how to measure port centre line

[busterno1]

I am building this sub

http://www.darcheraudio.com.au/rzaudio/rz52/12xls.htm

but rabbit has done a redesign on it so it is a down firing port [ he is unavailable at the moment due to illness ]

my problem is how to measure the centre line to make sure the length is right

if its not right how do i change the length of it do i change the size of the corner pieces .

maybe i should just make it & hope for the best thanks for any help

[Ron E]

length is approx:
56+2*(374-112)+112+374-2*112+fudge
=842+fudge

fudge would be due to the effect of the area under the enclosure, which is not super simple to predict. You pretty much need to cut and try. Moving the point of entry into the spiral is the only practical adjustment.