Ideal BLHorn type enclosure? (for Fostex FE168E∑ besides the recommended one)

[Fosta]

Hello all!

Recently being lulled by chance into the world of full range drivers and home made equipment (redundancy = plenty of time to spare ), I've been racking my brains to try and understand as much of the science as possible so I can develop my own ideas asap.

Before I go any further, I have already built the recommended enclosures as specified by the Fostex company and they sound fantastic. I also have the ishtek freeware and hornresp to play around with but understanding it is something else.

So my question begins here... what would be a good set of BLH* specifications for these speakers assuming there was no additional sub box, and constraining myself to an internal volume of 100litres max? I'm very proficient with CAD and building moulds so I was considering a venture into "organic form" avoiding the use of wood, flat faces and corners altogether.

*Reading into terry cain's BIB's, you can apparently reach lower frequencies, so should I be building similar to these designs instead?!?


For a BLH, I think the only details I'd really need would be what compression chamber size, throat size, horn length, horn mouth, and flare rate to use... from which I can design everything else.

The company recommended enclosure specifications are approximately as follows:
horn length - about 260cm
horn mouth - 840cm2
horn throat - about 113cm2 (same as sd)
compression chamber - 3litres (taking away about 0.8 for the volume of the actual driver)
flare type - steps up at intervals so I'm assuming some attempt at an exponential increase


Any help in direction or solid data would be much appreciated & one day/one year when i have completed the project, I'll post up a few images of my attempt.

Many thanks

[marekst]

Hi Fosta,
Welcome to the forum.
I understand you are planning to build BLH (organic forms etc) the way I did a few years ago, so here are my thoughts on the subject of solid data and horn shape.

What people do, is calculate optimal horn dimensions for a driver with advertised parameters, using mathematical formulas and than drastically change the calculated dimensions so the horn can fit into a box. And that is what people are repeating after each others until it becomes a "proven" design.
I decided that horn dimensions taken from the top of my head were as good as "solid" data. My ears and my drivers are different from yours (modified Lowther EX3) so the dimensions will not work for you, but this is what I got: L=3.5m; S2=0.8m^2; S1=75cm^2; CC=3l; Contour: Hipex M=0.8; Material: metal powder, fiberglass, polyester composite and solid oak baffle rings.

The length depends on how low do you want the horn to play and placement in the room. Because room boundaries change (extend) the effective length of a horn, calculation of exact and perfect length of the horn is questionable. Similar rules apply to mouth size. Too small a mouth reflects part of sound energy back to the source causing distortions (resonances, cancellations delays…) similar to the ones in badly folded horns. Therefore I made my horns big, round, not folded, with only one bend. For clarification: in a bend, inner wall forms considerable arc (Hedlund horn is an example), a fold has inner walls folded onto each other (like Big Carfrae). In fact, both of these horns are rounded but still square. If you think shape does not matter, just imagine a sound of Canadian Brass playing on instruments made with folded square tubes and boxy bells. If you can't do a horn round all the way, try to make the compression chamber, throat and first section of the horn round and symmetrical on axis of the driver, so the back pressure on the cone is even. Also, the shape of the back chamber determines how the sound waves are reflected back to and through the cone. You can find the influence of the back chamber volume and throat size by computer simulation. Try 2, 3, 4 and 5 liter chambers and different throats and compare the changes in horn output and cone movement. The back chamber can be reduced after horn is tested and horn can be tuned with stuffing to suit your ears.

You can find information, theory and calculator at James' site:
http://fullrangedriver.com/singledriver/hornshape.html
Or maybe some resident expert here or on James' site will supply you with perfectly compromised dimensions.

Good luck,

Marek

[marekst]

nt

[Fosta]

Thanks loads for your informative response.

I wasn't expecting what you said to match so closely to my concepts (reasons to avoid square horns, large curves etc...)

To give you an idea of what I already had in mind, I built the model you will see below in CAD to precise dimensions (using some of the free software available to calculate the correct rate of expansion). They are currently too big so I'm going to have to find a way of shrinking them down a bit before spending an aeon in the garage building them.

Form followed function in every respect in this design, yet the final look actually gave me a great deal of satisfaction to how they could look.


Ben

[rjbond3rd]

Marek, wow, incredible. And your room is more tweaked than most recording studios I've seen. Amazing.

Fosta, your image is not attached (due to the preview message function, I would guess).

[marekst]

Thanks,
If you like acoustic treatment, have a look at my back wall...
Marek

[rjbond3rd]

Hi marek, that is really amazing. So you are going for diffusion more than absorption? Is that a concrete floor?

[Fosta]

picture attached? possibly...

Ben

[rjbond3rd]

Both your shape and Marek's shape are both beautiful. Fosta, what material would it be?

[Fosta]

materials - anything that pours so hopefully resin base mixed with marble dust... to give that solid stone effect

We shall see. All depends on how i design the molds really, and remember this isn't going to be my final design though I'll say it probably isn't far off.

Hopefully I'll be able to find a very cheap CNC machinist to create a couple of perfect soft material molds... otherwise there's going to be a lot of elbow grease involved.