Seemingly, primary motivations for felt mats are reduced cost and facilitating DJ pitch bending and slip cueing.
Instead, my platter mat wish list includes:
good grip - speed stability cannot be ideal with slipping LPs
more thickness - 1.5mm felt mat provided e.g. with AT-LP140BX
wants vertical tracking angle at or below adjustment lower limit
minimize dirt and damage - felt attracts dirt and is hard to clean
vibration attenuation - including immediate reactions to needle tracing
and ambient e.g. feedback from loudspeakers
While I eventually intend to measure frequency response sweeps from test records with different mats,
initial measurements are phono signals from a needle resting half-way thru an idle LP stimulated by loudspeaker frequency sweeps.
These measurements are confounded by loudspeaker response, room resonances, turntable stand rigidity and chassis isolation, so only interesting for changes in response.
In this case, the turntable stand is 20mm MDF on engineered flooring over concrete slab, so minimal "trampoline effects".
Unlike traditional (e.g. belt-drive) turntables,
which typically have flat platter surfaces with LP contact determined by rubber mat ridges,
the AT-LP140BX platter has an edge lip with an abrupt surface recess reducing gradually towards the center,
so that support for flat LPs is only at edge and label even with a clamp.
Consequently, LP grooves in between are are free to respond as diaphragms.
Custom molded or highly resilient mats could fill that gap,
but (unless ribbed) risk driving mat debris into underside LP grooves.
As an expedient spacer for provided 1.5mm felt, a 295mm diameter mat was cut from tool box liner:
.. which raised LPs 3mm.
Using REW (Room EQ Wizard),
felt pad cartridge signal measurements of loudspeaker sweeps indicate that
microphonics are generally comparable with cover down or removed,
with most differences below 350Hz:
Sharp nulls suggest mechanically conducted vibration into platter from base cancelling
airborne energy into LP.
The extent to which acoustic energy travels up from stand through feet and chassis vs cover effectiveness,
despite mechanical coupling of cover to plinth to platter, remains to be determined.
However, sharp nulls suggest that they are comparable.
Similarly, benefits of mat isolating LPs from energy conducted into platter
vs damping LP vibrations by mat and platter area work items.
Here are some evidence from REQ (Room EQ Wizard):
Trace 3 averages 1 and 2, which are of provided felt mat over tool mat.
The orange trace of tool mat and green trace of felt mat indicate that
more is generally better, but dip around 5.5Hz for felt and 12Hz for tool mat may be meaningful.
The 12Hz dip may be cancellation broadened by tool mat damping,
but the 5.5Hz dip looks like a resonance.
By analogy with electronics, we know that there are two kinds of resonance: high impedance for parallel and low impedance for serial.
Felt may present a high impedance resonance @ ~5.5Hz to energy from platter,
while tool mat may present a high impedance @ ~12Hz,
but it seems more likely IMO that mats instead present low impedance (sinking acoustic energy from LP to platter) at resonances.
Here is more evidence:
This compares felt over tool mat vs tool over felt, so arguably the same series impedance between platter and LP,
but note dips at both ~ 6 and 11Hz,
which suggest that the tool pad couples better to the LP, supposing that energy is being sinked from LP to platter.
For clarity and before evaluating traditional mat alternatives,
measurements are wanted with better isolation, perhaps including
a cover that surrounds the turntable, not resting on the deck
rather than the included cover that couples to the plinth.
That sort of cover is seemingly wanted for most turntables lacking decoupling between platter and plinth...
These direct-drive "DJ" turntables are about 450x350mm,
while e.g. Thorens TD 160 plinth are about 480x390mm;
if covers of about TD 160 size were available at reasonable cost,
then one could construct an open-top box about 100mm high
in which to set a sprung turntable and e.g. attach Thorens cover hinges to that box.
I would prefer a semi-automatic direct drive with spring suspension,
but that wants getting lucky e.g. finding a Technics SL-1700MK2.
Instead, this Audio Technica has M6x2.0 threaded feet, so inserted springs between them and its plinth.
Installing nuts threaded down to expose about 1/4 inch locates small ends of $8 conical springs from Amazon:
Taller and softer springs would be better...
A hole saw cut four 2.25 inch diameter pucks from 3/8" plywood that fit plinth feet pockets,
with 1/8" deep spring pockets made by a 3/4" spade bit
and a 3/8" center bore to clear threaded feet ends.
If not semi-automatic, then at least tonearm lift at runout is wanted. The Q UP lift on my Thorens was much less expensive back in the day,
and no commercially available lift fits an AT-LP140XP or other DJ style turntables.
The AT-LP140XP has a steel deck, so a lift can be located by
Flat head screws are hot-glued to a pair of those squares,
and a pair of nuts on each screw anchor either end of about 7 inches of 14AWG copper wire.
Another 2 inches of wire are wrapped and soldered at right angle about 2.25 inches from one end. A pendulum is wrapped around one bent arm of that cross piece,
with enough random nuts hot-glued at one pendulum end to just overbalance the tonearm at the other end.
The other cross piece arm is bent to hold the pendulum's weighted end just over center,
so that the tonearm can easily topple it:
Spring Suspension MKII
Springs from McMaster-Carr:
Their spring rate is spec'd at less than half that of relatively tiny springs from Amazon,
which were probably overrated.
Upper spring seats are old Pioneer PL-950 feet.
Increased spring travel has no benefit in this application,
but the LP140XP is now much less affected by side impacts to its stand.
Springs could be still softer e.g. 5 instead of 6.5 lb/in: https://www.mcmaster.com/1692k4 Those would re-enable height adjustment by turning nuts on threaded feet
and further reduce plinth resonant frequency:
REW microphonics measurements involving these larger springs had two unexpected results:
resonances specific to the dust cover
unfortunate interactions with tonearm resonance e.g. @ 59Hz
Increased microphonic sensitivity below 10Hz would be inaudible, but impacts record tracking
Some measurements include open cell foam cut from packing material:
With stock feet, removing the cover generally measures worse;
those feet seemingly cancel 59Hz cover resonance:
As note during earlier measurements, dust cover up is worse than either down or removed.
With the dust cover removed, foam alone is worse than stock feet nearly everywhere.
The foam over washer over stock feet sandwich works less well
but shows dust cover resonance impact more clearly:
Increasing mass also reduces suspended plinth resonant frequency.
Inserting a 3/4 inch maple sprung shelf with a pair of foot square ceramic tile,
interleaved with expanded polymer sheets to avoid rattles, should improve isolation.
McMaster-Carr 1692k42 springs still have a half inch of travel.