Ok guys

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.

Wow 20dB 🤯.....here's the miracle sound-proofing material the world has been waiting for.......1/8 inch acrylic 🥳
And no proof required...🤣
Wait till I notify that village looking for it's inhabitant.....that he's been found 🤥

It’s a cavity resonator, it’s a Helmholtz resonator, it’s two, two! mints in one!!  🤗

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. 

I love it when he talks about walls, doors and sound......
Almost as if he wasn’t pretending that he had some knowledge of ‘acoustics’ 🤥

Mijostyn, most turntable covers have an opening or gap around the bottom. Thanks for taking the bait.