2x12 Closed Back Cab Design?

[boots]

Can anybody direct me toward a design for a closed-back cab for a pair of Vin30's? I've built open-back cabs before, but have never tried a closed-back. I'm hoping somebody out there has already done the math so I can just fire up the table saw and go for it! I'm thinking to do a removable half-panel for the back so I can use it in open-back configuration as well. I'll probably use either solid pine or birch ply with finger joints. What I really need are the inside dimensions.

Thanks,
Boots

[tedmich]

if you are wedded to a Celestion V30 a closed back (sealed) cab is a bit harder to dial in than a speaker with known Thiele/Small (TS) parameters. Dr Z has done it:
Z- Best 2*12 cab black

and you could copy its ported design. The bottom end will be amazing.
also see: Interesting info on Thiele Cabs - Page 2 - The Gear Page
Many good designs are at Eminence, but for Pro series drivers, not guitar speakers:
Eminence - The Art and Science of Sound

I'd favor a 2.75 ft^3 ported 2 x Deltalite 2510 for solid to 60Hz bass which will destroy small buildings, at about the same size as the Dr Z. for ~$200

[boots]

Thanks, looks like a lot of interesting reading for me on those links.

I did find a free software for designing cabs, but it looks like you need to know the TS parameters of the speakers you are going to use. I remember seeing somewhere that Celestion does not publish their speaker data - is that right? Does anyone out there have the data for the Vin30?

The cab will be for a friend who already has a couple Vin30's. Would I be correct to assume that the Vin 30 is not the best choice for a closed-back cab? Maybe I can talk him into going with an open-back cab....

If I were to copy the Z-Best cab, would I be close enough in assuming that the dimensions given on their website are outside dimensions? And that they are using 3/4 inch thick material? So I could therefore figure the inside dimensions by subtracting 3/4 inch all the way around?

-Boots

[Steve Conner]

As far as I know, Celestion guitar speakers are so non-linear that the T-S parameters are impossible to measure.

Also, guitar speakers in general have such high damping (due to strong motors and light cones) that they're pretty tolerant of whatever box you put them in.

Just make a box that's roughly half the size of a Marshall 4x12" and it'll turn out fine. 3/4 of an inch difference doesn't matter.

[fyl]

As far as I know, Celestion guitar speakers are so non-linear that the T-S parameters are impossible to measure.

You can measure any speaker. A testing rig is nice and fast but you may use basic bench equipment. Cf. T-S Parameter Calculator for instance.

Here are some measurements from a recent G12M-25:

Driver Properties

--Description--
Name: G12M-25 Greenback (8 ohm)
Type: Standard one-way driver
Company: Celestion International Ltd.
Comment: Classic series
Frame: Pressed steel chassis.
Voice Coil: 1.75 inch (44.5 mm) copper coil.
Magnet: 30 oz (0.99 kg) ceramic magnet.

--Mechanical Parameters--
Fs = 85, Hz
Qms = 9,
Vas = 47, liters
Cms = 0,012 cm/N
Mms = 29,22 g
Rms = 1,734 kg/s
Xmax = 1,2 mm
Xmech = 1,8 mm
P-Dia = 260, mm
Sd = 525,1 sq.cm
P-Vd = 0,063 liters

--Electrical Parameters--
Qes = 0,71
Re = 6,7 ohms
Le = 0,41 mH
Z = 8, ohms
BL = 12,14 Tm
Pe = 25, watts

--Electromech. Parameters--
Qts = 0,65
no = 3,92 %
1-W SPL = 98, dB
2.83-V SPL = 98,77 dB

Also, guitar speakers in general have such high damping (due to strong motors and light cones) that they're pretty tolerant of whatever box you put them in.

Au contraire, guitar speakers have very small motors and show high to very high Qts - above 1 is quite usual for Alnico, >.5 for ferrite. Big bad vintage drivers from Altec, EV or JBL are the exception.

[Steve Conner]

What drive level did you use to measure the T-S parameters? Try again at a different level, and see if you still get the same answer. I'm pretty sure that they change drastically with drive level in the actual application.


My assertion on Qts goes as follows:


People like electric guitar to be loud.

Guitar amps tend to be low powered.

Therefore, guitar speakers have to have high efficiency.

Therefore the cone must be light and the magnet must be strong.

Therefore they must have low Qts and a high resonant frequency.

[Doctordog]

Hello Boots,

I built my cabs with info from londonpower.com and Kevin's book on building speaker cabinets. I really like the sound. The short version is using a "detuned" cabinet. Essentially building a 2x12 and leaving 1 driver out. The advantages are that you have less weight, directional and consistent sound no matter what room you are playing in, the SAME volume as you would get from 2 drivers.
It works for me and others, maybe you'd like it too. The book is only about $15 I think.

