Tuntable not working with wood floors..Help

If it's an old house, the ceiling will bounce too, and it's Ugly, Ugly, Ulgly !!! Plus the TT will sway when you use it unless you add horizontal guy wires or post tension wires down to the floor -- more Ugly!! So forget that.

Elizabeth's shoring solution is OK. But instead of bricks, get four concrete piers (look like 12 in. cubes w/ sloping sides and a steel strap imbedded in the top to hold a post.)

Place two per joist on the ground in the crawl space under where the rack feet come down, and then using temporary 2x4s or framing jacks force the joists up in that area about an inch and slip 4x4 posts cut to length onto all the piers and remove the jacks.

ABSOLUTE BEST SOLUTION? A steel wall rack/shelf lag bolted into the studs or (even better) a masonry wall. Nothing better short of a reinforced concrete column poured directly into the ground (this solution works nicely for telescopes too, after removing the roof ;--)
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Not an uncommon experience for folks who have a turntable with suspension. Believe it or not, unsuspended tables can work far better on unrigid floors type than a suspended one.

If this is a ground floor you can go under the house and add supports to floor in the area where your tt is located. You can mount your tt on the wall as jjmali suggest, or in extremis you could build a support platform over your floor with the outside edges extending beyond the joists on eith side of your audio rack and with no contact with the cloor between the joists (so when you step on the floor where you rack is your tt/arm won't bounce up and down - unfortunately when you walk on the floor away ftom the rack it may still bounce.)

Personally, I'd either reinforce the floor under the turntable by adding support under the house. I'd probably use use piers or small jacks directly under the floor under the rack, or I'd get an unsuspended turntable. I did the latter (and years later I have two tt's and a new house with a stable floor).

Oh, I forgot to mention, you might just relocate your rack to an different area in the room where the floor has better support. Floors near bearing walls and corners are usually more solid although corners may not be the best place for acoustical reasons.

Anyway its a PITA, good luck.

Move the TT to a corner (where the floor joists are more solid), or wall mount on a wall rack (anchored into the wall studs.) Use an electronic stud locator to find the center of each stud.

Try hanging from the ceiling first. Really won't cost much to try. May be ugly, but you may be surprised how well it works. I had to do that in my apartment, and even though there was an apartment above me, worked well. Just got a platform and cable. It will sway slightly but it's cheap and easy.
Good luck.. I know how frustrating that can be