I was struck by a comment made in this post
in another message board regarding the equal loudness contour:
What this chart also tells us is that low distortion is more important than absolute output. At 32Hz, which is plainly audible, our hearing is what, ~40db less sensitive than at 64Hz, the 2nd harmonic.
That means, if I'm reading the chart correctly, that with 1% 2nd harmonic distortion at 32Hz, the 2nd harmonic is audible at about the same level as the fundamental.
To expound on that observation just a bit, let us first define deep bass as anything under 40 Hz. Yes, its arbitrary, but to me 40 and below sounds pretty deep, and it doesn't really change the overall point. It also makes the math a bit easier when compared to the lowest sound there, 20 Hz. Now then let us compare the perceptual difference between 20 Hz at 100 dB and an octave above, 40 Hz, its second harmonic. These are familiar frequencies and loudness levels for most of us here, and it also makes for clean math and is easy to track on that chart. According to the chart, at 20 dB at just below 100 dB is perceptually equal to about maybe 77 dB at 40 Hz. Let round this off to a simple 20 dB difference, or a power ratio of 100. What that means is you need 100 times the loudness of 20 hz to perceptually equal a 40 Hz sound. So, like Irv Robinson said, to even have 1% second harmonic order distortion at 20 Hz at 100 dB would be as loud to human hearing as the fundamental. Of course, that becomes all the more worse when you get 3rd, 4rth, and higher order harmonic distortions. The cleanest subs that I know of out there can not output 100 dB at 20 Hz with less than 1% harmonic distortion. The situation also gets worse below 20 Hz as the curve gradient becomes steeper in those ultra deep frequencies. I pity the guys chasing after a clean 10 Hz.
Now I'm not one of those smarty pants acoustic scientist people what with their eye glasses and pocket calculators and book readin', but I have to wonder what it would take to get truly clean deep bass, perceptually speaking. I also have to question how the CEA2010 distortion limits are established, and I am guessing there is a practical consideration there as no subwoofer would pass a 20 Hz tone at any audible frequency without an audible component of distortion as well. So, to those better versed in acoustic science, I am asking if I am not somehow overstating the audibility of distortion here? Because if this is correct, none of us have true clean deep bass except perhaps a few of the most extreme bass systems in the world.