speaker cab design question

[skyclad]

I'm designing a 1X15, 1X12 vented speaker cab to go with my blackface Showman amp, and wondered if anybody had info about how to size the cabinet/port for maximum efficiency and optimum frequency response. I want it to be loud and clean so am thinking about EV, JBL, Weber or some combination thereof. I hear lots of talk about Thiele/Small parameters but so far haven't found anything useful to read about it that's not too technical.

Any info is appreciated.

[Fresh_Start]

I hear lots of talk about Thiele/Small parameters but so far haven't found anything useful to read about it that's not too technical.

That stuff is going to be too technical no matter what. 

Even more than electrical circuits, my theory on cab design is copy something you already know you like.

If that doesn't work for you, there are a couple of cab designs on the AX84 website & one of them includes a link to the really long design discussion thread.  There are lots of threads on this subject on the 18Watt forum.

Of course, there's also a cab/speaker sub-forum here.

HTH

Chip

[tubesornothing]

To follow up with Fresh Start's advide.

I have built a couple of Thiele small based cabinets (I am not afraid of math).  Wasn't worth the effort at all.

Go to a music store with your miniature tape measure and a notebook.  Find a cab you really like.  Copy it as exactly as you can.  Make sure you note what type of wood they use as well.  Manufacturers sites have lot of info on their cabs.

[PRR]

For guitar range, a fifteen is "too large" to benefit from porting. A twelve won't need it unless it is inefficient (non-guitar speaker).

Venting works flattest when the box is much larger than the "natural size" of the speaker. For a good-efficiency 15", this is 2 or 3 cubic feet. A vent box for a 15" should be 4 to 16 cubic feet. Alternatively, you can under-box and get a big bass boom.

Are you really thinking about a 15" and a 12" in the -same- vented box? The problem is solvable: combine the cone's masses and areas to postulate an equivalent 19" driver. You think Small is too technical; I would not go near a combined-driver problem like that.

Venting does NOT help mid-band efficiency. All it does is boost a half-octave at the bottom. Above around 100Hz the box volume and vent tuning does not matter. What matters then is total cone area, cone area/mass/force ratios, and if your audience is all on one side, frontal area.

If you can find a good Ten (tough), a four-10 box is loud and may be clean. Working indoors, a large open-back two-12 on the floor is hard to beat for total acoustic power into the room.

[stingray_65]

I've recently buit the 12" cab from the AX84 site and stuffed a VERY efficient Eminence Wizzard into it,

on my 30 watt amp it doesn't sound any louder than anything else I've used (I do like the speakers voicing too, very "British")

where it stands out is with low watt SE amps

My last Tweed Princeton I built was as loud as Thor falling down a flight of stairs!

Those fella's over there put a lot of thinking into the cab and gave some pretty clear prints, it is ported as you were saying you were looking for also.

[6G6]

OK, I'm gonna play.

At some point, somebody, somewhere is gonnna want to plug a bass into that amp.
Why?
 It's just big enough and a 12 and 15 seems like it should work for that.
The thing is, a bass will shred speakers that have held up to years of guitar abuse.

A ported cabinet will work below 100Hz ad it controls the Xmax in that range.
The key to making it work with two, different sized speakers is a center divider.
By giving the 12 ~ 2cu ft and the 15 ~ 3 1/2, you can tune both parts to work.
Although Thiel-Small covers a lot of speaker parameters, I have done just fine by using
only the resonanat freq.
If you keep looking, there are simple programs on line that will let you plug in the box volume
and res freq and it will spit out the port size and may give you a choice of round or square ports.
EV, JBL, Altec and Emininence al have T-S parameters available, if you look.

[ChrisK]

Win ISD will do what you are asking: http://www.linearteam.dk/default.aspx?download=winisd, just design as two seperate boxes as stated in the previous posting.

[skyclad]

Thanks for the suggestions. I'd thought about two boxes in one and that sounds right - to limit the interaction of the two speakers. In fact I'm wondering now if two different sized speakers are even necessary or beneficial - the 15" force goes up to 6.5k Hz.

