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Unsolvable Woofer Pumping (Phono only)

I'm at a loss for trying to find the source of my "woofer pumping."  It's most noteworthy when playing something that is mostly/all treble, and the woofers of my Focal Aria 906s are going nuts (inaudibly, of course).  Turntable is a Debut Carbon with Ortofon 2M Blue.

Initially I was told it's an isolation problem, so I better isolated my TT, even put it right on the concrete floor to test!  Next I thought maybe a problem with the TT itself, so tried a couple others, no change.  So I figured it must be acoustic feedback, as with the TT stopped and stylus on a record, I could produce woofer pumping by tapping on certain parts of my stand...but it is also not this! I turned off my amplifier and recorded from the pre-out to a Tascam digital recorder and played that back afterwards and the pumping STILL happened! So I tried an Schiit Mani phono stage, no change in woofer pumping...I was sure it had to be my pre-amp...

So a local audiophile came over with a couple of pre-amps and we tried those.  The only time the problem went away was when the subsonic filter that one had was engaged.  So, I've ordered some Harrison Labs "FMODs" (20Hz high pass) to see if they will help.  If they do, I may order a KAB RF1 one day...but don't want to spend that much if I don't have to.
Any other ideas on what could cause this?!

tl;dr: Woofer pumping not caused by isolation, acoustic feedback, phono/preamp or a compliance issue...what's happening?!

branden_8091

Showing 4 responses by mijostyn

mijostyn's avatar
mijostyn
8,224 posts
 
Branden, this is called rumble. It is worse in your instance because of your speaker's design, little ported woofers. The enclosures are designed to push those little woofers as low as they can go. The end result is that they have to move farther to have any output down low. 
You have two fixes, a rumble filter or using a subwoofer with a high pass filter for the satellites. The subwoofer is a better solution. With a high pass filter on them your speakers will sound MUCH better and you will get better bass with a subwoofer system. Two subs or more will work best. You have to have a high pass filter for this to work. 
mijostyn's avatar
mijostyn
8,224 posts
 
And Branden the low frequency limit is 55 Hz because the woofers are too small to have any significant output below that that you can hear. It does not mean that they do not try to follow the waveform below 55Hz. They will unless you have a low pass filter on them.

Mike
mijostyn's avatar
mijostyn
8,224 posts
 
Branden, the woofers in any speaker including subwoofers will still dance to the rumble but with a larger driver in a sealed cabinet it will be much less noticeable at moderate volumes anyway. All records have some rumble built in. You may notice that some records are worse than others.
You would have to have a damaged turntable or an old idler wheel drive for it to rumble that bad. For all of us the only way to keep our main speakers clean is to divert everything under about 100 Hz to subwoofers.
For those of us with two way or one way loudspeakers this is actually critically important even if you do not use a turntable as low bass will Doppler distort everything else the woofer or speaker is trying to reproduce. For those who do not know what Doppler distortion is just have a friend drive by you at 40 MPH leaning on the horn. As the car passes you the horn will change tone. It will go from high to low. That is exactly what is happening when the woofer is flapping back and forth at low frequencies. 

mijostyn's avatar
mijostyn
8,224 posts
 
Branden, I have heard some very incredible systems using small stand mount speakers with subwoofers. You do not need any more than a two channel system to run subwoofers. The amps and crossovers are usually built in. Sure, if you are itching to get new speakers go for it. Focal's aere not shabby loudspeakers and to get a floor stander of that quality will cost a lot. I am an old fashioned two channel guy. Each of two channels has two subwoofers attached to it and my speakers are 7 feet 10 inches tall.
Newbee, cartridge tonearm mismatches can cause a lot of trouble but not like this. If the tonearm is too light the system will feed back when the right base note comes along. If the tone arm is too heavy the tonearm will start shaking when the right warp comes along and can even fly up into the air. When you are about to replace a worn out cartridge start adding pennies to the head shell and see what happens. Oil damping will do absolutely nothing for this problem as the cartridge is doing exactly what it is supposed to be doing, picking up vibration. It is not shaking, jumping off the record or feeding back. Do this. Take your speaker covers off. Put the stylus in any run out groove and turn the volume all the way up. Now look at your woofer dance. 
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