Originally Posted by bossobass
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.
I would dissagree firmly with the statement that distortion is inaudible at low frequencies. It is most certainly audible. The question is how it impacts our listening, and at what point it becomes either objectionable as with higher harmonics and operational noises, or impactful on sound character as wtih coloration from harmonic distortion. The hearing sensitivity curves work against us here, but room gain gives us a fighting chance.
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.
Myself and many others have done this measurement plenty of times and it correlates with theory quite well, making it rather un-interesting and unlikely to get published. The deviation which can be seen in some cases is within 2-4m where the size and configuration of the source can have a 0-2dB skew on what might be expected for different assumed origins (D=0).
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.
Vibrations, be it airborn acoustic energy or SONAR through water, will alwasy travel faster through a higher density medium. The incorrect part above relates to the transfer from air to that higher density medium. In general the transfer is very inefficient between mediums of greatly differing density.
Refraction is different from reflection, and it is the reflected energy we are concerned with. If you look at the problem from a virtual source perspective, it makes sense that you need a reflected energy of less than 50-70% to have significant impact on the total summed result. I only wish we could build walls that lossy, as our room mode problems would be much less severe.
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.
Agreed. My intention is to convey the concept, not the math. If we remember that every large source can be viewed as a tightly spaced cluster of discrete points, we can save much confusion when we consider less common or new ideas.
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..."
There is plenty of room for argument as to the subjective benefit, usefulness and pitfalls of greatly extended VLF response. Let's not confuse these issues with the physics of what is actually occurring. There is plenty of ammunition for argument already!