.......
AND if all that's correct, I'm not sure what value the actual TOF is when measuring with loop back (for the analysis I am doing). The measured delay (T2-T3) on sweeps I took NOT using loop back are the same 2.54ms as the loop back method. The only diff I can see is that for non loop back, direct/peak energy spike is referenced at "0" time on X-axis, while in loop back "0" time corresponds to the test signal, while shifting everything else (initial direct energy and reflections) out on the axis.
If one knows their speaker distance to mic, measure the difference from initial signal to reflected signals and do the math, what does it matter if you use loopback or non loop back?
Maybe I'm just being dense, but I don't get it.
Why are you worried about what the loopback value is? What color are the curtains in your kitchen? (There are ways to determine this, but its not germane to the issue at hand.)
If this is enabled and you have no additional processing enabled in the AVR, the time that the signal takes to travel from source to mic should be the Time of Flight. Period.
That's all you need. From this you can determine the time of flight converted to distance by multiplying the TOF by 1.13foot/ms. The time differentials between direct and indirect signals are of no practical value for this application. ...Maybe when you start setting delay lines...
What is so complex about this?
But let's take this two steps further. You imagine that you "know" the acoustic center of the speaker. Really? This is NOT always true! Is your speaker a signal aligned full range single driver speaker? Is it a signal aligned coax driver such as in a Bag End coaxial? Probably not. Taken top another practical extreme often encountered in pro-sound, what if the source is a multi-element array?
The acoustic origin of the speaker - the point at which the sound 'appears' to emanate from the speaker is NOT the tweeter (as so many want to assume) or necessarily the baffle. Confuse this a bit more by including a horn, where the acoustical origin is at some amorphous spot somewhere inside the throat, determinable only by measurement...And these are 'well-behaved' examples! We haven't even mentioned astigmatism in CD horns where the acoustic center moves with frequency, or the fact that many drivers literally offset at different frequency ranges and move forward to a different position only to move back to the normal excursion range in other frequency ranges (as Don Keele demonstrated at the Atlanta '91 Loudspeaker Design Seminar with the woofer removed from his reference B&Ws!).
If only we could simply 'define' the acoustic center of a speaker simply by looking at it!
The point is, if you already know the distance (by intuition), why bother? Because you are going to use the MEASUREMENTS to determine the actual vector paths of travel and points of incidence. And these times, and hence derived distances of travel, are referenced to the MEASURED values - the actual acoustic center - not to your intuited acoustic center. You wanted accurate...right?
And the use of the string is a basic conceptual device. It is not the recommended procedure to use as a rule. It provides a concrete demonstration and visual reinforcement of the concept!
Once this is accomplished, I would recommend that you use the blocking technique...unless you simply enjoy monopolizing other folks time and taking several days to resolve the individual reflections paths and points of incidence. In this way the process, once understood based on a good understanding of the concept, can be relatively quickly performed.
And I will go still further...if you are going to do this with any regularity, I find it difficult to imagine not using the Polar ETC and a laser pointer and letting a PolarETC program calculate the 3space coordinates and simply dialing these coordinates into a transit with the mic replaced by a laser pointer and letting the laser pointer point to the incident spot on the wall as fast as you can move the cursor to the reflection and adjust the transit coordinates!
This isn't intended to be a character building exercise. Its a necessary bit of busy work used to identify points of incidence in order to then facilitate treatment options in accordance to one's desired target acoustical response model. In other words, the first few times someone does this its exciting and fascinating. Thereafter its work...and some would rather focus on moving the goal forward and not simply standing in admiration of the hammer they use to complete various tasks - if you catch the analogy.
Thus, to regress a bit, the focus is NOT on the string. Its not on guessing the acoustical center of the source. (And what if the source you were measuring was flown 15 feet over your head? Are you into repeatedly climbing up there on a ladder or scissors jack with your string? And then there are the other points of incidence...ceilings come to mind...what fun.....) the focus is improperly configuring the test platform so that these issues become trivial and one less issue about which to worry - allowing you to focus on the larger issues at hand.
So, I have taken you a bit further than where your current focus lies, but i hope it puts this process into a bit better perspective. We use the loopback correction as we are relying on the
measurement to provide accurate values. Not because we want to add a bunch of approximate estimations into what is otherwise an extremely accurate process. And such errors, while perhaps not critical in a very short time frame and corresponding distance, become increasingly critical in longer times and distances, where the identification of incident points could easily be off by a few feet - significant enough to effect the treatment and response. And also significant enough to require what could be a pretty extensive period of re-measurement, re-placement of treatment and re-verification.
For some this time is money. For all its an avoidable pain in the backside.
The point here is not to see how many goofy variables we can introduce and explain (at least not from my perspective...and if it becomes this, there are quite a few neater 'anomalies' to explore...). The purpose is to properly configure the platform, make reliable measurements that avoid the amazing serendipitous journey of exploration, and to cut to the chase and determine the necessary information, namely the effective point of boundary incidence and to move on toe determining what treatment is most effective in obtaining the overall response that is desired.
So try not to get lost in the weeds and try to remain focused on the larger goal. Once that is understood there will be plenty of time to explore and to better appreciate the various associated minutia and variables that one can 'construct' that can be explored.