mass near 21-23g).

BUT some people does not care about resonance frequency at all, here is the thread about it.

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.

A cartridge with lower compliance (15cu) that i mentioned also has light mass itself (5.5 - 6.5g instead of 8g AT). Could be an ideal solution if the OP's problem is cart/tonearm resonance.

@starbusters look at Pickering 3000/SP:

Stylus Type: Nude Stereohedron
ORIGINAL STYLUS: D3000SP
Compliance: 15cu @10hz
Contact Radii: .0028 (71u)
Scanning Radii: .0003 (8u)
Stylus Tracking Force: 1 gram (+1/2g) (-1/4g)
Setting with Brush: 2 gram resulting operation tracking force 1 gram. Optional range is (+1/2g) (-1/4g)
Frequency Response: 10 Hz to 30 kHz +
Output: 5.0 mV
Channel Balance: Within 1 dB @ 1kHz
Channel Separation: 35 dB @ 1kHz
Cartridge DC Resistance: 600 ohms
Cartridge Inductance: 350 mH
Cartridge Color: Brown
Cartridge weight: 5.5g (6.5g with brush)
Load Resistance: 47-100k Ohms
Load Capacitance: 275 pF
SP version was made in 1989, discontinued in 1994