Building Vintage Enclosures - Help

[FrankRoss]

I'm nearing the completion of building my Bozak Concert Grand cabinets. I'm excited, but I'm a bit concerned:

Several speaker cabinet builders that I have spoken to strongly advised me to use MDF. I knew in advance that vintage speakers were built primarily out of plywood, simply because MDF was not available back in those days.

After months of pondering, my decision to use MDF was a difficult one. The advice given to me was that MDF was not only cheaper and easier to use, but also because of the fact that "insulation" would blanket the entire inside of the cabinet, it would null-ify any sonic differences between MDF and the plywood.

After spending countless hours (and dollars), I have recently, though quite by accident, come to learn that the use of MDF in vintage speakers is considered the worst taboo in hi-end speaker building/restoration, in that it will destroy the open "ringing" sound that was so wonderfully inherent to the originals.


Before I invest any more time and money into these boxes, and pull the drivers out of storage (there are a LOT of drivers!), I am asking the folks at DIYAudio 3 questions:


1. What is the truth concerning the use of MDF for vintage speaker cabinet building?

2. Is there a particular brand/type of insulation that I should use that will compensate for the use of MDF to make the sound more open and "ringing"?

3. Should I scrap the boxes and start fresh with plywood?


Each cabinet will consist of the following:

4 - 12" Woofers (8-ohm)
2 - 6" midranges (16-ohm)
8 - 3" tweeters (8-ohm)

[Cal Weldon]

Originally posted by FrankRoss
1. What is the truth concerning the use of MDF for vintage speaker cabinet building?

Hi Frank. I don't think you'll find concensus on this isuue but a lot of it has to do with what you are trying to achieve. If you are looking for an exact reproduction then you've taken the wrong road. But if you wanted an exact reproduction you wouldn't have used MDF in the first place so move on from there. Are you trying to get the best sound quality? Probably, that's usually the idea so MDF is fine. You can talk till you're blue in the face about the sonic qualities of MDF vs plywood. I don't care, there are so many proponents of both that one simply can't be "better" . I've made boxes with both and I prefer plywood, I like the "feel" of it better. 1" multi-lam birch in fact.

[i]2. Is there a particular brand/type of insulation that I should use that will compensate for the use of MDF to make the sound more open and "ringing"?[/B]

No. Use ploy-fil. Less itchies. The ringing comes from cabinet resonance. Best to avoid this and not try and duplicate it. If you go without bracing, it will sound like you're playing it through a tube amp. All warm and fuzzy but not that accurate. (oh boy, I'll get flamed for that one) Todays thinking dictates that we try and eliminate outside sources from colouring the music. Even if you ike the originals, it's not to say that the updated version won't sound better.

[i]3. Should I scrap the boxes and start fresh with plywood?[/B]

What, and waste all that time and wood?. Considering the layout of the drivers, I wouldn't worry about the cabs so much. If they are ported, I wouldn't worry at all. As a matter of fact, I've already stopped worrying on your behalf. Just make sure you are well braced, as that's a lot of air you'll be moving.

[i]Each cabinet will consist of the following:

4 - 12" Woofers (8-ohm)
2 - 6" midranges (16-ohm)
8 - 3" tweeters (8-ohm) [/B]

That should be enough drivers.

Now, a question for you:

Why didn't you come and ask before you started this? Are you one of those who won't stop at the gas station to ask directions?

Cal

[Hornlover]

The original concert grands were made from particle board. Most vintage speakers from the 60's on were. Go to the H.E.L. website, and look at the Bozak Concert Grand photos (photos 4 and 5, in particular), and you can clearly see the particle board in use. Even the JBL hartsfield, from the early 50's, were mostly particle board (called timblend, by JBL). The Jensen Imperial was plywood, but not for any 'ringing' quality. Good plywood is fairly well damped, and used with bracing, will be quite usable, and should not 'ring'. After all, we are building loudspeakers, not guitars.

http://www.highefficiencyloudspeaker...surePlans.html

[FrankRoss]

Cal - thanks so much for responding to this.

Actually, sometimes I do tend to overthink matters, and as for asking for directions - whatever can save me time/money lol.


I did ask this forum, and it seemed at the time concencous was to go with the MDF. Also, I had asked a couple of people in my area the way to go, and they pressed upon me the benefits of MDF. They told me that I was "thinking" too much about it and to just bite the bullet and go with the best material for speaker cabinets!


But recently, while shopping for CD Players at a high-end shop, I ran into someone who builds vintage speaker cabinets as a side job. He is the one who told me that I made a very crucial mistake in going with the MDF. It seems that he has worked with both materials, and told me that because Bozak, among others, designed drivers using materials of the day, going with MDF would just "kill and deaden the sound". Of course, I told him that several others have suggested that MDF would be an acceptable material. But he told me that whoever told me to go with MDF on ANY vintage speaker had no clue to what a true natural sound was, and that they were likely accustomed to speakers on a smaller scale, such as for home theater (quicker speakers). He did go on to say that most modern drivers/crossovers actually do sound better with MDF.

I know it sounds arrogant, but personality aside, I thought that he may have a point, especially if all of his vintage cabinets are made out of plywood. The owner of the store used to sell Klipch, Bozak, Jensen, etc. some years back and seemed to completely agree with the gentleman.


