what frequency do i tune my enclosure to?

I did a search, so please no flames.

I am building a new enclosure for a single 12".  Probably will be between 2.5cf and 2.75cf and will feed about 400 - 500 watts rms.  4" port between 6 and 9 inches long depending on what I tune it to.  What I'm trying to figure out is, "What frequency do I want to tune my enclosure to?"  My sub has a resonant frequency of 35 hz.  I have never heard of a "standard" frequency to tune to, such as the resonant frequency of the sub.  Is that the best thing to do?  I'm not sure if I want to tune lower or higher, which is why I have started this discussion.  Tuning to 35 hz is not a problem, in fact should be just fine.  I'm just looking for answers on the best tuning frequency for my sub / enclosure combination.

Anyone have the answer? 

35 Hz is a high Fs for a 12" subwoofer.  Without knowing what brand/model it is, from that much info I suspect this sub is geared toward SPL output with very little low end extension capability (SQ).  If thumping loud is not your utlmate goal this sub may disappoint you, but if SPL is what you are looking for, tune to 40+ Hz.  Tuning the port amounts to a combination of several factors:  the capabilities of the sub, your musical intent, the specific vehicle it is mounted in, sub location and firing direction in that vehicle....etc etc. 

Experimentation may be the key for you with this one. It may work out better tuned to 38-40 Hz then above 40.   A way to do this is to have a few PVC ports on hand varying in one inch length increments.  Before the box is completely finished (exterior carpeting and such), slide the different ports into the cutout and play a combination of favorite music type and test tones.  Use rope putty as a temporary seal.  You can leave these experimental port tubes sticking out ot the box if you wish, too, so that box volume remains a constant...it doesn't have to be pretty..  When you decide on the tuning, adjust dimensions and finish the box.  Note that experimentation has to take place in the vehicle, in the way you wish to ultimately install it.

Interesting information.  Thanks.

I am not looking for SPL.  I have decided on a single 12", but plan to get my volume (SPL) from the 400 or 500 watts of clean power.  My main goal is to go deeeeeeeeep, while still sounding anything but flabby.  A nice punch would be nice, but I know I can't have everything.  I'm hoping the clean power will help a little in that regard.

I am stuck with the enclosure size, as it will have to be built to the vehicle and still give me some cargo room in my hatch.  Am I correct in understanding your suggestion to lengthen and shorten the port while it is in the enclosure?  I understand you saying that the port sticking "outside" of the box will not effect the port length.  ie. a 9" length port is only a 5" port is only 5" of the port is inside the box?  It makes sense, but I have never experimented with that.  Should make it simple to test and tune.

No, that is not correct.  A 9" port is always a 9" port even if 5" is outside the "box."

What is the make/model of the speaker you want to use?

Well, that just made tuning a little more involved, didn't it?

Old sub, many years old.  Crunch ProCast.  Stop laughing.

Theile-Small Parameters

The last two items there, DV (diameter of vent) LV (length of vent) do not jive with many of the calculations I've made. 

I could just buy a better / newer sub, but I already have this and it should  perform OK.  I'm not looking to get stupid amounts of thunder, just looking for a little extra oomph.  If I'm going through the trouble of doing this, I want to make sure the enclosure is made to optimal specifications.

Thanks to everyone so far for your answers.

I don't understand that figure for driver displacement at all.  658 cu in?  = .38 cu ft?  Should be, instead, somewhere between .07 and .1 cu ft., I would guess.

You should do fine with 2.75 cu ft of internal box volume.  Deduct, of course, the driver displacement and port tube displacement.  With that amount of volume, tuning to 35 Hz with a 4" X 7" port looks pretty good.  Port noise is just about maxxed out at 90 ft/sec air speed (with 300 watts), but I can't see cone excursion with the given parameters.  I would like to know how that 6.6 mm of Xmax will do in real life with 300 watts.

Re: experimenting with port lengths:  I meant that you want to keep the same air volume in the box for the differing ports while trying them out.  If, for example, you were trying 5 different port lengths...3" to 7"...you would mark all the ports at the 3" mark so that the same volume is displaced with each one by inserting into the box to the mark.  Once you establish the sound you want, you recalculate box volume in regards to chosen port displacement.

procastspl.jpg

A 4" X 7" PVC port tube should displace about .057 cu ft.  See https://www.the12volt.com/caraudio/boxcalcs.asp#cyl

It is a pretty good sized basket and magnet assembly, but I also noticed the above average displacement specs.  All discussion aside about this particular transducer...

I'm still looking for an answer about tuning.  Is tuning a ported box dependant upon personal opinion or actual specifications of a specific driver?  How do I know I want a box tuned to 35 hz?  or 48 hz?  I guess I can try to phrase the question differently...  Can I tune a box too low or too high?  Ho do I know when it happens and how do I avoid it?  And, is there a specific frequency that a particular subwoofer wants it's enclosure tuned for?

Thank you to all.

After already replying about tuning, I used my lunch break today to enter data into WinISD to take a look at your sub.

...but hey!  You're welcome!

Read my thread on using WinISD, stickied on top of this page.  Use the program as I descibe to look at some of the different sub drivers that are in the database.  Don't just read through my article, use it to learn.  Download the program and work your sub up.  You'll start getting it.  You're not going to learn everything about venting enclosures through answers in a forum thread.

Double check your info on that 658 cu in thing.

(DYohn, after your post I entered 658 as sq cm Sd...Bl looks right with that entry.  It looks plausible like that.)  But , now that I can see cone excursion,  this sub will exceed excursion with more than 200 watts in that box (at ~60 Hz!).