Open back cabinets, Dumble style

[skyboltone]

I have no idea how HAD arrived at the size of his oval other than trial and error. That was his pattern I think. Anyway, I've been fooling with the numbers a little and have arrived at a formula that reflects what he did on the back of the 12" cabinets. When applied to 15" and 10" drivers it yealds a hole that "looks" right.

Anyway, VAS is a Thiele/Small parameter that discribes the displacement of the cone times the cone travel expressed in cubic feet. Multiply that times 1728 to get cubic inches.

Opening in square inches= VAS/50

Anyway, I'm going to give it a try. I've played with some speakers and it seems to look like the opening as I remember it from when he gave me the size of the hole for an Altec 417-8C back in the 70s.

Let me know what you think....voodoo?

Dan

[ODwan]

Skyboltone,

I recommend to get in touch with some high end hifi speaker building folk. They truly know this stuff. There are also books written on it, though I never read one. For me the problem is to get hold on the relevant parameters for most guitar speakers, especially older ones.

Timo

[skyboltone]

Hi Timo, actually I've been one of those HI Fi guys for many years. The problems are, as you say, getting the Thiele/Small parameters for older guitar speakers but also I have not been able to discover any scientific approach for open back cabinets.

I've related in previous posts that I used to know HAD when he was in Santa Cruz California and he was adament that the hole in the back of the cabinet was precisely worked out. A secret known to him alone I guess.

Anyway, if you know of a paper or article on open backs I would appreciate it. The formula I've posted makes some sense in terms of its method but I'm sure it is incomplete. I think it needs Vas but also Qts in some sort of relationship. I ain't smart enough to figure it out.

Dan

[jaysg]

I've related in previous posts that I used to know HAD when he was in Santa Cruz California and he was adament that the hole in the back of the cabinet was precisely worked out. A secret known to him alone I guess.

Do you know what, if any, credentials he has? I think of him as one of the old crew...secrets...sheesh. I've worked with engineers like that. They're useless bastards in the long run. People who can function in teams and share information do better, imo.

As to HAD's approach, it's easier to comprehend his results as derived from a basic understanding and a ton of experimentation. I could be wrong of course. I don't think it's worth the time to run calculations when you can cut cheap plywood. Listening tests are mandatory, regardless.

I find the closed back and front ported design more promising. There's a Bogner 1-12" like that, along with the O'Connor cabs. In the late 80's, both Clapton and Knopfler were running 1960A cabinets with two EV-12L's. Dollars to doughnuts, the empty holes weren't plugged.

[jelle]

Jaysg,

I am with you on the closed back and front ported design. I have a pair of Dynacord cabs from the sixties with a 15'' concert series jensen in them. Closed back and ported front. I have never heared a sound like this. Open sounding and very nice balanced IMHO.

I have build two 1x15'' bass cabs like this with a bass port. After tuning the lenght of the bass port's tube, the sound was great.

Are there any rules how to design a cab around a speaker?

Jelle

[skyboltone]

Hi Jay:
As to question #1 his credentials; He designed and successfully marketed what are arguably the best sounding guitar amps ever. They are the focus actually, of this forum. How he managed that I can personally attest was by countless hours of listening and tweeking the fiddly bits. Does he have letters after his name? Nope. The purpose of my post is to shortcut all of that fiddling. Plywood is NOT cheap.

As to useless old bastards, I proudly fall into that category as well. Hundreds of amp manufacturers who relied exclusively on MIT trained engineers and the like have come and gone while those run by doddering old fussbudgets with good ears hang in there. Experimenting. Ever play through a Standel? I have. In a 1000 person dance venue. Nobody even knew I was there. That's when I got one of Howard's modified Fenders with one 12" Cut like a knife. Standel was gonna cash in on the guitar boom but dollars to donuts they tried it by the theoretical approach.

I don't like the sound of ported cabinets. Never have, never will. They sound forced in the low end to me.

I don't like the sound of sealed cabinets. When classic guitar speakers are used in small cabinets they sound boxy. The 4 X Marshall cabs are too big for my purposes.

Besides, all speakers are purpose designed for optimum enclosures.

Open back cabinets like all the Fender and myriad other combos sound more open and less focused. This once again when used with classic musical instrument speakers. I like that as a starting point. Your comment on listening tests are spot on.

On this forum we pay attention to the tiniest details that HAD used/uses in his amplifier circuits. His speaker cabinets beg the same attention.

thanks for the reply
Dan

[jaysg]

I should explain that I've worked mostly in medical electronics and DoD stuff. How would you feel if the EE on a life sustaining device, wouldn't explain his pet circuit to anyone?

[ODwan]

Sorry, but I don't have info on open back cabinet design with T/S parameters. From what I understand the T/S parameters are not meant to be used for open back design. Me, I actually like the sound of cabs with front porting, if it's large enough. Open back cabs sound different each time you place them somewhere else (how close to a wall etc.). This can be great for tuning but can be a pain ita if you try to recreate your sound in different venues. To each his own!
As to HAD's credentials: I think in building guitar amps experience is the best teacher, whereas in other areas knowing the deep techism is most important.
A life sustaining device has to function. Period. But it doesn't have to sound good, has it? Anyone can make something that amplifies an electric guitar, but to make it sound good is the art. And what sounds good is in the ear of the beholder. There are many ways to get there and it is difficult to press it into formulas. In the end the ear is the judge, not your calculator or SPICE simulator.
Rant mode off!
I hope this thread doesn't ignite!

