Dustcover Blues


Most of you probably know that I have always championed the use of dustcovers on turntables even during play, the goal being to protect the record from the environment and shield it from sound. For the first time in my audio career I have stumbled into a problem with this and other than not putting the dustcover down I have not come up with a solution. 

Yesterday I was playing Herbie Hancock's Secrets and I cranked it on my favotite song. After about 30 seconds the room started to rumble. My subs were putting out a remarkably clean 20 Hz as if I were playing a test tone. Feedback! Just turn the volume down a little and it disappears. Turn the volume back up and within 30 seconds it starts up again. Did I screw up my cartridge set up? I veiwed the tonearm during the feedback and it was rock solid. Usually with low frequency feedback you can see the tonearm shaking. I played the resonance tracks on an Ortofon test record and both lateral and vertical resonance were centered on 9 Hz With the feedback going and the house shaking I wanted a better look at the cantilever. On lifting the dustcover the feedback stopped!  The dust cover is attached to the plinth which is isolated from the sub chassis (tonearm and platter mounted on this) by four springs. The resonance frequency of this suspension is 2 Hz. Nothing above 2 Hz can pass directly through to the platter and tonearm. What is going on here? Any of you scientists out there have a clue? My best guess is that I am dealing with a type of Helmholtz resonation. The dust cover is lowered on four hard rubber pads, one at each corner. There is a 1/16" slot all the way around. This combined with the weight and dimensions of the dust cover creates a resonance at 20 Hz. To get it going I have to turn the volume way up. 

Today when I get home I'll play around with it to see if I can figure it out. Any ideas would be appreciated. 

128x128mijostyn

You can see that up against the side wall the bass is way up

My surmise is that is your room boundary effect and may also be complicated by a standing wave (looks like you have cancellation in the center of the room). A distributed bass array might be really helpful. You'd only need a pair of subs to break up the standing wave. IIRC sounds like you already have them. If they are located by the Sound Labs  they are probably exacerbating the problem- they need to be elsewhere in the room.

Once the bass is evenly distributed you may not have this problem anymore.

I never had a dust cover for my Cosmos (it was serial number 0).

I suspect your alcove is a bass node. Have you tried putting the 'table in a different spot?

 

Yep.  Audiokinesis and Duke Lejeune.  Woofer array.

The question of whether the resonance has anything to do with Helmholtz is somewhat relevant, because if it were due to the Helmholtz phenomenon, then you could live on with your prior conviction about the unalloyed benefits of the dust cover.  You point out that there is a pathway for air to exit the space enclosed by the sealed dust cover and plinth, downward around the platter evidently.  But if that constitutes a Helmholtz Resonator, then sealing the space between the edge of the dust cover and the plinth ought to have changed the resonant frequency. Sealing the dust cover to the plinth changes both the area of the aperture and the net path length across the aperture, and those two parameters are in the equation for resonant frequency of a Helmholtz Resonator.  Face it; dust covers are not all good.  If you admit that, I will admit they are not all bad.

Works for me Lew. Obviously, I am having trouble involving the dust cover. It is a first for me and my music habits have not changed. Once I fix the problem the dust cover will be all benefit again:-)

@atmasphere , Actually, because there is no rear wall and I use dipole linear arrays my room is very well behaved. The turntable is adjacent to the listening position. The bass is due to the boundary effect. This happens in almost every room. The table handles this just fine with the dust cover open. I am not putting it in the middle of the room.

I already have four subwoofers soon to be 8. Although they meet the definition of a "swarm" system they are arrayed in a specific pattern for a reason. At any rate the problem is not with the subs. The problem is a resonating turntable and it only happens with a few records cranked to the max. Next weekend I will make that skirt to see if closing off the space under the turntable will stop the resonance. The dustcover is now effectively sealed. Sealing off the bottom will prevent air pressure from having immediate access to the interior of the table.

The bass is due to the boundary effect.

Yes. But it appears you have a standing wave; if you were to break that up the boundary effect might be a bit reduced (that's how it worked out in my room anyway...). From how you've written about this, it appears that if you make just a bit of an improvement the problem would be gone since it only happens with certain cuts at higher volumes. I forgot to ask- what sort of stand are you using for the turntable?

Look at my system page and you can see where the turntable is. It is on a granite slab sitting on a built in walnt equipment/record cabinet (which I made) which rests on a concrete slab. The uprights are 1 7/16" plywood. All the shelves are braced. The entire affair sits in an alcove in the side wall. 

On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down.