"The Flatback Banned" OmniMet

[IslandPink]

I was in Denbigh keeping away from all the tourists !

[Cressy Snr]

Been chomping at the bit to get a bit further with these flatbacks, but decided to leave them until tomorrow, when I'll have the house to myself.y
However just to check I'm on the right lines, have been listening this afternoon, once more, to the standard Omnimets tilted back at the wall, with the drivers right up against the room boundary..... And the verdict is

Right now I can say without fear of contraception, that these speakers, in terms of an ability to play music convincingly in a domestic living room, are a mission accomplished. So the build of the proper flatbacks can continue apace.

[Cressy Snr]

Need to get my finger out with these things. Have made no progress this week with them although I had intended to have had them built by now.
Owston waits for no man.

[Cressy Snr]

Been on a bit of a creative thing with these speakers today because of a (possibly fortuitous) mistake I made with the cutting of the front panels, that only came to light when I was dry fitting the pieces together.

Due to the increased backward rake of the front panel, it needs to be longer than the back panel to reach the floor at the same level as the rest of the panels and I had cut the damn' thing the same length as the vertical back panel.
Now I could have shortened the back and side panels to compensate, but I never do things that simply. One of these days I'll learn that speakers are square-cornered for a reason. My production engineering skills are, shall we say, a tad underdeveloped at the moment.

So what did I do? I only went and put a 10deg downward slope on the side panels so that the finished cabs will tilt the main driver 10deg towards the listener, away from the horizontal, so that the front panel hits the floor lower. Now we are getting a bit Shahinian Compass like with the main driver orientation.
The cabinet shape is getting a bit arty, radical and highly "designer" I'm loving it!

Watch this space

[IslandPink]

Due to the increased backward rake of the front panel, it needs to be longer than the back panel to reach the floor at the same level as the rest of the panels and I had cut the damn' thing the same length as the vertical back panel.

You have my sympathies, brings back a number of painful memories !

[Dr Bunsen Honeydew]

You will lose that wall interface. It will no longer be 2pi.

[Cressy Snr]

I'm doing aesthetics over technical stuff with this now.
Function follows form with this variation. It wasn't supposed to until I discovered the cock up, but there you go.
It'll be an interesting empirical experiment if nothing else.

Sometimes a slip of the saw takes you to interesting places and I can always get another sheet of cabinet material if the music suffers: I have a hunch that it might not, though, because after discovering the balls up with the front panel, I put a little forward tilt on the existing speakers, to see what would happen; nothing did. The music remained intact, so a 10deg down tilt, off the horizontal is not the deal breaker it might have been. That's the beauty of my speaker designs, The truncated pyramid concept is pretty forgiving. They tend to be plonk and play and as long as they go near a wall, they are perfectly happy.
Plonk and play is their DNA.

[Cressy Snr]

It'll be interesting I think, if I bring the flatbacks and the "normal" OmniMets to Owston for a comparison session.

[Dr Bunsen Honeydew]

I am going on Roy Allisons work. Which came originally from work done with the AR LST and continued in the 10pi the last of Allisons work before he left AR. Pi being free air, 2pi being one wall or surface, 4pi being corner. AR originally with the LST and later in in the 10pi used tappings on a transformer (coil) to adjust dispersion. His final work I gave you with the Patent, even 10degs changes the angle of coupling / dispersion. The Shahinians were not based on the same principles.

[Cressy Snr]

Loving it yesterday, I have abandoned this project.

It is far too difficult to make and for the difference a few inches closer to the rear wall makes, to the sound, it is not worth the extra effort. More difference was made by changing the valves in the amplifiers than was made by putting the main drivers a few inches nearer the wall.

All I've done really is to transfer the compound angle business with the insides of the horn, to the outsides of the cabinets instead. So by solving one problem I have created three more. The cab is simply not a viable proposition, so I've returned to the "pure" design. Compared with the difficulties of this one, 4 sided taper is much easier and quicker.

Sometimes you have to admit to yourself that what you are proposing is not going to work. Nothing wrong with that.

Load of wood is going to the tip now. At least I have some oak veneer for a proper one in the future.

[Cressy Snr]

That's better. Wood gone.
Bit of an abberation the flatback. I was just not comfortable or happy with it in the end.
It had lost the purity of design that the truncated pyramid semi-omni has, and in the end, the assymmetry was just too jarring.

A simple rectilinear column one, with a single internal divider to form the QW resonator, would have made more far more sense as an alternative model in the series, that would have got the main driver nearer the wall. The flatback with tapers on three of the faces was just being silly, if I am really being honest with myself; a step too far. It has now been put out of its misery.

FFS I already have a design with the "proper" OmniMet that works brilliantly, so why not stick to that? I know how to build it quickly, I know the dimensions and angles inside out, so making a set of accurate templates and jigs for repeatability and consistency, has to be the logical step from hereon in. Sometimes one has to accept that when one has something this good, then that's that, you go with it, or you are an idiot.

From now until Owston, I will busy myself productively by putting the balanced mains unit into a proper box, and leave the OmniMets as is. They are well capable of speaking for themselves at the upcoming event.

[Cressy Snr]

Bit of an abberation the flatback. I was just not comfortable or happy with it in the end.
It had lost the purity of design that the truncated pyramid semi-omni has, and in the end, the assymmetry was just too jarring.

I've been busy with a ground up redesign of the flatback speaker, on order to produce something to complement the original OmniMet:

Still with the QW transmission line bass loading and the same drivers.
The up-firing main driver is right next to the rear wall.
No inset base, so construction is much more straightforward.
A sloping front panel angles the tweeter up towards the listener.

[IslandPink]

Load of wood is going to the tip now. At least I have some oak veneer for a proper one in the future.

Looks like you should have stacked-up that spare wood in the hallway , like I do !

[Cressy Snr]

Could have done that, but the stuff was already cut into various trapezioid shapes, that were no use whatever for anything else.

These new cabs are built. Just need the flush trim bit in the router to clean up the edges. After that, The driver holes, port and terminal tray hole can be cut into the cabs, then the drivers can be wired in along with a cap to the tweeter.

I'm starting simple again with the cap, because the crossover characteristics could have (almost certainly will have) been altered due to the closer proximity of the main driver to the rear boundary, so I'll be starting out again with that aspect of the design.

The cabinet shape is different too, so I am not expecting the responses to be the same. I'll be doing all the voicing all over again.

Obviously I have a baseline to start from as I know how the bass/mid and tweeter work together, but that's as far as it goes.

It should sound nice once it has been properly tweaked up. Hopefully, if the voicing goes OK, I'll be able to veneer the cabs in the English oak I have upstairs, and we'll have two different semi-omni designs to put out there....fingers crossed.

[Cressy Snr]

All orifices have been cut in the cabs, so it's now a matter of wiring in the drivers and crossover, then setting up the raw plywood mules in the living room and voicing them up.

In the yard, with a bit of wadding at the narrow end of the line, the cabs propped on plywood slats so that the port can exhaust down into a narrow gap around the base and the drivers just sitting in place with the terminal tray lightly tapped halfway home, the system passes my "tap on the bass cone" test: dull thump that stops dead with no resonant quality to it, so I'm confident we are going to get something reasonable out of them in the first instance.