The equal loudness is about sound level, not feel. Here is a good post by Ricci from Data-bass, I pulled it from this thread.
Originally Posted by Ricci
Bosso is correct that the more solidly built concrete or brick constructed room will exhibit more gain and more even gain below 20Hz. I have done the same measurements and they are remarkably similar. So yes you will get more SPL in the deep bass. Everyone is on the same page there, but that is not all there is to consider.
I moved from a 4500 cu ft very lossy wooden frame room with suspended floor over crawl space to a 3500ish cube room, half submerged below ground level, with only 1 wall and the ceiling that are not either concrete or brick. I had the exact same system in both rooms. I have measurements of that system from both rooms and just as Bosso posted the newer more solid room shows much better gain below 20Hz and the headroom available we'll just call "enough" for me. The system is measurably more powerful in the new room. Subjectively the really deep bass "ULF" experience (<25Hz) is dramatically different and was far better in the old flimsy wood room despite less actual gain from the room. This is due to the floor shaking with the deep bass and giving tactile feedback to your whole body through the seat. In the old room I could literally vibrate a quite heavy coffee table that weighed probably 50 or 60 lbs across the wood floor with Pulse at REF level. It was scary to sit in there and watch movies because the structure was actually moving beneath you. in the new room the gain is better so the same system actually has more output capability delivered to the seats but the cement slab floor resolutely refuses to transfer energy even at much louder playback levels than used in the old wood room. I get less tactile transmission in the new room at REF playback than I did at -15 in the wood room. In the old room with the wood floor I could actually feel the frequency of really deep effects through the seat. The best part was I had no trouble getting this effect at even -15 below REF so movies did not have to be cranked to enjoy tactile and powerful feeling LFE effects. In the concrete slab room this effect is a very pale shadow of what it used to be even playing movies back 10dB hotter and that is with a double 18" directly behind the couch to attempt to get some transmission into the seats. It's just not the same and I miss that old wood floor.
Now consider that below 20Hz the average hearing rapidly diminishes and we switch to other senses to perceive sound that low. Most of that information comes from either very high SPL or more typically objects or yourself physically shaking. My experiences in these 2 rooms with the same system has convinced me that as far as perceiving frequencies below 20Hz and especially below 15Hz goes, the direct transmission of the sound as vibration to your body through the furniture or structure is far more effective than pure sound transmission through the air. It requires significant SPL to be able to get even a sense of the sub 15Hz stuff without direct tactile effects and this means that it not only requires a lot of firepower to reproduce, but that if you typically listen at more sane playback volumes like -20 to maybe -10 from REF, the content below 15Hz might peak at maybe 85-95dB unless you are running a rising response or the bass hot, so even if reproduced it is unlikely to matter much subjectively unless you have some direct structural or furniture vibration. Not to harp on this anymore but in my old wood floor room sub 15Hz effects were an event easily noticed at even modest playback levels and it was quite apparent when I cut them out with a high pass filter. In the new concrete floor room it is still noticeable with some of the same tracks but the level needs to be louder and it is far more subtle when the high pass is inserted. It’s not all wins for the wood room though…With the majority of the bass range of let’s say 25-125Hz the more solid concrete and brick room is better both objectively and subjectively to me.
So all of that said the summary of my personal opinion is that the tactile effects and shake from a suspended wooden floor or riser makes far more subjective difference at sub 20Hz frequencies than the SPL gain from concrete or brick boundaries. So much so that I would say it easily overshadows any extra gains you would see from a smaller full concrete block room let alone one or two boundaries. After the bass system itself the floor structure or structure the seating is on should be the number 1 modifier to look at for this speculative ULF calculation rather than overall room construction or even size. If a concrete wall is a 1.26 then a suspended floor should be like a 7.8 modifier. The construction of the walls and ceiling also have an effect on the apparent low frequency gain as does the size and shape of the space, including the placement of the speakers and yourself within it, but we are constantly in contact with the floor and any vibrations transmitted through it. If I could build my perfect room all structural boundaries would be inert, dense and massive, except for the floor which would be a wooden suspended floor.
YMMV. This is all just my opinion.
I would urge everyone to try to audition a powerful deep extending system on a suspended floor at some point. It can be eye opening if you have not had that experience. I have had the same system on both concrete and wood and I know which one gave me the more visceral and intense sub audible bass presentation.