Nat P port length question

[Johnnz]

Today I followed the calculations in the 3" Precision Port instruction sheet and calculated two different values. One based on the original Nat P thread that recommends a tuning frequency of 28Hz for a 50l box, and one based on Opt E's "Yet another Nat P" thread which goes for 32Hz tuning. The lower value of Hz gives a port length of approx 12" whereas the higher tuning freq gives approx 9" port length. My particular cabinet design makes calculating volume rather tricky "Ambitious cabinet design for Nat P" I did make a concerted effort using Sketchup and a special plugin I downloaded to do so which gave me about 48 litres. However, there is obviously a loss from drivers, xover, wiring, dapening, bracing etc.

From the length supplied in the precision port box I was able to cut both lengths described above so will be able to listen to both and see if I can tell a difference.

Here's a picture showing how close the internal flare is to the inside wall of the cabinet. Not ideal according to the instruction sheet - it should be one port diameter away from cabinet walls. (note this is with the longer 12" port length installed)



My main question is, how much will the following factors be detrimental to the sound quality from A. Not knowing the exact volume of the cabinet, and B. Not having sufficient gap from inside flare of port to cabinet wall?


Now I have to work out how to fit the xover in there too!

[Wheels]

From playing around with WinISD and UniBox if you are within 5% on your box calculations; I don't know that you could hear a difference. I'm sure some people could comparing them side by side, but I seriously think room effects will have a larger impact in the low freq response.

Now the gap, I can't guestimate a dimension from your pic, but I personally would not run it as close as it appears to be in that pic. I would be concerned about the air "stacking up" between the port and the wall and having some resonance issues due to the reflected wave firing immediately back into the port. If you really want to run that tuning, I would build a small pyramid that is square with sides the same diameter of your port, and tapers off from nothing at the edge to the peak being half the distance from the wall to your port. then mount it square behind the port. My only thought for that is to try to redirect the air from just hitting the wall and firing straight back down the port and not letting the box resonate. Or you could just cut a board on an angle to try to reflect the air up into the cavity.

Not real sure how well that would work, or if its even necessary... but thats what I would try

[EdL]

Maximizing Performance from Loudspeaker Ports

https://www.aes.org/e-lib/browse.cfm?elib=11094

See figure 10 & 11

I faced the same situation with my Modula MTM. I included "port spreaders" attached to the rear wall of the cabinet...some referred to them as "nipples"...

I admire your craftsmanship.