Building BIG infinite baffle boxes for vintage drivers

Hello!
Wondering if anybody has any advice/experience in building big (10000 cu.in.) infinite baffle boxes suitable for vintage 15" coaxial drivers(Altec 604's, EV 15trxb's, etc.). I have four different pairs of drivers to mount in them (they should be big enough) and would like to switch between them with a switcher box.
I think I will have to block off the ones I'm not using at the moment to prevent them from acting as passive radiators and transmitting the back wave out the front.
Anybody ever tried this?
Anyway, this is my first post, other than the introduction, and I am curious how many of you are into vintage gear.
I am always looking for old interesting gear and also have lots of tubes that I'd be willing to trade, sell, etc.
Peace
Lee
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
Radiolee,

When I think of infinite baffle speakers, I think of Bozak. The boxes the size of refrigerators with 4 12" bass speakers mounted in some chaotic fashion. The Altec's and the EV's were mostly ported speakers. The 8H version of the 604 will work in a 4cu.ft. (6900cu.in.) ported enclosure. I don't know how it will work in an infinite baffle. The fs of the 604-8H is 28Hz and I believe f3 = fs in an infinite baffle enclosure, but the cone motion is that of the speaker in free air.

The enclosure you will need for an infinite baffle would have to be a number of times larger then 10,000cu.in. = 5.8cu.ft. The larger Bozak's were 3 or 4 time that volume.

I guess I wonder why you want to go with an infinite baffle configuration. You could make a box sized for a ported alignment with a removable baffle, and make a baffle for each speaker. Then you just need to replace the baffle and speaker assembly.

For what its worth,
Rodd Yamas***a
 
Hello again!
Well, I'm no tech, but my understanding is this.
Ported enclosures, bass reflex cabinets, acoustic suspension, etc are all ways of dealing with the back wave of the driver, but ideally an infinite baffle is the ultimate way to eliminate the back wave completely. In checking around, somewhere I read that 10k
cu. in. is the minimum volume necessary with drivers like this to eliminate loading the enclosure and thereby creating resonant frequencies.
(I'm sure that's way too simplistic, but bear with me!)
I've used these drivers in lots of different enclosures and love the coherent sound of coaxial drivers, but have yet to hear them in a sufficiently large closed box.
Thus this project.
Roddyama, I see a difference in the Bozak setup in that they were pushing all four drivers at once, and thus needed a larger box. I will only be pushing one driver at a time in each box.
I thought I had done the math correctly, but now realize that the box I'm envisioning is more like 27 cu. ft. (3x6x1.5 ft).
Also, these are early 604's, so they might spec differently than the later versions.
At any rate, I appreciate the input, and bow to the obviously high level of knowledge you guys have!!
Thanks!
Lee
 

psarin

Member
2001-11-09 6:35 am
Mars
have a look at the new ProAc 'Futures' speakers

it might be useful for you to have a look at the design and construction of the new ProAc 'Futures' series of speakers - they are an interesting trapezoidal open baffle with 'swept sides'. (they do use a box for the sub-bass unit).

there is also a lot of information on open baffles over at the full-range driver forum on james melhuish's single-driver website (http://melhuish.org) and also on j esmilla's website
(http://users.starpower.net/je2a3/open.htm)

overall though, i would recommend that you make your baffle pair with a single driver cutout (fitting the largest driver, or maybe even larger) and use adapter plates to mount each of your driver pairs in turn.

i have experimented with altec 407B's (an 8 inch sweet full-range unit) and also the ubiquitous RadioShack 40-1354A's in open baffles to great satisfaction. i generally mount drivers in these generic baffles i have made (4'x6' sheets of 1/2" lexan supported by 2"x4"'s to get to know them better (over ~ weeks/months) before building an enclosure for them.

hope this helps -

pradeep

my latest - http://web.mit.edu/~sarin/www/tlb.html
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
Radiolee

27cu.ft. is much more like it for an infinite baffle box. Don’t misunderstand me; I do not what to dissuade you from your vision. I was just curious.

