Off axis for a measurement mic is going to be a well understood question. You have a small diaphragm omni. There is a very very tiny directional effect due to the diameter of the diaphragm.
Checkout the performance of a well known measurement mic here.
https://earthworksaudio.com/wp-content/uploads/2017/07/M50-Data-Sheet-2017.pdf
Nope, it's not that simple, and the effect is not "very very tiny". The measurement mic you linked is exceptionally good, but it's not the one
@amirm is using. He's using a
Microtech/Geffell MK 255. Eyeballing the
specs, at 12.5 kHz and 60° it's about -3 dB down. That's significant. In contrast, the Earthworks M50 has virtually zero loss at this frequency and angle. (Maybe we should buy
@amirm a better mic...)
Eyeballing
@amirm's
reference axis measurement spinorama, we're about 4-5 dB down at 12.5 kHz. According to the microphone specs that's consistent with a ~70° measurement angle, give or take. In order to get that angle from the tweeter while measuring on the Neumann-specified reference axis of the KH 80 (a ~5 cm vertical translation from the tweeter), the microphone would have to be about 2 cm away from the speaker. (This is a rough back-of-the-envelope calculation - I'm sure other factors would come into play like waveguide shape, measurement post-processing etc.)
2 cm seems insanely close, but keep in mind
@amirm often says the NFS measures so close that it's easy to make a mistake and have the mic hit the speaker! Furthermore, other measurement points will be at an even steeper angle from the tweeter (e.g. when measuring near the bottom edge of the speaker), amplifying high frequency error due to microphone directivity.
If the particular mic in use had enough of an issue it would be reasonably easy to incorporate a correction during processing.
And how do you propose to do that? The NFS can't tell what angle the sound is coming from. You can't apply a correction if you don't know what the angle is.
One would assume Klippel have worked through this question.
Indeed. As I explained earlier, I suspect the conclusion Klippel arrived at is: "we'll just tell people to always measure on the tweeter axis, this way the analyzer knows what the angle is relative to the tweeter and can behave accordingly". In other words, they mitigated the problem not by adding more "math" or "magic" to their system, but by baking in the assumption that the measurement axis is aligned with the high frequency sound source. If this assumption breaks down, we get the results that
@amirm showed in the first post of this thread.
(One thing that I find a bit worrying is that the problem of microphone directivity doesn't seem to be mentioned
at all in any of Klippel's whitepapers.)
To be clear, I'm not saying this is a significant problem. As long as one is aware of the assumptions that the system is making, we're all good. This is the point of this investigation: understanding the limits of the system and what happens when it breaks down, so we know what to expect in the future. We've already learned quite a bit, but there are still some small areas of concern that need elucidating. (Also, according to the results of
@amirm's expriments, speakers that don't have a single well-defined forward-firing tweeter are impossible to measure correctly with the NFS. That's good to know, at least.)