The discussion regarding THD in the first 2 octaves is an absolute non-issue. Noah and Mark are correct, but I have always said that, in either case, the harmonics of these frequencies are inaudible. They are inaudible. They have zero negative effect.
There is also the measurements that I've taken at 4Hz, 5Hz, 6Hz, all at the same dBSPL at the LP and all with extremely low HD showing up on the graph. If there was 100% THD, the harmonics peaks would be there on the graph.
You may as well argue the audibility of group delay at 5Hz.
It doesn't matter the condition. From an extreme mathmatic perspective, a sound source ALWAYS drops at a rate of 20*Log(D/Di).
I understand the theory involved, but I haven't seen any measurements that bear it out. The evidence suggests that ULF does indeed travel much farther at an intensity that contradicts the law. If the explanation is that higher frequencies are attenuated by refraction and/or absorption, the formula for the law is incomplete and has no application in HT discussions.
GP tests such as Ilkka's sweep of the LMS5400-18" L/T, expanded to 1Hz, could easily nail this one. A measurement at 2M and at 16M should show a perfectly uniform drop of 18dB across the entire BW. End of story.
To date, there is no such measurement comparison that I can find anywhere.
Any surface that is large relative to the wavelength involved can be viewed and considered as a miror of the source. If the complex situation of a room is viewed as an array of virtual sources instead of a ray tracing problem, we can get a different perspective on the behavior we can expect that makes it much easier to conceptualize. Once you have an array of spaced sources, we can evaluate the problem based on wavelength vs. spacing. Mathmatically the result is the same for any given point, but it makes for a lot of messy math to observe what is reasonably obvious to anyone who has modeled and tested the interactions of multiple sources such as pro audio loudspeaker arrays.
I suspect that this is the correct explanation. The room creates virtual sources and those virtual sources vary in intensity according to their refractive properties and are frequency dependent.
Of course, again, the evidence shows that this theory is also suspect. ULF appears to be less affected by walls and in fact travels better through the earths crust than it does through air. This makes it hard to understand how a 4" wall in a typically constructed home offers any sort of refraction as frequency decreases, much less offering more refraction.
In the end, I am less interested in the precise mathematical formulae involved and much more interested in the results. I'm not a physicist. I'm a HT enthusiast who has a library of source that has content that's intended to be reproduced as part of the A/V presentation.
I continue to be taken to task over this by the weakest arguments imaginable. "It's impossible with conventional drivers. Flat isn't loud enough. There's too much THD. The source is mistaken, it's just artifacts. It adds nothing to the experience. There isn't enough juice in a 20A circuit. The signal chain rolls off. GP measurements to below 10Hz won't offer any useful information. There was a bus driving by. The HVAC skewed the measurements. It's one of the walls. The box is too small. Ported is better..."
Mark is right, this is just a silly thread jack. I've gotten Mark so befuddled that he doesn't know where to start. I apologize, Chas. This is a great thread that should have all of these first octaves posts deleted.
My original intent was to lead to the suggestion that a FR graph for each amplifier be included because reproduction of the LFE channel, especially DD TrueHD and DTS Master Audio, includes information that a SW amp should be capable of amplifying. It seems that watts at 20Hz vs rated watts is all that matters, which is cool. It's a great thread.
Back to amplifier tests...