2. analog
  3. 192567-1647

Some thoughts on dust covers

Over the course of time there have been many discussions concerning the subject of dust covers.  They tend to revolve around the central question:  Should the dust cover be down or up while playing records?  Some of these discussions have been nasty, consequently I have refrained from participation.  It is hoped that I can provide some common sense that was given to me by someone of unquestioned authority many years ago.  During college and after, from 1970 to ~1980 I worked in HiFi retail, selling high end lines of audio equipment.  One of these lines was Thorens.  Sometime around 1977 or 1978, if memory serves, Thorens introduced their new TD126, as a top of the line TT with their own arm and I sold the first one at our store to very good customer.  He came back very unhappy after the first night of frustration with it.  The problem was that with the dust cover closed some of his favorite records were hitting tangentally on the very back were the platter came closest to the dust cover when it was in the closed position.  I called the manufacturer's rep and he set up a three cornered phone call with himself, the Chief Engineer of Thorens at the time, and me.  I don't recall the man's name, but it doesn't matter, it is what he said that matters, then and now.  The Chief Engineer explained that the problem was caused because the hole in the offending records was slightly off center so there was an eccentricity as such a record rotates about the spindle.  The solution was simplicity itself, the dust cover should be removed always when playing records.  That the intent of the cover is to protect the turntable when not in use.  I pointed out that we lived in a semi-arrid environment (San Diego, CA) which is dusty to which he replied that if the environment was too dusty for records it should also be considered unhealthy for people to be breathing the air.  He recommended are filtration, not dust covers to address environmental concerns.  The rep asked about air bourne feedback from speakers and the Thorens guy laughed and said that if that was a problem in a given system, relying of the dust cover was a very flimsy and ineffective solution and that proper measures should be instituted to provide meaningful distance and isolation to ameliorate the problem.   So the often offered extremes:  a) Always play your records with the dust cover down, or b) put the dust cover away in it's box and never use it, should both be recognized for what they are are - not solutions at all.  First principles:  Identify the problem(s), seek solutions and alternatives, prioritize.

billstevenson
lewm
13,150 posts
 

Like Bill mentioned in his OP, of course there is such a thing as acoustic feedback that could in the worst case affect the cartridge, but the cure for that is to move the TT or the speakers so as to prevent the phenomenon. Furthermore, as I mentioned earlier, acoustic feedback of sufficient magnitude to affect the cartridge is also going to energize the massive undamped dust cover with possibly worse consequences. No experiment is necessary, ltmandela.

pvnasby
9 posts
 

I have a Rega P8 and recently had the opportunity to play with a Naia. Can confirm that Rega ships their $13,000 TT with the same janky dust cover the P8 comes with, if that tells you anything.

richardbrand's avatar
richardbrand
212 posts
 

@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.

ricmech23
107 posts
 

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. 

richardbrand's avatar
richardbrand
212 posts
 

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.

More to discover