Proper method when rack is not on cement floor

I'm moving my 2 channel system into another room. Right now it's in a room that has a wood floor over cement (with area rug), so I have the rack spiked to the floor and everything seems okay. The new location is still on the first floor of the house, but the floor is over a 2-1/2' crawlspace, so it's pretty much a hollow cavity under the rack. Again it's a wooden floor, but with a rug. Is spiking under the rack still advisable for this sort of arrangement? If not, what is the preferred method?

The rack will be in a corner location in this other room, if that helps.
I've had good success using my rack spiked to a wood suspended floor. Since you're over a crawl space, you might consider one more step: install some extending jack posts under the floor joists in the crawl space to add more rigidity to the floor. They were a big help here, even with my rack at a wall location near a corner. They also helped with the overall tonal balance in the listening room by reducing some of the floor flex. Here's a sample 3' post from Home Depot.
Rushton, that link takes me to a sort of general page on HD. I see no jack posts there. I did a search and found a Tiger Brand selection in HD. Are these the ones you were referring to?
Sorry about the bad link. Yes, the Tiger Brand posts are the ones I used and was trying to show you.
No problem. Is the idea to "push up" a bit on the jack once it has been put into place? That is, to put some counterpressure on what is probably a sagging floor? If so, how is that done with these jacks?
Yes, that is the idea. The jacks are very heavy steal, with plates at top and bottom, and a threaded rod at one end. They telescope to several interim lengths, you choose the length that is just short of your overall height from floor-to-joist. Then you turn the threaded rod until tight between floor and joist, and turn a bit more to either add just a touch of support or a bit more beyond that to lift out a sag. By using a number of these, you can end up with a very rigid, very strong, floor.
Okay. Just have to find someone willing to get into that crawlspace! I'm too big a guy to squeeze into something like that. I took another look at it, and it's more along the lines of two feet or so.
Tony, Rushton's idea is a good one, but let me offer one more bit of advice to take that a step further. When installing the floor jacks, mount a 4' x 4' on top of them and use three jacks. The jacks themselves should be spread out but spaced so that they are directly underneath floor joists. Even though the 4x4 will lift and support the flooring, it is better to have the jacks directly underneath the joists for what should be obvious reasons.

By using the 4x4, you lift abd support that whole section of the floor in a somewhat even manner, making it even more rigid. Placing large sections of flat patio type stone under the floor jacks also spreads the load out over a wider area and keeps the jacks themselves from sinking into the soil.

This whole project shouldn't cost you more than about $30 - $40 at the very most and should make a substantial difference in the solidity of your flooring and what you hear. I just hope you've got a good friend that is both skinny and limber that works cheap : ) Sean
That's a great suggestion, Sean. Thanks. I can easily get a 4x4 cut to the right size at my local lumberyard. But, I have some questions...

The room where the equipment will be placed is 11' deep x 17' wide. The floor joists run from front to back, parallel to the 11' depth. So should I use just one 4x4 stringer that's about 16' long, set about halfway back into the room? The room is actually the back extension of my media / family room; the front part is about 12 x 17, and is over a full basement. That's where the media display and the front speakers are located. Yes, I understand that's counterintuitive. I should brace the flooring under the crawlspace and put the speakers and display over there. Tried it, didn't like it. It also did not meet the WAF. So I have to do what I'm doing. Given that my speakers are actually in a different part of this combined space, how much improvement will be secured if I do the bracing, but really have only the equipment in this area?
I would take steps to shore up the area underneath the speakers and / or rack areas as best possible, wherever that might be. Once you've got those areas as solid as possible within reason and WAF, anything else would be icing on the cake. As to the benefits of doing this and / or reinforcing other areas of the floor, that would depend on the solidity of the floor to begin with. The more bounce that it has, the greater the benefit would be in terms of stiffening it up. Sean

If I understand it, you have an unusual situation where the speakers are in one end of a room with a suspended floor over the basement, and the components are in the other end of the room over a suspended crawl space. Your concern is whether spiking the equipment rack in the component end would be advisable. Before you can answer that, you need to determine the magnitude and frequency of the room and floor resonances, to see what if anything you need to do.

Here are some ideas for how you might approach this.

1.Measure the room response at the listening and component positions. See if there are large peaks below 120 Hz., as these are the ones with the most energy and likely to disturb your components the most.
2.Feel the floor at these frequencies and see if the floor gets to vibrating significantly at any of them. The floor resonant frequency may be different than the room resonant frequency. Floor resonance will be very noticeable through your feet or bottom, especially at the center of the suspended floor sections.
3.Walk throughout the room while playing music with deep base and listen for peak areas, typically near walls or corners. Put your finger tips on the equipment and feel for the nature and extent of the vibrations being introduced.
4.Record all of these observations.
5.If you are lucky, the room modes will be well behaved and spread uniformly, and the floor will be solid (the component space has 11 ft. floor joists, which if not too far apart may be more than adequate). In this case, the spikes are fine and you will not need to do anything.
6.More likely, you will have both some difficult room modes and some specific floor frequency resonance that can shake everything in the room. Dealing with room modes is a popular topic here and you can find plenty in the archives. The floor also can be dealt with in a number of ways.
7.You can stiffen the floor from underneath as described by Rushton and Sean. My suggestion would be to first place a 4x4 beam perpendicular to the joists in the center of either or both floor areas where the resonant amplitude is the greatest, second, place one under the speakers which are the source of the energy, and third, place one under the equipment rack which reacts to the resonance, in that order. The first one may be enough.
8.If posts are intrusive below, you can eliminate them in favor of adding blocking between the joists (if the basement ceiling is exposed), or take 2x6s and lag bolt them perpendicular to the joists. The 2x6s work very well if the basement ceiling is dry walled and you don’t want to disturb it. Start with a single 2x6 in the center of the room. This may be enough without any posts.
9.Repeat 1-3 as you progress. When you quiet the floor it will be obvious to both your ears and the measurements taken. Since the speakers are no longer putting energy into exciting the floor, they will much more quickly respond to exciting the air and sound much tighter. Since the rack is not shaking as much, any shelf damping under components will be more effective. It is a win - win.
10.As you stiffen the floor, you can expect more of your speaker’s bass energy to get into the listening room and propagate further into the house. This may necessitate more in the way of room treatment, such as tube traps etc.

So this may be the beginning of a longer journey. Sometimes a change for the better opens up a whole string of possible improvements. Only you will know when enough is enough. I hope this is helpful.
Zargon, much thanks for the detailed information. I should be able to do all that you mentioned (as needed). It's just a question now of getting it all in place.
Lets us know what you learned and how it ended up.

Have fun!