Originally Posted by
RMC
Sootshe,
Something didn't seem right to me with the numbers presented in post #5 and had another look. This is not to criticize your work, i found it a pretty clever try for someone who "never could get my head around calculating the port area for different size & shape ports." Simply trying here to avoid yourself a possible vent mistake with additional work.
On the first attachement for round port imperial measures are used and you get Lv 1.45". Then for slot port, which is basically a rectangular one, you get Lv 1.43". So both the same. This is not unusual because what matters is port area regardless of vent shape (round, square, rectangular or triangular).
Then you use the METRIC version of the port length formula instead of the imperial version to go with the above. You end up getting the same Lv 1.43 or so as above, but this time in centimeters. Inches translated to cm should give a higher number as there are 2.54 cm per inch, that seemed like a red flag to me. Something else had to give.
On the second attachement, i did some math on your double ports with Dv 7.9 cm each and they don't meet requirements compared to the imperial ones shown. They are way too small diameter having less than half the port area of the imperial ones. Btw port sizes don't add directly (e.g. 2 X 4" ports not equal to an 8" port, instead equivalent to a single 5.7" port). There's formulas in audio books for e.g. to calculate these things.
The imperial unit ports mentioned in that post have the following areas: 6.5" = 33.2 sq in, 3" X 11" = 33 sq in, basically are the same. On the other hand, your two 7.9 cm dia. ports are equivalent to a single 11.17 cm dia. port, with an area of 98 sq cm = 15.19 sq in (compare the latter to the 33 sq in above its less than half). Seen differently but with same result, a 7.9 cm dia. is equal to a 3.11" dia., two of 3.11" dia. equivalent to a single 4.4" dia. = 15.20 sq in, so again less than half.
On your second attachement, since your port area is too small vs imperial ones, then your software calculated Lv is also too small. Reason: by increasing Sv (vent area) you would also need to increase Lv to keep tuning frequency constant...