Ceramic speaker enclosures

[Josephjcole]

Well I've been out of the speaker building loop for a little while now as my Wife and I moved up into the great midwest and built ourselves a pottery. Things are starting to feel as though they are slowing down a bit(as much as possible while running your own business), so I'm begining to think a little about speaker building again.
I would like to build myself some high efficiency three way speakers. I hear good things about open baffle. My listening space is quite small though and I don't think I could get enough distance between the rear of the speaker and the back wall to make the open baffles really sing.
So I'm thinking about trying some ceramic enclosures to reduce the box modes and hopefully reduce diffraction as well. My thinking is to make the shape similar to the B&W shaped tear drops, but cutting off the last several inches to make them something along the lines of an aperiodic transmission line.
For the three way I would love to build a ceramic enclosure for the 6-8 inch mid (audax PR170m0?), and possibly for whatever goes above that as well. However I'm still a little ways away from being able to afford everything for the three ways, and I would like to work out the ideas for the ceramic enclosure.
I'll start with making some ceramic enclosures for some 4"tangbands that I have lying around. My idea is to glue a MDF disc onto the front of the enclosures to mount the drivers to. Using MDF since it is easy to sand and shape, and then paint. This could also give some room for filling in with wood filler (or something else ... bondo?) trying to get the smoothest transition possible from MDF to ceramic enclosure.
Our next firing is mid march, so I'll probably throw them in the next week or so. Questions: Are the proportions of the "aperiodic transmission line" critical? I wouldn't think so as they not for bass, but merely to flatten impedence (this is an idea I'm "borrowing" from planet10 to give credit where credit's due) and reduce reflections. Also would there be much benefit to making them unsymmetrical internally? Options would be flattening one side, or adding in a small divider internally, possibly filling this cavity with "Great Stuff" after firing (somewhat time consuming to build up an internal wall though).
Anyways I'll be making these fairly soon, and post pictures as it progresses. Right now though I'm all ears as to suggestions, concerns, and ideas.
thanks
Joe

[planet10]

Joe,

MDF is easy to shape, but isn't great for baffles.

The aperiodic mid-range TL is not my idea, i'm just being an advocate for their use. Size it bigger rather than smaller, a diminishing cross-section is the key (and the progressive damping).

dave

[picowallspeaker]

Originally Posted by planet10

Joe,

MDF is easy to shape, but isn't great for baffles.

And ... making a flat surface could be the easier job when building
a curved enclosure using ceramic .

[Josephjcole]

I know that on larger enclosures Baltic Birch is the material of choice by many around here. My main speakers are made out of a combination of baltic birch plywood and Walnut.
However on a small baffle, maybe 8" in diameter max for the tangbands, I wouldn't think the choice of wood would have such a large impact. I defer to your expertise in this area though Dave. Do people here think something like baltic birch would make a noticeable improvement on the front baffles?
Two other questions: Dave, you mention diminishing cross section is key, do you think something like the picture I attached is going to be a problem. I had invisionned it with a increase in cross section to start out so as to provide a more gentle continuous curve starting from the driver itself to help reduce diffraction. Will this initial increase in cross section cause troubles?
Next question as my budget is short and I don't know when I'll be able to afford a high quality 6" driver for a nice high effeciency three way, I'm considering testing this out with my main speakers now as well. As they are right now they are a 4" fostex (108) in a sealed enclosure inside a MLTL with an eminence delta 12lf. The crossover is at 300 Hz. If I build a enclosure like the one attached for the fostex driver. How tall of a stand can I build without running into issues of the system sounding like two seperate sources instead of sounding like a single point source? In other words how far apart can two drivers be with a 300Hz crossover and still function well together (3rd order acoustic x-over) ? I'm guessing some where around a 1/4 wave length? or ~11". Does this sound reasonable?
thanks
Joe

[planet10]

A baffle is the weakest panel in the box because of the big hole... and MDF won't reliably hold a screw.



dave

[tvrgeek]

Not as exotic, but I use "ceramic plates" in a sandwich for my subs. I glue 12" or 16" ceramic floor tiles between two 1/2 inch sheets of plywood. It makes a cabinent that is far more rigid than my prototype of four layers of 5/8 MDF. Lighter too.

I am not sure I would go ceramic for mid-range and up because it is so rigid, its resonance frequencies will be in the audible range. That is why MDF is so popular; it is not very rigid so the resonance can be below the useful range. In my sub I push the resonance frequencies above my useful range.

Very interesting options in shapes, but how many you would have to built before you get the dimensions just right after the shrinkage in the kiln? Boxes still have their place.

[Josephjcole]

Originally Posted by planet10

A baffle is the weakest panel in the box because of the big hole... and MDF won't reliably hold a screw.

dave

Fair enough MDF is out....

Originally Posted by tvrgeek

I am not sure I would go ceramic for mid-range and up because it is so rigid, its resonance frequencies will be in the audible range. That is why MDF is so popular; it is not very rigid so the resonance can be below the useful range. In my sub I push the resonance frequencies above my useful range.

Very interesting options in shapes, but how many you would have to built before you get the dimensions just right after the shrinkage in the kiln? Boxes still have their place.

Tvrgeek, I'm not too worried about resonance from the enclosure for two reasons. One, I believe having wood glued on the front baffle will help damp any ringing before it begins. Two the clay body we use is much different than commercial porcelain you are maybe thinking of, it is a much more "open" body, meaning that it has a sand like material added to it. It does not ring in the same way a commercial porcelain plate will.
And as far as shrinkage and sizing goes, I believe as I have some wiggle room as far as size goes, since I am using these for midrange and not tuning the boxes for a certain frequency. Also although we are just starting our pottery now I had been working as a full time production potter for the seven years before this, throwing exact shapes for other people, often making matching sets. I know our clay body shrinks 12% from wet to fired, so I feel pretty confident that I can make what I want. I'm just trying to get suggestions so I can figure out exactly what it is that I want
thanks for the advice so far
Joe

[planet10]

Originally Posted by Josephjcole

diminishing cross section is key, do you think something like the picture I attached is going to be a problem.

A strong taper reduces the length of the line required to hit the target. The taper also shifts the higher order resonances up making it easier to kill them.

Ideally the length corresponds with a 1/4 wave resonance = the driver Fs, but because of the aperiodic damping and a higish XO it is not a real problem if it isn't.

An intial expansion should actually work as a HP filter as well, you can see a similar concept used in Scottmoose's Olson-stype stepped horns. Each bend has an expanded area and acts to keep HF out of the horn mouths.

B&W did a lot of research to end up with the shape you are using... theirs has to be longer because they are half-wave.

The crossover is at 300 Hz...How tall of a stand can I build without running into issues of the system sounding like two seperate sources instead of sounding like a single point source?

When we were prototyping Tysen we started with the woofer at the floor. Measured ripple was high (XO ~333Hz), and flipping the (separate) woofer enclosure the other way to get the woofer as close as possible to the mid-tweeter cleaned things up a lot. Sounded more coherent as well. My advice would be to get them as close together as practical.

dave

[planet10]

Originally Posted by Josephjcole

into the great midwest

PS: what consititutes the MidWest? Looking at the map i always put it in a vertical strip that includes Colorado... but i get the feeling it means more eastward?

dave

[wakibaki]

What about horns? Can you make a big horn? Throw one on a wheel I mean. Or even smaller horns. I guess firing them would be the problem when they got big. Plus the bigger they are, the more problems with flaws. Still, I'd be interested to know. I saw some large items fired outdoors in a primitive kiln. I think it was wood fired, but it might have been charcoal.

w