Sealed vs Vented measurements, equal slope?

I'm trying to decide whether to go with a sealed or a vented box. I have a sealed box for two 12" subs so what I did was to put 1 sub, and cover the other hole with a board. Then I measured the frequency response of the box. Then I made a hole for the port, used a piece of PVC pipe, and repeated the measurement, but The results were strange.

I expected the plot to have different curves, but they seemed to have the same slope. The ported box had the same response at around 45Hz, and 3dB higher at 20Hz, but that's it. It didn't look anything like 12dB/oct vs 24dB/oct. In WinISD with the same size box, same driver, but different configurations, the plots were quite different as expected.

I think this may be due to the fact that I used a way too small port. I just used a 40mm pvc pipe (because I have hole saws for 40mm but not for 110 :LOL:). Yes, chuffing was unbearable, as expected. I was thinking of repeating the measurement with a 110mm port. But I'm not sure about the physics of this: does port diameter have any effect other than air speed? Does a too small port get "saturated" somehow and stop performing? I thought ports only provide a "load" so they can be any size as long as they meet the required length to provide the needed load. My 2x6.5" Yamaha NS-50F, for example, have only a 40mm port.

It could be also that the sealed measurement is wrong because the box may be leaking a little?
Small ports suffer from compression under higher loads, meaning they don't scale up linearly with rising power input. This is well documented.

Small ports also generally have lower sensitivity. This has not been documented, except by an extensive study that I did in German (probably not much use here in an English speaking forum). Thiele I believe has a loss model ("QL"), which comes close, but has never been empirically tested / verified.

Leakage in BR boxes, actually speaker boxes in general, totally ruin the response. That's probably your main problem here. The Helmholtz resonator will only perform properly if the "spring" of the air volume in the box has a constant stiffness; even small leaks strongly influence that interaction.
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