For 45RPM record plastic adapters to fit into.
Bad idea then, bad idea now .
Bad idea then, bad idea now .
As mentioned, the recessed area of the platter is intended to accomodate the thickness of the label, that way the record has better contact with the platter. However, most records (at least in the USA) have a raised lip, which was to keep the record surfaces of stacked records (using the record changer on vintage turntables) from rubbing on each other when the record dropped. But that negates the idea of having the surface of the record in contact with the platter. So platters should have both a recessed area for the label and a recessed area for the raised lip.
I wonder why most modern LPs are still produced with a raised lip since modern turntables don't include record changers and I doubt many people use the changer on vintage turntables. Records should be flat all the way out to the edge.
As for recessed areas on modern platters, two of my turntables (both with acrylic platters) do not have recessed areas and the other turntable (acrylic/vinyl sandwich platter) does. My hunch is that the platters made without recesses were done that way because it's less expensive to manufacture. That and perhaps the company doesn't think it makes any difference in playback quality.
For me, the label and raised lip of LPs aren't even a consideration since I use Herbie's Donut platter mats on all three turntables, which provide full contact of the record surface and the platter no matter what the platter/LP combination. If you're interested, the Donut is available here:
Herbie's Donut mat (Towards the bottom of the web page.)
Terry9; when you mention both recessed areas, are you referring to the label and the record lip? If so, the manufacturer is to be commended.
As for your comment about aftermarket mats, I've also found that to be true, but not universally. Some platter mats can noticeably improve the sound of some platters; it depends on the materials and what you're trying to improve. For example, with a metal alloy platter I was able to greatly reduce background surface noise, improve bass control, and increase perceived dynammics using an aftermarket mat. But the same platter mat used on a platter made of an acrylic and vinyl laminate only dulled the sound. I also use a platter mat on an acrylic platter but the improvements aren't as noticeable as with the metal alloy platter.
And as for the Herbie's Donut, it's not the typical platter mat; it's a very thin (0.5mm and 0.8mm thicknesses available) elastomer "donut" that is essentially neutral. I use it to adjust for different record thicknesses so that changes in tonearm height (and thus SRA) are kept to a minimum. If there is a change in performance caused by the mat, it's not discernable by me. It does however, noticeably improve the sound caused by incorrect SRA. The Donut helps to maintain SRA between thick 180g-200g pressings and thinner 110g-140g pressings. If your tonearm height can be easily adjusted then the Donut isn't necessary.
Yes, recessed for label and recessed for lip. Nottingham Analogue is much under-appreciated IMO.
I discovered them shortly after I bought one of the first Lynn Basics, which was almost as good as Lynn's best-of-the-day, but for 20% of the cost. The first TT I heard which was vastly superior to the Basic was a NA, so I sprung for one of their premium tables, which I have to this day. After a re-plinthing and re-arming, it is quite something.
To top it off, NAS is a joy to deal with.