Exactly. What's interesting about the BMR is that its dispersion between 2 kHz and 4 kHz is actually wider than from 1 - 2 kHz. That's because it transitions to its first rocking mode slightly below 2 kHz. If you measure the BMR monitor fairly far off axis, say 45 degrees and wider, a sag develops in the 1-2 kHz region. That's just the inherent response of the little BMR moid.
Thank you for inform me on this, I have some thinkings after read the graphs. Directivity of a speaker is the hardest part to understand and design, the only way to really know is use simulation or build prototype, so my sayings as follows are just speculations.
Normally when woofer cross to tweeter, the directivity will narrows, hit lowest point at crossover frequency, then climb. For this speaker it is not the case, so saying BMR actually widen after 2 kHz is a good point. But at 3 kHz there is a 2.5 dB dip range from a bit over 2 kHz to 4 kHz, it is a diffraction of cabinet. I have looked at BMR measurement from Hificompass, the directivity behave like a normal driver without any widening of directivity at low treble. Erin used normalized direcitivity graph, which means the directivity will be compared against on axis that has a 2.5 dB dip. So now here are two hypothesis, one is there is a diffraction which may increase directivity after 2 kHz. The other is the normalization against on axis, from the color contour plot the darkest red will become 0 to 5.5 dB rather then 0 to 3 dB. Both are due to diffraction, but different reasons.
I think a better way of representing graph for color contour plot may look like this:
We can see that if we artificially remove the dip in a smart way that there is no diffraction at any angle, the direcitivity will be flatter. That blue line shows the directivity line when diffraction removed, but only valid from 2 kHz to 4 kHz. I know there is still widening even the directivity looks like blue line, the green line is horizontal line, the small white line represents the widening of directivity. I think maybe it is because the diffraction that widens the directivity.
In the end if you followed to here and can understand the point I made, thank you for following along. One thing I want to say to Erin
@hardisj is definetly add 3 to 0 dB color in the graph to remind people there is a dip on axis. The other one is when there is diffraction, what can we do to make the contour plot represent a better directivity? Maybe add a few graphs at off axis like 10, 30 or 50 degree where there is no on axis dip. When measurements gets more technical people start to forget what they are looking and what it means, showing them the limitation and odd behavior of measurement will make them understand the speaker and measurement in a more complete and smarter way.
OMG it's happening!
This is going to be amazing especially with this potentially improved BMR driver out of left field
I just got the curved rosewood's though
