Red is 4pi, blue is EWave.
I want to point out some things that may not be immediately obvious to everyone. Hopefully this is helpful.
JBL usually shows distortion this way in their data sheets. I measure distortion like this too - an SPL curve of the fundamental plotted on the same chart as an SPL curve of the distortion. Another common way to chart distortion is by percentage, which looks decidedly different.
For more examples of distortion plotted as SPL, look at the distortion charts in the post below:
This post is a comparison of various basshorns, not necessarily the topic, but the thing I wanted to do was to show several SPL charts of this type, with distortion plotted along with the fundamental. The charts in the post above show a fundamental and THD+N, bascially 2nd and 3rd harmonics combined. You can also split them out, plotting 2nd and 3rd individually with a tracking VCF, as skywave-rider has done. Either way, this kind of chart shows actual SPL, not percentage. To calculate percentage, use the chart at the link below:
Another thing worth mentioning is the noise floor. As the fundamental drops, so does the distortion, sometimes below the noise floor. Once distortion is below the noise floor, obviously, you can't know what it is. You can only know it is below a certain level.
Notice that distortion basically tracks the fundamental in most cases. Where you see SPL rise in the fundamental, you'll usually see a corresponding rise in the distortion chart. This doesn't mean distortion went up - It means the
acoustic output went up. If the distortion tracks the fundamental exactly, then distorton is flat and even. This is an important thing to remember when looking at SPL charts that show distortion.
You'll see a sort of baseline for each speaker where distortion is fairly uniform across the passband. Distortion rises dramatically when excursion rises quickly, like below the passband in a horn or reflex box. That's one of the key things to look for - if distortion goes way up at the low end, that's an indication you're using the device too low. This shows a lower limit of the device.
As I said earlier, you'll usually (hopefully) see that at some point, distortion drops below the noise floor. If the low end of the scale doesn't show a big rise in distortion, then the device is high-passed above the point where excursion has become excessive. The device basically will not distort in the passband, at least not any more than its nominal amount.
When you look at skywave-rider's chart above, you can clearly see this trend. Both crossovers high-pass the horn above its cutoff, so distortion down low doesn't rise (as a percentage). Where you see distortion SPL rise, that's because the fundamental rises. The distortion tracks the fundamental, basically -60dB straight across. This is a very low level, approximately 0.1%, indicating to me the drive level was probably just a couple volts.
