An idea: A T-line with both ends open

And then you proceed to share your 60hz ‘sub’ woofer, 🤡

Also, it is a BP4 enclosure and not a BR or BP6. Below tuning, a BP4 performs as a sealed enclosure.
Measure a sealed enclosure with a F3 of 55hz outside on the ground and then measure the same enclosure in a vehicle. Betcha the F3 is WAY below 55hz that HR cannot predict at 0.5pi.
And remember, the enclosure was designed for an extended cab COMPACT pickup. The cabin is SMALL which BOOSTS cabin GAIN.
Tuning a BP4 below 50hz gives you that "1 note" bass in a car. It's cool for SPL, but not SQ.
I've been trying to find an article where a guy built a FLH for a Subaru wagon. I think his tune was around 70-80hz. He kept raising the tune to get rid excess low bass (or add midbass).

Lets @$$-U-me that the teal line below was from my 1996 Ford Ranger SuperCab's transfer function (TF).

TF @ 80hz = 0dB.
TF @ 40hz = +14dB.
2pi BP4 @ 80hz = 131dB.
2pi BP4 @ 40hz = 118dB.

BP4 131dB + TF 0dB = 131dB @ 80hz.
BP4 118dB + TF 14dB = 132dB @ 40hz.

So with a 74hz tuned BP4, I went from a ground plane F3 of 55hz to an in car F3 in the mid to low 30hz range.

TF @ 30hz = +16dB.
2pi BP4 @ 30hz = 111dB.

111dB + 16 = 127db @ 30hz.
131dB - 127dB = F4 @ 30hz.

GET OFF my 2009 BP4 😆🤣

https://www.diyaudio.com/community/threads/transfer-function-measurement-and-use.350963/

I'd be curios to know the enclosure tune of the red line below.

1726853180900.png
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Also, it is a BP4 enclosure and not a BR or BP6. Below tuning, a BP4 performs as a sealed enclosure.
Measure a sealed enclosure with a F3 of 55hz outside on the ground and then measure the same enclosure in a vehicle. Betcha the F3 is WAY below 55hz that HR cannot predict at 0.5pi.
And remember, the enclosure was designed for an extended cab COMPACT pickup. The cabin is SMALL which BOOSTS cabin GAIN.
Tuning a BP4 below 50hz gives you that "1 note" bass in a car. It's cool for SPL, but not SQ.
I've been trying to find an article where a guy built a FLH for a Subaru wagon. I think his tune was around 70-80hz. He kept raising the tune to get rid excess low bass (or add midbass).

Lets @$$-U-me that the teal line below was from my 1996 Ford Ranger SuperCab's transfer function (TF).

TF @ 80hz = 0dB.
TF @ 40hz = +14dB.
2pi BP4 @ 80hz = 131dB.
2pi BP4 @ 40hz = 118dB.

BP4 131dB + TF 0dB = 131dB @ 80hz.
BP4 118dB + TF 14dB = 132dB @ 40hz.

So with a 74hz tuned BP4, I went from a ground plane F3 of 55hz to an in car F3 in the mid to low 30hz range.

TF @ 30hz = +16dB.
2pi BP4 @ 30hz = 111dB.

111dB + 16 = 127db @ 30hz.
131dB - 127dB = F4 @ 30hz.

GET OFF my 2009 BP4 😆🤣

https://www.diyaudio.com/community/threads/transfer-function-measurement-and-use.350963/

I'd be curios to know the enclosure tune of the red line below.

View attachment 1358532View attachment 1358533
Open a window (and then the doors) and the cabin goes from a Helmholtz resonator to a stubby 1/4 wave resonator maybe?
 
Clearly, and unquestionably a 1/4 wave resonator, and one with the port exit and driver in such close proximity that you even get the full wavelegth cancelation notches(a lot like lazy shaped ‘tapped horn’ as you mentioned
 

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Open a window (and then the doors) and the cabin goes from a Helmholtz resonator to a stubby 1/4 wave resonator maybe?

Something like that, and you get MORE SPL.
That's why SPL competitions have sealed kick panel, sealed windshield, 1 window down, and 1 door open measurements.

It's a box inside a box type of deal.

Your car is a BP4 enclosure with a sealed box in the trunk and a window down.
Your car is a BP6S enclosure with a BR box in the trunk and a window down.
Your car is a BP8P enclosure with a BP6P box in the trunk and a window down.

BANDPASS RULES!!!
 
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This goofy thing is full of them too (after a nice big notch of silence based on the full wave length between the driver entry /high pressure ends it seems ? (Ignore the access hole)
 

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Well, we did prove that you built an 8th order enclosure based on your measurements and physical design.
I can’t seem to figure out how to make a decent ‘8th order‘ . except for two parallel qw resonators (separated in length, by their odd harmonic interval) and then complimented by a very large series exit qw resonator …. But it requires the very nice PA drivers and a ridiculous sized box that is so far beyond reasonable that it’s just mental masturbation in horn response
 
I can’t seem to figure out how to make a decent ‘8th order‘ . except for two parallel qw resonators (separated in length, by their odd harmonic interval) and then complimented by a very large series exit qw resonator …. But it requires the very nice PA drivers and a ridiculous sized box that is so far beyond reasonable that it’s just mental masturbation in horn response

There is an 8th order thread on this forum (also check the HR thread when DMB added the function to HR). I think they model a small BP8 with good performance.
 
@ICG I didn’t understand anything until I could look at it with horn response and the measurement mic. Im so much less confused now, but still trying ‘everything’ possible to figure it out (especially things based on 1/4 wavelengths so I can ‘easily’ identify them