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The JBL construction guide suggests a 2 cu. ft. enclosure for a single 2206H tuned to with a 4" duct 7.6" long. Two questions:
What should the length be if I chose a 3 in. duct instead?
The 2206H data sheet shows good response tuned to 50 Hz in a 2 cu. ft. box. What would the corresponding lengths be for a 4 in. and for a 3 in. duct for 50 Hz tuning of the 2 cu. ft. box?
Application is for HT with LF crossed over to sub at 60 to 80 Hz. Thanks.
John Y.
Hi John
Here's an online vent /duct length calculator . The numbers that this web-site spits out seem to be slightly "longish" when compared to my "MacSpeakerz" software. ( No big deal ).
Giskard did a full workup ( a couple of years back ) of the comparative numbers calculated by the various software packages - for duct lengths. Hopefully you're not too shocked to hear that they all had a bit of variance. If you care to fuss about this, you might search to see if that "overview" exists within this present forums' archives.
:cheers:
Earl K,Quote:
Originally Posted by Earl K
Thanks for your timely answer. The online calculator shows I need 2.37" for a 4 in. duct in my 4 cu. ft. cabinet. Quite a variation from the 7" + recommended by JBL. The online calculator also shows about 3/4" length for a 3" duct. ie., I don't even need a duct, just a hole.
I may invest in BB6 just for the experience in working with this. I think I will go for a 4" duct and plan to vary the length experimentally until I am around 40 Hz. It isn't very critical when the cutoff to the HT sub is considerably above that (60-80 Hz).
I couldn't find any Giskard reference to general comparison of the various SW packages. Maybe he can assist in pointing to it. Thanks again.
John Y.
I suspect it's really buried . Most likely in a thread that dealt with tunings for either 2235 or 2234 or 2245 . That's a huge search area .Quote:
I couldn't find any Giskard reference to general comparison of the various SW packages. Maybe he can assist in pointing to it.
Always a good investment .Quote:
I may invest in BB6 just for the experience in working with this.
- I can't explain your results from using that calculator . It shows me, 3.85" for a 4" diameter tube in a 2 cu' box giving a 50hz tuning. That's a wide variation from your results. I do notice that you started this thread talking about a 50hz tuning ( in 2cu' ) and now you seem to be looking at a 40hz tuning ( in a 4cu' ). BTW, 7" plus, is in the low 40hz tuning range / the online calculator suggests 7.66 for 40hz in a 2 cu' enclosure - my software suggests 7.728" ).Quote:
The online calculator shows I need 2.37" for a 4 in. duct in my 4 cu. ft. cabinet. Quite a variation from the 7" + recommended by JBL.
Personally, for this 12" / & according to the "look" of the SIMS I ran - I'd go with a 55 hz tuning. That's plenty low for surround ("helper") speakers / especially when subs are present .
:cheers:
Giskard's analysis should be trusted more than mine and if he discredits this response I would trust his remarks. Regardless, when I did calculation comparatives on port sizing I found two programs to be pretty much right on - BoxPlot and Unibox. Unibox gave quite a few more data entry points which leads on to believe it may have more comprehensive capabilities. Box Plot was pretty darn easy to use and gave straight forward answers.
As you would expect - while Unibox seems more complete, its answers are a bit more complex.
Both are available on the web - I suggest a google search. I use both (plus WINisd) when building boxes. Each have their own strengths.
Quote:
Regardless, when I did calculation comparatives on port sizing I found two programs to be pretty much right on - BoxPlot and Unibox.
- Does this mean that the "Fb Predictions" from these programs matched " Fb Reality " ?
( Meaning, a "projectbox" was actually measured for its' Fb after the build and found to agree with the prediction . )
:cheers:
Earl, I can explain my results. I went back to the online calculator and got exactly your results. I must have entered a 4 cu ft box, which will give the results that I presented. Chalk that up to senility. http://audioheritage.org/vbulletin/i...lies/frown.gifQuote:
Originally Posted by Earl K
The JBL recommendation is 7.6 inch length for a 4 in. duct in a 2 cu. ft. box tuned to 40 Hz. Good agreement with the online calculator data.
Why 40 Hz? This exercise started out with a design for a center to complement my 4648-A mains, which are tuned to 40 Hz. I wanted the same tuning for the center, which consists of dual 2 cu. ft. boxes. I thought I could just apply these box sizes to my surrounds and have a theatre system configured as "large" all around. Then I could put the sub down as low as 50 or 60 Hz, giving me more flexibility, if I wished.
The 2 cu. ft. center channel boxes that I have almost completed are actually 2.3 cu. ft. without bracing, duct, driver or insulation (which I see makes a huge difference in duct length). I shall be able to adjust the duct length by experimenting before laminating a panel of 1/2" or 3/4" to complete the front baffle. If I follow the same dimensions on my surrounds, I can work off of results of my center design.
John Y.
I did not do as you suggested - what I did was take standard 'cookbook' calculations and did the manual calculations and compared to the programs. That means I did both the box sizing and port calcs (with several others). Since the manual calcs held true (mostly - way too many variables for the DIYer to precisely manage - construction materials, air leaks, stuffing properties, etc.) for the senario you mentioned, I make a broad assumption that the mentioned programs replace lengthy, mistake prone manual calculations.
As in all cases - a final measured box is the desired goal and both programs have been within my tolerance range for my ear for previous projects. On my current project with 2225 (the one whose measuements I did take) the Fb was designed for 40hz (same as the JBL enclosure guide) and it measures at -3db 41.5 hz on one box and 40.5 hz on the other. Not much that my ear can hear different and pretty close for me. Of course, measurements in your own room may vary:D
- Okay,,, that just means that the mentioned programs and your manual calcs./ "share" or utilyze the same calculation "formulas" . I remember TimG has stated in the past that MacSpeakerz gives similar results to "UniBox" when suggesting port dimensions.Quote:
I did not do as you suggested - what I did was take standard 'cookbook' calculations and did the manual calculations and compared to the programs.
- Well,,, Fb & F3 are two distinctly different terms . Your 3 db downpoints (F3) will be influenced by Fb and perhaps some "LF room-gain" . I'm glad things worked out for you . :DQuote:
On my current project with 2225 (the one whose measuements I did take) the Fb was designed for 40hz (same as the JBL enclosure guide) and it measures at -3db 41.5 hz on one box and 40.5 hz on the other.
:cheers: