Printable View
Hi,
I'm building a pair of Altec 620 cabinets for a pair of GPA 604-8H coaxials.
I'm making the front baffle from 2 thicknesses of 3/4 inch MDF, so 1&1/2 inch thick.
I'll be using the same slot port as the standard 620 cabinet, but I'm wondering if I need to alter the dimension to account for the extra baffle thickness?
Cheers.
Yes, you will because the baffle thickness is basically the port length.Quote:
Originally Posted by Sootshe
Since port length is increased a little it will tune the box a little lower. In order to keep the same box tuning frequency then you have to increase port area a little, which tunes higher, in order to go back to where it should be.
Ah yes, that is what I'm after. I never could get my head around calculating the port area for different size & shape ports.Quote:
Originally Posted by RMC
What size would the slot need to be if I increase the baffle to 1&1/2" ? or can you point me to somewhere I can calculate this?
Some have suggested I use a 6" circular port, but again, I don't know how to calculate the length.
Cheers.
Found these calculators on line & they seem to be accurate.
I worked backwards from what I want to achieve....so, I determined that the 620 cabinet box tuning is 40Hz by entering the slot dimensions & changing the tuning frequency until I got a slot length of 3/4".
Then I changed the slot dimensions until I got a slot length of 1&1/2".
From my calculations, with a front baffle 1&1/2" thick, I should need a slot of 11" x 3" to maintain the original 40Hz tuning.
The second attachment shows the results when using a round port.
For this you need 2 x 7.9cm ports 1.4" long......or 1 x 6.5" port 1.4" long.
I prefer to use the rectangular port option to maintain the original look of the cabinets.
:)
Cheers.
Attachment 90402Attachment 90403
I like the idea of a larger port, but if you were -really- wanting to keep the stock look (and port aperture), you could simply cut a larger opening in the inside panel... as a thought.
How much bigger? Good question, but I would expect a 1" rebate all around would get you close enough... 2" if you're worried about it (otherwise it could approach some sort of one-sided flanged port).
What a great idea....thanks so much.Quote:
Originally Posted by grumpy
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...
Quote:
Originally Posted by RMC
Yes, I see what you mean.
I'm not very good at understanding these port calc's, but my maths is usually prey good.
Swapping around from one port calculator to another must have got me confused with all the varying parameter options.
I went back & had another look at that particular calculator & came up with the attached 2 options.
Thanks for the QC check.
Attachment 90409Attachment 90408