I'm enjoying the discussion involving Hop, and Jamie, and Bass about how much <20Hz SPL is needed to add perceptible weight to the sound. It's something I have spent a lot of effort trying to figure out, without too much success. But, FWIW, I don't think that the Equal Loudness Contours help us too much with that, for two reasons. First, as Bass pointed out, we aren't necessarily trying to hear bass frequencies in equilibrium with other frequencies. They are just providing some heft to what we actually are hearing.
The second reason is that frequencies below 20Hz are at least as much tactile as they are audible. As frequencies drop lower, we have more-and-more difficulty separating what we are hearing from what we are feeling. And, there is good evidence that we can feel low-bass tactile sensations at much lower volume levels than would be required for acoustic equilibrium.
I read one study that indicated that people can actually hear <20Hz test tones at volume levels only 10-20db above the noise floor in the room. The average noise floor is probably about 40 or 45db, so that suggests that listening to <20Hz tones in isolation, we might be able to hear them at 60db or so. Of course, during normal listening, we don't hear <20Hz tones in isolation. Instead we hear complex sounds, with some fundamentals below, or well below 20Hz. And, the complex sounds, and harmonics of the fundamental <20Hz frequencies keep those lower sounds from standing-out clearly. They just provide a kind of foundation for the bass.
I listen to a lot of jazz music, and I was thinking of an analogy that any jazz enthusiast should get immediately. Most jazz combos consist of at least three instruments: a piano, or other solo instrument, to establish the melody; a drum set to provide rhythm and variety; and an upright bass to establish a foundation for the music. There may be more than three instruments involved, but the drums and the upright bass are pretty common denominators, regardless.
Sometimes, when I'm listening to a jazz track (typically not smooth jazz or something bass-enhanced) I may be well into the track before I even notice the upright bass. It's simply providing a rhythmic bass foundation, perhaps at about 50 or 60Hz, that is almost completely covered-up by the more overt sounds of the other instruments. I may find myself deliberately listening to hear the upright bass chords, and even then they are usually still subtle. (Sometimes, the upright bass gets to do it's own thing, and then the bass chords do stand out.) But, there is a reason why virtually all jazz combos have that upright bass, because without it, the music would sound thinner and less important. It is a back-up instrument most of the time, but a very important one.
I think that is a very good analogy to what <20 bass provides in our music and HT systems. Most of the time, it isn't extremely important sounding by itself--it's just a presence, a weight that makes the other sounds seem more real. After all, there is <20Hz content all around us all the time, outside our HT rooms. And, most special effects in movies rely heavily on low-frequencies to establish a foundation for the action. Occasionally, there will be content where the <20Hz frequencies are really emphasized--some organ music, or some bass-enhanced music, or some special effects in movies.
But, most of the time, the <20Hz content just establishes a foundation for the frequencies that we are more aware of hearing. Play two pieces side-by-side, and I think that most people would have no trouble at all distinguishing between the one with the heavier bass and the one with the lighter bass, even if the heavier bass content were not very overt, or in equilibrium with the higher frequencies that we hear more easily.
GUIDE TO SUBWOOFER CALIBRATION AND BASS PREFERENCES
* The Guide linked above is a comprehensive guide to Audio & HT systems, including:
Speaker placements & Room treatments; HT calibration & Room EQ; Room gain; Bass
Preferences; Subwoofer Buyer's Guide: Sealed/ported; ID subs; Subwoofer placement.