An item you will have to investigate is if the Altec or EV driver suspensions have the ability to be used in an infinite baffle arrangement. A driver designed for infinite baffle enclosures will have a suspension that will support the full motion of the cone, particularly at driver resonance = fs where excursion is at its maximum. Drivers designed for ported enclosures, like the Altec’s or EV’s will generally have a “looser” suspension and depend on the air in the enclosure to control its motion. Check with one of the contacts you and Cyclotronguy were speaking about to see if your drivers can be used in this manner.

You could still do the multiple baffle arrangement with one driver in each baffle, but this time, mount all 4 baffles in the front of the same box. You could have rear covers for the speakers not being used to eliminate unwanted pass-through and interaction. The front of a 27cu.ft enclosure would certainly support 4 - 15” woofers with room to spare.

One last note: A transmission line configuration will also (theoretically) dissipate the back wave of the driver. This base configuration has been modified to include other variations of the concept the do use the back radiation in hybrid designs. I don’t know enough about TL designs to go into them any deeper. I have already set myself up for a flaming.

Duck!!
Rodd Yamas***a
 
Hello again!
Thanks for the encouragement, and please, don't worry about offering differing opinions! Audio is great fun, but I've also found over the years that it CAN be an emotional subject (DUH)!
That is an excellent suggestion on checking the original specs on each driver to see if the suspension is right for infinite baffling.
Now onto the box-
I'm thinking a skeleton of 2x2's with additional diagonal bracing. I'll line the inside with 1/4" Luan, the outside with 1/2" plywood. I'm hoping the air space between will isolate the vibrations of the back wave. I could conceivably foam in between as well.
To stiffen the box from the outside would take at least 3/4" plywood and would weigh (and cost) a ton.
Rodd- what do you think of blocking off the unused drivers from the outside?
It'll be interesting to see if the unused drivers act as passive radiators and whether it makes any difference if they're sealed off or not....
I might be nuts- I'm going to end up remodeling my entire basement to accomodate this little project!!
Thanks for the input!!
Lee
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
Hi Radiolee,

I was thinkin' 2X4's for the main frame, but 2X2's should work. I also wouldn't go less then 3/4" Ply or MDF for the walls and probably thicker with that large a box. Go to this thread and see my sub-bass/end tables.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=4840

I built a stereo pair. Each box is about 4.5cu.ft. and I have nearly 40ft. of bracing in every corner and across each side. There is also more then 150 screws in each box. The walls are all 3/4" MDF and the baffles are 1.5" MDF and plywood laminate. I guess what I'm saying is, when in doubt, brace the hell out of it.

I don't know about the luan, it will have its own resonant frequency, and you won't know what frequency it is until you hear it rattling inside the box during your favorite song. Your best to try and keep the walls of the box as thick and as stiff as your back can afford. You have a big vision. You have to know that it won't come without a price. It's no little project.

The problem with blocking the drivers from the outside is that they are still free to move and react to the pressures inside the box. Theoretically, all of the back radiation of the active drivers should be absorbed by the shear volume of the infinite baffle design. Maybe if you stuff it to the gills (which you're suppose to do anyway) with dacron (best bet for price vs absorbtion) it will work according to theory, but I'm just thinkin' how the sound fill a room, making things rattle throughout the house. If it can do that, I'd hate to hear what going on inside the box.

My thought, for what they're worth.;)
Rodd Yamas***a
 
I get a lot of vintage -- alnico -- drivers thru here (only a single 604 and it was without a cone). When i think of an infinite baffle thou, i don't think about a box, but a big flat baffle or folded baffle with an open back -- these do have dipole cancellation at some frequency. Always worth consideration is cutting a hole to mount a baffle in and using the next room as an enclosure...

27 cu ft is almost big enuff to be a room in its own right thou. Building a non-resonant box this big is a serious challenge, and no matter how you do it will be heavy. A structure made with thin well braced walls (non-integer, non-parallel bracing). Building a stressed wall (ie band it) would also help with reducing the weight while retaining non-resonance.

Unless you plan on filling any air space with sand i would stay away from a cavity.

And i would design for a set of interchangeable baffles, as opposed to 4 drivers in a single cabinet. Each one of those drivers, the holes you have to cutout to put them in and the large variation in position will strongly affect the outcome.

