| Oops.. too big! [message #36118] |
Sat, 27 April 2002 05:40  |
SteveBrown
Messages: 330
Registered: May 2009
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Grand Master |
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Remember that old saying, measure twice cut once.. good, no one has to rub my nose in it :-). So I somehow ended up making my 3pi Thermionic cabinets 400 cu in. too big. The external dimensions of mine end up giving a volume of 12070 cu.in. but the plans call for 10890 cu.in. When I account for the fact that I used 3/4 material and the plan uses 5/8 material, the inside volume difference is about 400 cu.in. The first question is, will this need to be corrected, is it a significant error? Second question is, righ now I'm about 3,400 cu.in. too small for a Theater 4pi. Would it be possible to add stuffing to bring up the volume and go that direction?
Thanks for the help!
Steve
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| based on my vast experience with 2 boxes:) [message #36121 is a reply to message #36118] |
Sat, 27 April 2002 07:56  |
Sam P.
Messages: 307
Registered: May 2009
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Grand Master |
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I would say "go for it"...if you use R13 to cover the entire rear wall, the bottom wall, and one side wall...my 2.5 cu.ft. "net Volume" boxes have consistently "tuned" as if the boxes were at least 3/4 cu.ft. larger than their actual physical size. Mine were acting like the "apparent volume after R13 stuffing" was increased by over 30%. Even after reducing the port tubes to half the original length, my tuning (50Hz) was still several Hz. lower than "Pi alignment specified". I recently removed a portion of the R13, and feel the tuning shifted a bit higher. Probably closer to 52Hz. Fb now. I had started with 1.2cu.ft. of R13 'glass inside the 2.5cu.ft. box...may have been overdamped...twin peaks (at Fl and Fh) were only 20 ohms. Sam btw, go easy on the silicone seal holding the port tubes in...a beetch to pull loose from a 1/4 in. deep recess...and I'm pretty darn sure adjusting the length is often needed...only a careful z plot will tell you where you are for certain.
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| per weems, you [message #36124 is a reply to message #36122] |
Sat, 27 April 2002 08:25 |
Sam P.
Messages: 307
Registered: May 2009
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Grand Master |
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need to identify three freq.s using a z plot test setup. The lower peak is Fl, the upper peak is Fh, assuming a vented reflex enclosure. Then Fc, a single peak is found with the port sealed. Fb = sq.rt.[Fl^2+Fh^2-Fc^2] Fb is the box tuning of the whole system, after all is said and done.
A 'scope can also be used, the elipse will go into a flat line at Fb. Of course, your accuracy depend upon your sig gen calibration, that's life...but the sig gen cal can be verified against line freq (60Hz), so how far off will you be to either side...not much. Sam
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