Some thoughts on dust covers

@lewm 

Household-wide, I normally subscribe to the equilibrium theory of dust - eventually enough leaves to balance the amount arriving!  But if you do use a dust cover while playing, the air volume inside the cover is pretty much a closed system so once the dust settles, no more arrives.  Most seems to end up on my stylus which I can now see with my new Zeiss loupe.

@noromance

I am not surprised you can hear a sheet of paper inserted under a record. A lot of sound energy is generated by the stylus and some of this energy hits the bottom of the record.  If the record is well-coupled to the platter, the energy will pass into the platter and be dissipated.  With paper, it is more likely to bounce back and affect the stylus.

At least, that's the theory behind the Achromat which is basically a vinyl mat infused with tiny air bubbles to absorb vibration.  Vinyl couples well with vinyl!

I am surprised at the lack of acoustic feedback I am encountering. But whether you consider a dustcover to be a massive undamped structure or not, it provides more attenuation than thin air!  It cannot create more sound energy than hits it, and some is absorbed and some reflected.

Of course, if the record surface and the cartridge body are both displaced equally by impinging vibrations, there is no relative movement to contribute to the signal output by the cartridge.

My Pro-ject tt user manual specifically states the dust cover can pick up resonance from the loudspeakers and pass those resonances into the plinth, therefore remove the dust cover while playing. 

It is pretty obvious most turntable manufacturers would rather not have to deal with dustcovers!

Thought I'd try to measure the attenuation from my cover, so I dug out my Denon Audio Technical CD and played some test tracks.  At the same time, I ran a Sound Meter application on my Android phone and put the phone on my platter mat near the pickup.  I could watch the phone while raising and lowering the dustcover.

On 1000-Hz test tones, the attenuation was about -13-dB with the cover down, while it was only about -3-dB down on white noise   On pink noise there was barely any difference!

I must point out that my plinth and table lean towards the massive side.  The more the mass, the less the amplitude generated by a specific acoustic energy level.  I find it interesting that Pro-ject don't want airborne vibrations to enter their plinth from a dustcover, but are presumably happy for them to enter the platter directly.

Acoustic feedback might make for nicer sounds, and could even explain why so many like records.  But it does not make for accurate playback if that is your thing.

I don't know why I keep posting on this, because it is one of those questions where every one of us is already convinced of the efficacy and rightness of his or her current practice.  We are all pretty much old farts and set in our ways.  Suffice to say, what I have already said, anything anyone else wants to do is fine with me.  I am operating based on my own past experiences, and so probably is everyone else. But please please don't accuse others of (1) not having a system good enough to hear what you hear, or (2) being a Philistine and so able to tolerate the horrible problem that you have seen fit to prevent.

Am l am correct in thinking that Rega and Project turntables are light weight minimalist designs? They certainly look like featherweights in the hi-fi boxing ring….

Maybe that’s why they shy off covers? Highly likely they already know that their design structures are not massive enough to resist external forces unlike decks with heavier more substantial plinths.