A confusing Topic which keeps coming up. Similar to the Two speakers thing.
We see recommendations on websites to point the mic upwards and to drive both speakers.
Both are valid, but only in exceptional contexts.
It is normal to point a Free Field mic at the source of interest. Dual sources can be very confusing, especially when reflections are being considered.
Most of the common measurement mics are Free (Direct) Field. They are at their best pointed Directly at the object of interest.
The Cal File which comes free with each individual Dayton EMM6 corrects (calibrates) the response to flat when the mic is used On Axis in a Free/Direct/Near Field.
EDIT, it appears they now supply three Cal files 0/45/90 degrees. How perfect.
Similarly the UMIK-1 from MiniDSP comes with a Cal file specific to the actual mic supplied.
Given the small price of either of these, it really does not get any better than this.
The generic Cal Files for the ECM8000 at REW try to do the same job using a generic response averaged over several actual mics. Home Theater Forum and Systems - HomeTheaterShack.com
These generalised files clearly cannot deal with individual mic variations which are far from the average.
The Dayton and MiniDSP are clearly better choices.
The most accurate response for the UMIK and the Dayton (using the 0 degree Cal file) will be Mic horizontal, pointing at the area of source of most interest.
The Dayton has the advantage that you can point it upwards to hear the rooms and speakers more equally (e.g. for 5.1) but still get decently flat response by using the 90 Degree Cal File.
For simple L/R or Surround Level balance, it is convenient to point up (or down if carpeted). This will (mis) treat all incoming horizontal HF equally. Pink Noise and an SLM are also convenient for this work. REW provides these.
Mics in USA ANSI standard SLMs are designed for Diffuse Field. There are very few standalone Diffuse Field measuring mics. When using a Diffuse Field mic, follow the instructions, point it upwards, perhaps leaning towards the source, e.g. 70-80 degrees.
Confusingly, the FR graphs seen in advertising for the ECM8000 suggest that it should be quite flat up to 10K when pointed upwards.
The problem with this mic is the differences seen between individual mics, as seen in the above HTS link.
EDIT A GS has recently tested an 'original' vs a new ECM. The new one has a very wild response.