Based on the above graph and what others have posted in response to me, the above graph is good to about 28Hz and then the sub rolls off. As one drops get below six dB down, the hearing difference drops off and we need additional amplification in the lower octaves to be able to hear them equally so with the measured drop off, yes, it's clearly going be noticeable as a 10dB drop is generally perceived as half as loud.
In the above graphed example, the difference between the peak at 45Hz and at 17Hz (20dB down), the 17Hz frequency note is about 1/4 as loud as the 45Hz note. Now to the phon graph; perceived sound graph.
Note on the above graph, the 70dB level, for a 40Hz note and note that a perceived, equally loud 20Hz note is 20dB higher. It doesn't take a lot of amplification to raise the sound pressure level from 70dB to 90dB. But if one is hearing a 40Hz note at 90dB then one must raise the amplification of the 20Hz note, by 20dB more to 110dB to equal the perceived loudness of a 40Hz note. Now one is outside the limits of a single RW-12d.
(for this reason we have three RW-12d's)
We don't have three subwoofers so we can play it loud but so we can enhance the impact of the lower notes, equally. After much effort with our three RW-12d subs, below is the result of this hard won effort and make note of the small null around 80Hz and I have been posted to in reply to my questions, that even this four dB null is noticeable. The graph qualifies as usable to 20Hz.
When our above graph is compared to steve nn's graph and the results of the Phon Graph studies, one can more easily see the difficulty is getting lower octaves played equally loud in one's Home Theater space.
(I've been posted to that the Phon Graph is taken into consideration at the time of mixing so a flat graph is suppose to take the Phon Graph into consideration)
Nothing in my above is a knock on steve nn's graph, nor is it a knock on the RW-12d, which we have three of. In my above I'm simply trying to answer your question as to the interaction between a room's acoustics, a subwoofer sound reproduction system and our hearing. The interaction of which is complex indeed.
Hopefully, the above, partially explains the need for multiple subs and lots of amplified large drivers to get one where they want to go, a room full of impact bass.
-Edited by BeeMan458 - 5/26/13 at 5:53am