Ok guys

Halcro, I have a decibel meter. My SOTA does 18 db down, the SME 25 dB. It has a thicker cover. The difference does decrease a bit with louder volumes. These figures are at 95 dB. Most good  hearing protectors come in at 25 to 30 dB and what are they? A thin plastic shell stuffed with a little foam with a seal fitted. Geoffkait, a Helmholtz resonator requires a throat (opening) Do hearing protectors resonate? Sorry, no mints at all.
I keep meaning to run a test record with a frequency sweep to compare attenuation at different frequencies. I will report when I get it done. Of course I can not be sure what other dust covers will do. Depends on the design. 
 halcro, most house walls are pretty resonant just bang on one. Now go into the next room. What happens to the sound? And the door is probably open. Now close the door. What happens. The dust cover is another room with a closed door. Some miracle ha. 
You guys crack me up. Assumptions are the mother of all ---- ups. If I say the word my post will get deleted. Assumptions based on mythology are fairy tales. 

Wolf how is a dust cover going to be a bass trap? Do you know what the wavelength of 20 Hz is?? The wavelength of your dust cover is 800 Hz or so. But none of this determines how your dust cover resonates. If you have a flimsy dust cover directly attached to the same platform your tonearm and bearing are mounted to and that platform is not sufficiently heavy, stiff and non resonant. You might indeed have a problem. Correctly made dust covers and turntables like the SOTA and dust covers that are not directly attached to the turntable improve the sound by attenuating the noise (music) in the room by about 20 dB depending on the cover, like having hearing protectors on. 
Now If you have static in your dust cover lifting your tonearm you need to throw the whole mess away and start over. Better yet switch to digital only. But then I guess you'll get paranoid about something else.
If you really think your dust cover causes sonic deterioration you also need to throw the whole mess away and get a decent turntable. Protecting your records is the first priority, sonics second. But Like I have said repeatedly all the turntables I have had in the last 40 years have sounded better with their dust covers down. I guess I know how to pick them or is it just dumb luck. 
I love you guys who complain about static and spend thousands of dollars on record cleaning junk. I use conductive sweep arms and dust covers, get better sound and I never clean a record I have owned from new. Never have to. The secret to cleaning records is don't let them get dirty. 

Bob, There is a best way to keep your records pristine and then there is everything else which has severe shortcomings for one reason or another. Obviously you can't spill beer on them or throw them into a corner at the end of play.
Uberwaltz actually enjoys listening to noisy records. I do not. For most of us who try to keep our records pristine by not pouring beer on them etc, the main enemies are static, dust and a worn stylus. The only way to deal with static is to discharge the record while it is playing by supplying a path to ground near the source (the stylus). As for dust the only way to deal effectively with it is to keep the record discharged and the record shielded from the atmasphere (sorry Ralph, I had to do that:) buy keeping it in its sleeve and using a dust cover. Anything else is wishful thinking and I include in this category Zero Stats and record brushes. This is not IMHO. 

Lewm and unberwaltz, you can be as hard on me as you wish. The sun rises in the east and sets in the west. Playing records creates static electricity. Static electricity attracts dust, dust damages records, dusty records get noisier and noisier. I hate noise. I hate worn out records.
Using a Zerostat or conductive brush before play discharges the static electricity from the last play. As soon as you put the stylus down new static electricity is generated attracting dust not just on the surface but into the groove. None of this is my opinion. I promise I will put IMHO anytime it is just that. Going farther as you put your staticy record away more dust is attracted and you drag all that dust into the record sleeve. You might as well throw your records into a sand box, oh IMHO. 
I hardly ever have to clean my stylus. If you have to clean your stylus before every play your records are filthy. 
I have no idea who Kenny G is. I generally do not listen to either Rap or Pop music. My wife has dragged me into country but just barely. I like Alice in Chains, Wayne Shorter and Stravinsky.

stani? When you put on hearing protectors does your head rumble and your tongue vibrate?
Lewm, tell me your system is in a clean room. That would be a $250,000 upgrade. Records that have dust ground into them do not sound so hot.
A dust cover has to be hinged to be useful. Again the best way to do this for unsuspended tables is to have what I call a platform dust cover. You have a lexan plate on which your turntable sits. The dust cover is then hinged to the platform. If there is limited space behind the turntable hinge the dust cover at the top with the back fixed to the plate. This version requires a prop to keep it open. If you have plenty of room behind the table the you can hinge it at the bottom and the dust cover will counter balance itself so friction hinges will do the job. You can use any thickness of lexan you want. I would bet that 1/4" lexan would drop sound 30 dB under the dust cover. That would be like putting your turntable in another room. These guys will do anything you want,  https://www.displaycasej.com/custom-audio-covers. 

