The following post by SAC seems to me to very well summarize everything being discussed here, from value of measuring with ETC to treatment methods:
"Thus, if I understand you correctly, assuming hardware loopback (automatic device correction) is used to account for hardware latency/propagation delay, if one does not apply FM's minimum phase origin translation, then each energy peak displayed in the ETC will correspond to the actual Time Of Flight (TOF) from the acoustical origin (source speaker) to the measurement mic.
Folks, for our acoustic purposes here, that is exactly what we need to be using.
Normalization relative to the direct arrival point, Td, where time=0 is assigned to the Td, is NOT useful for our particular application here. While for delay settings and for a few alternative purposes it is very useful feature, is it NOT useful for our purposes in this application.
So, if you are using FuzzMeasure, (and if I understand the implementation properly),please be sure to enable the Automatic device correction (hardware loopback in order to compensate for hardware latency/propagation delay) and do Not use the minimum phase setting. If this configuration is followed, the time of each energy arrival will correspond to the actual time of flight.
In order to determine the actual distance of travel for each direct and indirect specular energy path, one multiplies the actual TOF x 1.13 ft/ms or .344 m/ms for the distance in feet or meters respectively. This will be useful if one is not using one of the blocking methods for identifying the correlation between ETC and thee various specular paths.
If one is not empirically determining the path(s) of the indirect signal(s) by blocking, and want to know the actual distance of the indirect signal, you need to know the Total time of Flight (TOF).
This information may make the process a little easier to visualize as you can now know the actual time and the actual distance traveled from acoustic source (your speaker) to the measuring mic.
Thus if you choose to determine the specular energy paths in the most basic and accurate manner, you can, depending upon the amount of separation in time of the various reflections, more easily determine which boundary surface correlates to each energy spike. And from there, (omitting a few mechanical steps*) you can determine precisely the location of the center of the incident region.
And, assuming one knows the acoustical model** they are working to satisfy; one indeed knows where to place treatment. The type of treatment is then determined by the effect one desires to create.
Absorption will damp the reflection.
Reflection will redirect the energy which will effectively cause incidence later in time at other location(s) - in other words, it will both reduce the gain of the spike and it will effectively be 'moved' to a later time.
Diffusion will do two things. One, it will decrease the gain of the spike. And two, it will break the primary reflection into 'smaller' reflections of lower gain and spread them out in time. Thus you will have a nesting of distributed lower gain spikes, generally with the distribution in time being skewed to a later time.
(See graphics below)
Thus, if you know the target acoustical response desired, reading and interpreting the ETC correctly will indeed provide information as to the precise point of incidence, and this point is where treatment is applied to mitigate said energy in the manner desired. And knowing the acoustical response desired, you can appropriately choose which kind of treatment is useful at the location.
Note, I say what type of treatment may be useful rather than what is necessarily best. The reason is that there are often multiple ways to achieve similar results depending upon the context of your space. For instance, if your acoustical model is that of a NE room, then you will most generally want to use absorption to reduce the reflection. On the other hand, if you are building according to the LEDE model, and you want to preserve the energy while simultaneously controlling its dispersion; then you may want to employ either reflection or diffusion in order to create the acoustical response appropriate for that portion of the ETC response. But a comparison of the actual ETC with the acoustical model template will help you to determine what behavior is optimal with respect to time and gain for the given incident point in the room.
Does that make sense? The ETC provides a total picture of the specular response in the room - from early arrivals to the 'last' of the energy, be it totally damped or a decaying diffuse soundfield. And this is all done with respect to time.
The ETC allows you to see exactly what kind of energy distribution you have currently, and allows you to select and precisely place the treatment you have chosen in order to create the effect you desire - be it damping, redirection, or diffusion. It also allows you, upon repeating the measurement, to see the precise impact the positioning of the chosen type of treatment has. from this you may be satisfied, or you may want to further refine the positioning in order to insure the proper response is accomplished.
Oh, and one more important point here. It will also show you if, and to the degree, that your treatment does not act completely in the manner you suspect. In particular, this is most common with absorption, which to many folks surprise, will often exhibit a stronger degree of reflection than anticipated - especially if the angle of incidence at the boundary is great. you can also determine the actual degree of diffusion versus scattering a diffusor or a scatterer such as a poly-cylinder exhibits. With this information, you may decide to modify or use another type of treatment if , for instance, the resulting reflections are not sufficiently diffuse.
But in any case, if one becomes proficient in using the ETC, the days of blindly assuming a treatment based simply by virtue of its name, performs exactly and solely as one expects, should be over. you will discover that absorbers have a reflective quality. And that diffusors exhibit an absorptive component (often more than one would like if your goal is to diffuse and retain said energy!) and that they may also act as reflectors (especially is the incident signal is perpendicular to the unit). In other words, you will not only know what is happening within the room, but you will quickly learn a great deal about the real, as opposed to ideal, behavior of the various treatments.
And with this combined knowledge of both what is happening in the room, having the information of where to place treatment appropriate to your acoustical response design goals, knowing what specific affects your choice of treatment actually achieves - while becoming aware of any residual artifacts of the treatments, you will be well on your way to creating the response you desire...."