I have to admit that I have not been interested in the BR-Port's directivity in such detail too (I try to avoid BR-constructions at all times). We have
@napilopez to thank for this!
Would, naive as I am, distinguish two cases. One is the general midrange "mess" that comes from the rear radiation of the woofer chassis. Since it is not possible to simulate the BR port without radiation from the driver, in this case I placed the membrane directly to the BR channel (an approximately 2'' diameter) inside the speaker - this should reflect the "evenly" distributed sound in the loudspeaker cabinet..
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The second case are the BR-Port resonances. The BR-Port resonances are created in the channel, so I would, as you suggest, make the sound radiation directly aligned with the baffle (for these frequencies).
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Let's take a look at what the computer has spit out as a result. Shown are the spectrograms normalized to the axis frequency response with +-180° horizontal for our two cases with an approximately 2'' diameter BR-Port, the cabinet is about 9'' wide. (Please note the scaling of the x-axis, only the frequency range 200-3000Hz is considered):
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At least these simulations confirm the listening experience of
@napilopez . In both cases one can assume that the midrange frequencies, around 1kHz, are attenuated by about
12dB 9-12dB with a rear BR port (compared to the front one). Of course without considering possible reflections from the front wall.
I re-enacted your case in Axi-Driver (on an infinite baffle). I got a slightly different result, but even there the membrane is omnidirectional up to more than 2kHz (+-90°).
It seems, the influence of the baffle plays a major role.
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