That all said, i have just set aside a set of really nice 5" FR Corals that i am going to play with in a big box (at least for a 5 -- something like 6 cu ft) for a similar kind of experiment.

dave
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
When i think of an infinite baffle thou, i don't think about a box, but a big flat baffle or folded baffle with an open back

I guess I didn't consider the open back approach, also eluded to by psarin earlier. The Bozak like any earthly size box would only be an approximation of an infinite baffle. And there are many examples from the 60's and before that used open back boxes. There have also been many di-pole speaker designs (although most had a closed bass box) over the past 30 years, and some even sounded exceptionally good. I've just always had a problem with the concept of a dipole speaker. True, it's a bias, but its more. It has to do with the whole Bose, Allison, room interaction idea, that the reflective surfaces in the room become part of the speaker. I agree that the room has a great influence on the overall sound, and to a small extent, you can control some of those influences, but to a greater extent, you cannot. This is, I think, where I have the problem, with a design that depends on uncontrollable influences (outside of the design) to deliver the sound to the listener.

This is interesting (at least to me) because I have never articulated this idea to this extent before, but I think it is the reason I went with horn speakers and their controlled dispersion.

'Nough of that.
Rodd Yamas***a
 
roddyama said:
I agree that the room has a great influence on the overall sound, and to a small extent, you can control some of those influences, but to a greater extent, you cannot. This is, I think, where I have the problem, with a design that depends on uncontrollable influences (outside of the design) to deliver the sound to the listener.

In this case a 27 cu ft box is almost a small room in its own right, and its walls could have a significant impact on the sound.

Your comments on dipoles (which apply to bipoles as well) are well taken... i always preface any discussion recommending them with "if your room can support them...". I was fortunate enuff to have been able to design my room before there was any hifi in it, and it supports dipoles (my ESLs) and bipoles (currenttly installed) very well. Horns, with their specific compromises, are a very valid solution if you want to take the room out of the equation as much as possible.

dave
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
Dave,

Did you re-mount the Accoustat speaker elements, or is that (on your site) the original Accoustat speakers?

How far off are the Pearls?

I don't quite get how the sonotube will work attached to the neutrallized pressure of the push-pull box. Please explain.:confused:

Sometime you just feel like Curious George.
Rodd Yamas***a
 
I have a friend who built a pair of cabinets each the size of a family refrigerator, made of custom pressed higher density particle board 2 inches thick, and loaded with Altec 15" and horn Hf, although I do not know which models.
These just plain do not exhibit any cabinet resonances, and interior echoing does not seem a problem.
I understand that they are using sheeps wool as damping material, but as I have not seen inside these cabinets, I do not know how much or where placed.
These cabinets are a serious 4 man lift, and once placed are there for life. :)

Also, many years ago I went to a party at a house set in a small steep valley.
In the house were two open backed speaker cabinets the size of a fridge that you stand inside of, and fitted with a single 12" driver with a paper cone moulded with 6 or 8 protrusions like the bottom of an egg carton and a whizzer cone.
These cabinets were hellishly efficient, and filled the valley very nicely although powered by a little 10W/ch Rotel amplifier.
The sound still sticks in my mind as being very nicely relaxed and free sounding, within the limits of the drivers and amplifier, of course.

Regards, Eric.
 
mrfeedback said:
Also, many years ago I went to a party at a house set in a small steep valley.
In the house were two open backed speaker cabinets the size of a fridge that you stand inside of, and fitted with a single 12" driver with a paper cone moulded with 6 or 8 protrusions like the bottom of an egg carton and a whizzer cone.

Sounds like Olsen's RCA FRs. I saw and heard a pr of these in my Uni days. Quite impressed i was.

I'll look for pictures after a sleep.

dave
 
roddyama said:
Did you re-mount the Accoustat speaker elements, or is that (on your site) the original Accoustat speakers?

This is a set of IIs converted to 1+1s. I wanted to try that ever since Paul & Stan (PS Audio) showed me a pr they had converted, on a visit to their Santa Maria, CA factory.

How far off are the Pearls?

Either them or something similar this winter i hope.

I don't quite get how the sonotube will work attached to the neutrallized pressure of the push-pull box.

It is not push-pull, it is push-push. Both drivers pressurize the box at the same time. 2 8s is like having a single 10, but with no reactive forces loading the box.

dave