I feel pretty silly right now, I spent four hours yesterday measuring my JBL 520Cs, and had no idea that Erin measured them a week ago
I am including his measurements in this post (with credit) as well as mine, so you can compare and contrast.
This is Erin's measurement of the vertical polar response
This is my measurement of the vertical polar response
When I listened to the speaker subjectively, I found that the treble was shockingly "crunchy." I have owned some speakers with very good waveguides, including the Gedlee Summas, and my 'hunch' is that the waveguide on the 520C is making more High Order Modes than is ideal. The Geddes speakers were basically inspired by him owning a JBL speaker with a diffraction slot, and Geddes trying to figure out why the treble sounded sub-optimal.
With that in mind, I stuffed some polyfill into the waveguide, producing the measurement above. The polyfill reduces the output of the compression driver by about 1-2dB.
This is Erin's measurement of the horizontal polar response
This is my measurement of the horizontal polar response, with polyfill in the waveguide
Here's my distortion measurement.
The performance of this speaker is a bit puzzling, because it measures quite well but that tweeter does not sound great. I'm not sure if I was just fatigued, or if the speaker could be improved by reducing the output of the tweeter by a decibel or two. I am inclined to say "no." I think the quality of the tweeter and waveguide simply isn't great.
I am not Greg Timbers, the designer of this speaker. But my hunch is that the "crunchy" sound of the treble is caused by a couple of things:
1) I think the waveguide has more HOMs than optimal. This isn't a deal breaker, because you can reduce HOMs. See my article on diyaudio titled "The HOMSTER"
2) The thing that's more difficult to treat, is that the tweeter seems to have a giant resonance in the midrange. Danny Ritchie observed that in his CSD measurement of the tweeter along, pictured above. I didn't do a CSD measurement on the speaker because I haven't cracked it open to measure the drivers individually.
The compression driver in the Studio 520C is basically a polymer dome tweeter with the world's tiniest neo motor, and a relatively basic phase plug. From what I can see, I think that they intentionally under-sized the motor to accomplish two things:
1) By undersizing the motor, it creates a resonant peak at the low end which increases output. Basically the same idea as putting a big woofer in a box that's too small; you get a big peak on the low end, and increased power handling because the enclosure is too small.
2) Undersizing the motor reduces costs dramatically
The Studio 520C is a really peculiar speaker, I can't recall the last time I've seen a speaker that measures this well, but sounds this "crunchy."
I have some JBL Control Now speakers that I bought ages ago, and I'm kinda tempted to measure them for similar reasons. The Control Now speakers suffer from a similar malady - they sound good but not great.
If you're looking for speakers in the price point of $100-$200 each, I would rank my purchases in this order:
1) Behringer B2030A
2) Kali LP6
3) JBL Control Now
4) JBL Studio 520C
5) Infinity IL10
Having said that, I think the 520C has a lot of potential. The cabinet is astoundingly good. It's as good as speakers costing 3X as much.
Also, keep in mind that there's absolutely no reason that you would use this center channel speaker as a center channel. The woofers are way too far apart to work as a horizontal MTM. The Studio 520C is basically a pretty decent vertical MTM that you can get on Amazon for $135 that happens to be marketed as a center channel. I would never use it as a center channel, it should be oriented vertically, as all MTMs should.