 I've read recently that a "detuned" cabinet with a baffle opening roughly the same area as the speaker cutout, passes most of the frequencies and makes efficient use of the amp's electrical power. Like a 2X12 cab with one of the speakers removed. I think this is the tact I will take with the cab for my Showman. I'm not a frequency lab, but if it turns out any good I'll post results here.

sky

[Frankenamp]

Most Vintage instrument speakers were designed to be used (more or less- sweeping generalization here) with a two to three cubic foot enclosure. If it was even designed. Most early speakers were designed by ear more or less- if it sounded good, then they would try to duplicate the electrical parameters- mostly following a 'recepie' of sorts... Any box in that sixe range will work for guitar. Bass will be more problematic- the laws of physics are less forgiving with lower frequencies (hence Thiel & Small).

Your biggest problem will be with dispersion. A 15" starts 'beaming' below 2K and by 5 or 6 Khz, it is a bloody flashlight. Aluminum domes and whizzer cones help spread the highs a bit, but the biggest limiting factor is cone diameter. Which is why a good 8" or 10" sounds better for guitar. 12's are better than a 15, but not much. You will not be getting anything of value by combining two different sized (12 & 15) speakers in one cabinet. Here's a little experiment: get a couple of cheap speakers (thrift shop phonograph specials will do) just two smallish speakers on separate boxes. Play some (mono) music outdoors and adjust the angle of the speakers to make the sound spread out. try toeing them in, straight ahead and splaying them out at various angles... Listen for the high frequency and mid frequency dispersion- which angle has the best 'spread' of even sound coverage, and report back. You will be surprized-  at the results. Your eyes and your ears may not agree, because what looks best, and what sounds best are two different things.

I should have asked this question earlier... Why do you want to put a 12 and a 15 in one box?

[skyclad]

I've always liked 15's for guitar, and especially in this case as I'm going for a big sound. Perhaps a 15" plus a 10" would compliment each other better than the 15" and 12". As for focusing the high frequencies, I have found that a Weber Beam Blocker or similar device solves that problem. It's rumored that Stevie Ray used duct tape over the grill cloth of his 15" Vibroverbs.

My purpose for getting the Showman was to go for a loud, clean, warm and Fendery tone that I could use as a platform to build my stage sound on. I do use a few pedals, and I use various clean tones much of the time as well. I'm hoping the two driver cabinet with detuned ports is efficient (loud), covers a wide frequency spectrum, and is portable. (I'm building a dolly onto it like the old 8X10" SVT cabs) It's not that I play that loud, I just need the extra watts so I can run the amp on 4 or 5 and have enough clean power to keep up with the rest of the band.

Frank I haven't had a chance to try your two speaker experiment, but had planned to make the top speaker (10") lean up a little, like a Marshall slant cab. I'm guessing that the best spread of even sound coverage occurs when the speakers are splayed apart from one another to some extent. I'm also guessing that whatever effect is achieved by the various speaker angles, is lessened as distance from the speaker is increased. Am I correct? At any rate the sound from the up-splayed top speaker reaches my ear a little better.

And anyway, I like dreaming about this stuff.
Let me know if you have any more ideas . . .

[Frankenamp]

Sky: Your intuitive answer isn't quite right... (it LOOKS right, but it isn't) This is one of the reasons that JBL, University, et.al. went from primitive line arrays ( like Plexi50's yard sale score of some '60's era Standel columns) to those big clusterfreakin monstrosities of 80's hair band fame. The big names conveniently forgot a lot of solid sound engineering because people without a good grounding in acoustics wanted a big pile of speaker boxes (comb filter hell and other time alignment problems) that looked like it could blow you off the stage.

You will find the best dispersion with the two speakers close together in a 'V' configuration. this brings the acoustic centers of the two speakers the closest. This will make the time differences the least they can be, which makes the sound clearest. Here is a good example of how to make a decent sounding guitar cabinet without looking too strange: http://billfitzmaurice.net/XFCabs.html

A less effective solution would be to do a split cabinet similar to what you envision with a smaller (ten or so) directly over the 15. I forgot who posted it but someone described a trick (similar to the old JBL 2301 acoustic lens) http://www.jblpro.com/pub/obsolete/acoustic_lens_family1.pdf which consisted of a circle of open cell foam with a 2" hole in the center. should be better than a beam blocker...

Or you could do sumthin like this:
http://www.thewho.net/whotabs/equip-sunnamps.htm

[skyclad]

To quote the Fitzmaurice site:  "Guitar players themselves have never had a problem with looking funny, but they do draw the line when it comes to how their gear looks." That's for sure! I'm interested in the cross fire design - will have to consider that. The 2X12" open-back configuration satisfies PRR's suggestion as well. I'd be the only kid on the block with one of those. Not sure how the acoustic lens would work though.