I was playing with my dust cover up and down last night when it dawned on me. There can be a significant delay in the sound getting to the turntable depending on where it is in the room. If the delay is long enough this causes an echo which can give you the sense you are in a larger room which might appeal to some people. A euphoric distortion. I would rather listen to the acoustics on the record. 
Well for once VPI and I are on the same page. Go figure.

Mc, silly you clean the brush off between plays. Once in a blue moon I'll clean the brush with alcohol. It won't smear anything and it will keep any incidental dust away from the stylus. The sweep arm is only 20 bucks. Spend some money and experiment a bit. Who knows. You might actually like it. 1++ edcyn! 

Lewm, I also own an SME and it benefits also as does the Linn LP 12
All suspended turntables. I suspect the Air Force 1, 2 and 3 will also benefit. My opinion is based on suspended turntables. They are the only type of table I have ever own except for my very first one (TD 124). I would never own anything else even though my equipment cabinet rests on concrete. The SOTA is a brilliant turntable for the money. It easily runs with the SME, a table that costs 4 times as much! The only down side is that it does not take a 12 inch arm and some arms like the Tri Planar will not fit. I also must warn that Putting a 4 Point 9 on the newest version of the Cosmos is apparently difficult. An A goner ordered one with a 4 Point based on my opinion and SOTA is having a hard time getting it to fit. I think they will shoe horn it in but just barely. SME's and Origin Live arms fit perfectly. Unfortunately, SME arms are going to get hard to find because the company will only sell them with their tables, a stupid decision IMHO. SOTA is thinking about making it's own arm. Lets cheer them on!

Mc,  https://www.sleevecityusa.com/Antistatic-Record-Cleaning-Arm-p/tac-01.htm. If you get one let me know there are a few tricks to setting it up right. 
Guys, as I said before the SOTA sounds better with its dust cover on and down as does the SME. It is like taking the blur out of a photograph. I have done the fore mentioned experiment and I can routine tell when the dust cover is down blind folded. If your turntable sounds worse with the dust cover down then have a platform dust cover made or get a new turntable. Not using a dust cover is not an option. Record safety is paramount. Using a conductive sweep arm will help but still incidental dust will fall on the record not to mention get into your tonearm and bearing.  

Not using a dust cover is a sure way to destroy your records but it is great for the record cleaning industry. I never clean my records. Don't have to.
As for sound that depends on the turntable and the dust cover. This may be the reason there are so may opinions. Most turntables do not come with a dust cover.  Those always sound better without if you catch my drift. A suspended turntable like the SOTA always sounds better with the dust cover down. The sub chassis with the arm and the platter are floating inside so the dust cover is not connected to it. The dust cover attenuates the noise in the room 20 dB or so, like having hearing protectors on.
Unsuspended tables with a fixed plinth might have a problem with the dust cover attached directly to it. Close your eyes and have a friend or spouse raise and close the dust cover to see if you can identify when the dust cover is down. If not, you are in business. Same if it happens to sound better. 
Playing a record generates thousands of volts of static. The record becomes negatively charged but dust is positively charged. So the charged record pulls dust into the groove like a magnet. If you use a dust cover and a conductive sweep arm this does not happen at all. Records do not get noisier and you never have to clean them. Preserving your record collection is paramount more so than even sound. 
Making a dust cover for odd turntables like a VPI Avenger or an SME is not hard at all. You just hinge the dust cover to a plate on which the turntable sits. If you can not do it yourself these guys would be glad to. 
https://www.displaycasej.com/custom-audio-covers  A turntable covered in dust and grime is a travesty. Precision instruments should not be treated this way. To me a turntable like the Clearaudio Statement is worthless because you can't put a dust cover on it. But I can put a dust cover on an Air Force 1:)