JB

[boots]

Wow thanks everybody, for all the good info. I have a lot to learn about cab design. I think I have decided to do a rough copy of either a Bandmaster 2x12 or a Marshall 2x12 and hope for the best. I want to make it with a removable panel on the back so it can function either as a closed or open back. I gather that the exact dimensions are not terribly critical, as long as I am in the ballpark.

It may take me a while to finish this project (what with life being busy and all), but I will post a photo of the cab when it is finished. I would love to quit my day job and build amps and cabs all the time.... but alas, I suspect my hobby shall remain a hobby.

Thanks,

Boots

[bob p]

the problem with 4th order butterworth alignments, as Steve mentioned, is that bass response as f(input power) is a very nonlinear function. there is a tremendous falloff in bass response with large signals. i can absolutely guarantee that every one of those hifi-looking frequency response curves that's plotted for a TS array was plotted with a small input signal.

if you think TS arrays have unpredictable response for guitar, you should try using them for bass. the bass rolloff with large signals is so huge that essentially TS arrays aren't what they're cracked up to be. if they were the magic bullet, then the small size that they offer would have resulted in the extinction of large cabinets for high SPL SRO systems. we all know that hasn't happened.

fwiw I have been using EVM drivers for bass and guitar since the 1970s when EV first introduced their TL series cabinets. i was on the ground floor working with the design engineers on bass cabs back then. i still use one of the original prototype, bare-naked plywood TL-606 cabinets for my EVM-15L, which doubles as a bass/guitar cab.

its interesting to note that many of those thiele ported cabinets whose pictures are in the gear page thread are nothing more than a copy of the original 1970s EV cabinet designs. i think that EV marketing department completely missed the boat on that subject -- they were so interested in selling speakers back in the 70s that they gave away the plans to make the cabs. a lot of other companies made a lot of money buying EV speakers and selling the cabinets. by the 90s EV figured out their mistake, but the TL series designs and the folded horn designs had already been released to the public domain. today people are still using those plans, especially the TL plans ... like mesa and boogafunk. that boogafunk is definitely built from an old EV TL-series blueprint. you can even see the 3-way port that's convertible to a 2-way port if you add the center plate.



what amazes me is how stoopid some people are in deploying those cabinets. those cabinets were designed specifically to match the T-S parameters of individual EVM series speakers. the port sizes tune the frequency response of the cabinet to precisely match the speaker. today, those cabs are commonly used for all sorts of speakers beyond those that they were designed for. thatis particularly dangerous -- you never want to put a driver into a TS enclosure that was designed for something else -- many people fail to realize that the lowest frequencies that the cabinet produces are produced by the port, not by the speaker. cone excursion increases exponentially as you go below the box's tuning frequency, which makes it very easy to exceed Xmax if you're not using proper EQ, or if you're not using the intended driver to match the box parameters. just taking a random speaker and slapping it into an existing TS cabinet is asking for trouble. if you don't crunch the numbers, its probably best to stick with open back and closed back cabinets. otherwise any adventures into the LF limits of the box could spell disaster.

[J M Fahey]

Hi Steve, sorry but although your deduction sounds logical , in fact it's the opposite.
Guitar speakers *do* have a high total "Q" usually around "1", i've seen as low as 0.89 for big modern speakers to as high as 1.6 on "vintage" very-light-cone weak-alnico undoped speakers.
On PA types it's usual to have a Q of around 0.45, with some very good ones going as low as 0.39 or even 0.35.
Fact is, high Q means low damping and viceversa.
Also, low Q speakers benefit most from properly tuned enclosures, specially Thiele aligned ones , which is quite an ample specification or statement, given that Thiele and Small describe *a lot* of very different alignments for every speaker.
In fact, you *can* design and build a "T&S" box for any high Q guitar Gelestion, Jensen, or whatever, simply you will gain little ..... which is also accurately predicted by T&S !!!!!!!!!!!!!
I had (like many), a honeymoon period with those theories, measured everything on sight, built boxes so tuned, adjusted them both in the lab and by ear, the works.
¿End result? Now I build mostly closed box designs, period, except when some client (generally a Strat or P90 LP armed Bluesman) specifically asks for that open and at the same time deep sound that open back cabinets give.
I sometimes still tune closed cabinets but for another reason: for dropped tuning death/gothic heads I tune for 70 Hz; for bass players to 40/45Hz, but not for "flat response" (far from it), the idea is to minimize excursion on the lowest notes.
¿Do I get a "bump" there? You bet I do ... and they love it.
T&S must be revolving in their graves.
PS: I *never* mention T&S in my advertisements, I think it would not help me at all and besides, purists would start calling me names, none of them very flattering.