Timo

[jaysg]

I suppose discussing credentials is off track re: semi-open back cabs. It's just that I've gleaned from first-hand accounts that he used to like to impress clients, that he really knew what he was doing. It may be that Vas/50 is a legitimate Dumble standard. If it was derived precisely, then what exactly was the goal? It looks like you're lightly damping cone travel. Beyond listening tests, how would you know how much damping you wanted?

My experience is with guys who repeatedly put poor designs into products thinking that they were being clever by saving a few components. They were tight lipped about these features. Eventually, better engineers had to reverse engineer some of these circuits and redesign them for reliability.

I compare HAD to innovators who built big and small companies. He could have done that, but apparently has no such interests. Remember Mike Soldano's Yamaha amp?

[rfgordon]

I've been building guitar speaker cabs for a while now, and I'll toss in my bit. I've tried to work out the Thiele-Small numbers, but since the guitar has such a wide frequency range (compared to a woofer), you end up with numbers that tend to relate to a small part of the overall voice. Now that can work, if you want to bring out something in particular. I've have to say, however, that my best results have come from cuttin' wood!

I have come to prefer a few things:
1. Pine cabinets are great for most folks, but since pine can vary quite a bit in terms of hardness, all pine is not created equal. Harder, denser pine equals brighter tone. Aspen is a bit more neutral than pine, and because I use clear Aspen, it's extremely consistent. It also takes a stain much more consistently than pine, if you want to do that.
2. 1/2" Aspen ply is my main choice for baffles.
3. The new neo magnet speakers move the baffle to a greater extent than conventional (heavier) magnets, due to the delta in inherent damping properties of a basket attached to a heavy magnet. Thus, for neo speakers I have taken to using 3/4" solid, not ply, Aspen baffles. Man, you use this, plus a tone-ring, and a 15 neo speaker and you've got jazz tone that'll make you wanna play George Benson licks all night!
4. Celestion V30s may self-destruct in an improperly ported cabinet. Found that out the hard way, only to be told by Celestion that plopping one in a closed,ported cab is, indeed, a frowned-upon endeavor.
5. I find that most 12" speakers sound pretty good in cabs about 10" deep. I like to fit the back in three horizontal parts, with the middle one on a piano hinge so that its opening can be tailored. Not all cabs are happy in all venues.
6. I have also made some 1x12 cabs with a full-width shelf port approximately equal to 1/2 the speaker area. This yields a very nice, open, almost open-backed sound, but never gets woofy.

[skyboltone]

I agree that he probably used an empirical method to make the hole. Experimenting with one speaker. He started with the Altec 417 series after having rejected JBL because of cone alignment problems at high SPLs. I remember him cussing them and their 4" voice coils. When Altec tanked he switched to the EV 12L.

What I'm trying to do is look at the parameters and reverse noodle to get a starting point for experimentation with other speakers. I see no reason why Thiele/Small can't be used for rear firing ports, sized not for bass enhancement but for phase alignment at the front of the cabinet.

I will fully admit that it may be a red herring altogether! The basic theory is to separate the front and back waves or to put them in phase with each other at the front of the cabinet. An infinite baffle works best (theoretically) to separate front and rear waves. I imagine his (or my) intention with the rear baffle hole is to slow down the rear wave so that it emerges in phase with the front wave. That's what happens with the ported enclosure except that they only work at certain frequencies. They act as a low pass filter. I think they sound boomy. Guitar speakers (in my experience) lack sufficient flexibility in the surrounds to make great sealed enclosure speakers. At least without giving up a good deal of low end. Once again though, it depends on what the engineer's intention was when he/she designed the speaker.

There is a story told amongst Altec snobs about the 417 series speakers. Celestion was creaming all other manufacturers in the market place. Altec hired Scott Leslie (son of the Leslie speaker inventor) to design them a speaker to compete with Celestion. They wanted it good, and they wanted it fast. He used all his know how and the resources of Altec to make the prototype speaker in about 4 months. He then gathered all the managers together in a sound room and played his guitar for them for about 30 minutes. Then he turned the amp off and said, "That sounds like shit", and walked out of the room. A couple of months later he blessed the design that became the 417. It's a black art I tell you.

I come out of the petroleum industry. There, they don't pay any attention to the engineering staff regardless of our input. Accountants are the most important then the regulators. We call it the difference between global and detail thinkers. It's a mutually exclusive team concept; always at war.

And I detect no flames here by anyone. I hope nobody's offended.

Dan

[skyboltone]

[quote="rfgordon"]
5. I find that most 12" speakers sound pretty good in cabs about 10" deep. I like to fit the back in three horizontal parts, with the middle one on a piano hinge so that its opening can be tailored. Not all cabs are happy in all venues.[quote]

Great post Rich, gotta like that louver too.

Dan