Thanks for your ideas.
sky

[Frankenamp]

If you try the Fitzmaurice design, you wouldn't have to worry about the acoustic lens.

[skyclad]

There are some interesting designs on that site, and I purchased plans for the XF212 and Jack 12. I had originally thought to build a vertical cab but now, for the sake of a wider dispersion of sound, the horizontal design seems best - even better if sitting on the floor and tilted up. I'm also considering a cross fire design in a piggyback style box with tilt back legs and head cab clamp on top.

Some of the suggested construction methods are great ideas. Dura-Tex cab covering and polyurethane adhesive are two examples. Others I didn't like so much - the cab corners are simply butted together without even the benefit of a rabbet or dado. I'll probably use 3/4 plywood with box joints for long life and durability. In the XF forum, Mr. Fitzmaurice rules out any ports, vents or dissimilar drivers.

Many thanks for posting that link here. My knowledge and understanding have expanded because of it.

[Frankenamp]

I use Duratex in a manufacturing facility- it's one of the few things that stands up to the abuse from low-dollar labor that doesn't care what they tear up. The Jack (of all trades) is an update of the Omni 10 which was a popular mid-grade PA speaker which killed the usual Eon style speaker-on-a-stick Probably the best bet of all the designs if you are scared of building one of the DR series. The cabinets are designed so that a novice with a skil saw can have a successful project. I used dadoes (routed) to construct a TT (2X8) sub that is bloody loud with an old 60W Adcom (the particle board from my prevous 2X12 sub is now blocking weeds from sprouting in my garden). I think that if you can, nicer joinery is a plus- just be very careful to take the changes into account when you make the cuts, Baltic Birch is getting expensive. PL Plus is a very forgiving adhesive and strong enough that the cabinet has a chance of surviving a fall off of a pickup doing 60 (not recommended). Use 1/2 Baltic Birch. Do not use 3/4" lowes or home repo white ply (chinese junk that delaminates if you look at it wrong) 1/2" Radiata (south american) pine is one of the few approved substitutes. Several volumes have been written as to why... Some guys even had good luck with 10mm (3/8) plywood. the more plys the better. Now go make some sawdust

[skyclad]

Here's an update on my crossfire speaker cab. It's a piggyback style cab and will employ some tilt-back option. I'm using a pair of EV 12's which are very heavy, so I used finger joints, 3/4" baffles and braced the baffles in the rear and sides. The first photo shows the rear baffle brace which is wedged in between the baffle joint and rear panel braces. This feature adds structural rigidity without adding lots of weight. Can't wait to put the speaks in and fire it up!

sky

[skyclad]

Here's my 2X12 crossfire cabinet. The sound is big as I'd hoped - open and very wide. It's somewhat bright, but that will mellow as the speakers get broken in.

This design is from Bill Fitzmaurice's site http://billfitzmaurice.net/, where there are some very interesting speaker cabinet designs - not all for guitar. The idea here is to cause the sound to be diffused and dispersed over a wide horizontal area, avoiding the high frequency laser beam effect typical of many flat-front designs, especially with the EV speaker. In my opinion it works. I think everybody's going to be able to hear my guitar better, without the usual increase in volume.

The wheels are too close together and it wobbles when it rolls, but that can't be helped. The handle needs to be moved up for a lower center of gravity - it wants to tip back when rolling. This was my first time using PL Premium adhesive and Duratex, a textured, roll-on cabinet covering, both available from http://www.speakerhardware.com. The PL is definitely a winner once you learn to control the mess. I don't think Duratex looks as good as tolex, but it has the advantage of being able to touch up easily (and I have already).


Many thanks to BF, and to all who helped with this project. I know I'll enjoy this for many days and nights to come.
paul

[jjasilli]

Nice 2 X 12!

Vox made a ported cab with 2 large speakers, 15" or 18" I believe.  Maybe you can get some ideas from that.

EDIT:  I think this is the cab I was thinking of:  http://www.voxshowroom.com/northcoast/vox/cabinets/t60.html 

[skyclad]

Thanks Jjasilli

the pictures you see above - this is the result of the cabinet I was looking for when I started this post back in February. This is what I decided to build. That's not to say I'll never build another, but for now this fulfills the desires I expressed in that first post.

sky