I guess if the sound is only slightly different, I will go with what I have. It was after all, a lot of work. But if the sound is drastically different, and I can't here the timpani ring and cellos resonate, I'd rather ditch the cabinets and start all over again - especially before I sink money into very costly veneer.


Btw, the cabinet are not ported.


Again, thanks for your response! I won't go crazy with this, but any other thoughts/opinions would be greatly appreciated.

[Curmudgeon]

For me, the big advantage of MDF is that it is self-damping. There are many different grades of MDF, but all are self-damping to various degrees. Cabinet vibration occurs at specific frequencies; see the acceleromater tests in the Stereophile. The peaks never vary, so there is a consistent coloration, which for me becomes very irritating over time. I would suggest shelf bracing; "shelves" from wall to wall, with 4 large cutouts, so you have the perimeter and a cruciform shape remaining in the center. Thiele and many others have used this with great success. Two shelves, unequally spaced (for example 20 and 34 inches from the bottom) will help a great deal in reducing cabinet resonance. A bit of knuckle testing will indicate areas you might want to add a bit of added thickness; I use non-rectangular additions, asymmetrically placed.

"A while back" I heard a new Klipsch Horn, plywood, with the Emory Cook Earthquake record, and the plywood really sang; by today's standards the bass was highly colored. Certainly with bracing and damping plywood can work, especially when you start talking about 1" Baltic fir(?) but for me the MDF simply is much easier to use. I would suggest sealing it w (I use water based polyurethane) as many grades of MDF absorb water from the air.

Of all the (hopelessly unaffordable) speakers of the time, I preferred the larger Bozaks; in large part because they did not suffer from horn peakiness or cabinet "talk". From what I remember, the tympani WILL sound, and with the MDF, JUST the tympani, and not the cabinet.

[kenseger]

Frank,
First off, wonderful Bozak info can be had at the Yahoo Bozak_speakers group.
Some basics - on all of the classic Rudy Bozak enclosures the idea was to confine and eliminate the back wave, period. Various myths about intentional resonances I find hard to believe based on the construction of the Concert Grands & Symphonies and Rudy's writings. Depending on the model, the Concert Grand had either one or two 2"x4"s going from the front panel to the back and one 2"x6" going between the side panels. Add to that dampening thick heavy insulation stapled to the sides, top, bottom, and back - plus a hanging curtain of the dampening material a few inches behind the woofers. This same material was inside the hemispherical internal midrange enclosures, which by the way are not coaxial to the midrange, but offset. Think about that clever idea for a second. Also, the joints in the five sided box were tongue in groove, not just abutted and reinforced. Also the front panel had on edge 1"x4"s to add stiffness, and the front panel was made of two layers of 3/4" what-ever-it-was. So if Rudy wanted cabinet resonance, he sure made a bunch of expensive errors.
Perhaps "MDF" is like colloidal silver? The name stays the same but the actual material described by the name changes over time? I can take pictures of the insides of both my Concert Grands and my Symphonies and let you decide what the material is.
On the cabinet, stronger is better and bigger is better. Remember that Rudy sold "panel systems". All of the drivers and crossovers were wired up and mounted on a panel and you mounted the panel into a wall using the next room as a back chamber. See www.hifilit.com for pictures.
Ken
P.S. More later about the crossovers, X vs. Y vs. Z tweeters, why you can't rebuild a Bozak driver, arrayed tweeters vs. line tweeters, vs. single tweeters, et cetra.

[planet10]

Like Cal said... you will not get a concensus on plywood vrs MDF... myself i won't use MDF for building vintage or modern speakers... it does have its uses in bases, spacers and deflectors...

I'd agree with the guy at the hifi shop, but given all the work you have done, i'd go ahead and assemple what you have, but you may have to add more bracing to work to negating the energy storage issues inherent in MDF (this is part & parcel with the self-damping Curmudgeon mentions).

With a speaker as big as the Bozak, one of the reasons for using plywood vrs MDF has little to do with sonics, but that it would be just that much easier to move made of ply.

dave

[catapult]

Originally posted by Hornlover
The original concert grands were made from particle board. Most vintage speakers from the 60's on were. Go to the H.E.L. website, and look at the Bozak Concert Grand photos (photos 4 and 5, in particular), and you can clearly see the particle board in use. <snip>

http://www.highefficiencyloudspeaker...surePlans.html

As the pictures Hornlover linked show, the original was clearly made of particle board. So, if the goal is to match the original, MDF will be a better match than plywood.

[sreten]

Originally posted by FrankRoss

I guess if the sound is only slightly different, I will go with what I have. It was after all, a lot of work. But if the sound is drastically different, and I can't here the timpani ring and cellos resonate, I'd rather ditch the cabinets and start all over again - especially before I sink money into very costly veneer.

Hi,

The sound will not be drastically different.

Originally posted by catapult

As the pictures Hornlover linked show, the original was clearly made of particle board.
So, if the goal is to match the original, MDF will be a better match than plywood.

If anything as stated above MDF will be nearer the original.

You do need to copy the bracing shown in the photos.

/sreten.

[Salas]

What kind of drivers and crossover are you planning to use?