Foam Core Board Speaker Enclosures?

[turk 182]

hey guys
i don't know if i missed it or if there's another resource on the in's and out's of the design of these.
also wondering how critical is the relationship of driver to final enclosure size is as in trying a variety of say 3" or 4" drivers just for comparison

[xrk971]

Cocleus suggested this glue many posts ago. Works very well apparently. I did not get that is why he said to wet the parts first.

[xrk971]

Originally Posted by turk 182

hey guys
i don't know if i missed it or if there's another resource on the in's and out's of the design of these.
also wondering how critical is the relationship of driver to final enclosure size is as in trying a variety of say 3" or 4" drivers just for comparison

I use MJK's papers on back loaded horn theory and design. Scale the throat opening to match piston area Sd. Basically adjust depth of channels if you want to stick with Sebastian P's template, or mod your own spirals like I have to for a 2 in driver. The overall enclosure size determines path length (freq) and mouth area or expansion ratio which determines efficiency. I don't know what to do about design of split other than to choose two frequencies that have to blend in resonance peaks, maybe one at odd multiple of length and one is even multiple?

[turk 182]

xrk971
do you have a link or url for MJK's paper or will a google search lead to the promised land!
by Sd do you mean speaker diameter?

[xrk971]

Sd is the effective piston area typically in Thiele/Small driver parameters. MJK's website:Quarter Wavelength Loudspeaker Design

[klampykixx]

Originally Posted by xrk971

Klampykixx,
Interesting design, glad you were able to get us a picture. Am I correct in assuming that this picture shows 1/2 of the box only with the two drivers loading a pre-chamber that has a channel that leads to the mouth which then goes past the faces of the drivers? You mentioned that you tested it prior to capping it with the lid using glue since you wanted to put the amp inside. Until you glue or seal the channels air-tight, the performance will be very poor - not even close to what it could be. The reason is that the smallest leaks will collapse the establishment of the fundamental standing quarter-wave which is where you get your bass boost from. If your path length is 73 cm, this gives an approximate fundamental tuning frequency of 117 Hz (assuming speed of sound is 342 m/s). If you cap the lid off properly with glue, you should get a good amount of bass at 117 Hz or close to 100 Hz.
A couple of design considerations to think about: (1) what is the effective piston area (Sd) of your two drivers combined and does that area equal the throat area leading from the pre-chamber? You can trim the foam-core board to make this so if it is too small, this will let the horn "breathe" better and increase the bass; (2) make a cutout panel on the side to install the amp and have access to it after the lid is on (inevitably, you will need to access your amp for connections or to debug); (3) Use lots of stuffing in the pre-chamber and some loose stuffing on the snaking channels to absorb the reflections and higher harmonics otherwise you may end up with lots of distortion junk coming from the horn besides the bass; (4) add some bracing in your pre-chamber to keep it from flexing under the pressure of the two-drivers. If you haven't already, download and read MJK's papers on horn theory and back loaded horn design (htpp://www.quarter-wave.com) - very helpful info.
Good luck and keep us posted.
Regards,
Xrk971

firstly, thanks for all the help, not only from yourself, but the rest of the forum also. this is probably the only forum where i dont get shunned for my ideas, and actually given good quality constructive help. so thanks DIY

regarding sealing it up, the amp i plan is a little 5v chip amp, 3w+3w off ebay. $3.50!! o_0 hehe cheap as. lol

your question on the throat area, definately could be opened up alot, just based on simple math, two 70mm circles are 24.1cm^2 and the throat is definately not even half that. so i could probably trim it down all the way back to the first bend before it has a big enough throat. however, this will shorten my horn path significantly, so what im thinking is to trim the first section on an angle, so its tallest towards the first bend, and shortest at the throat, so the actual throat is quite long and thin (hope that makes sense.)

i do plan on stuffing it, so thats a definate.

and one problem i have, is i have already bolted in the drivers, so might not be able to get the blade in behnd them to trim the first panel... :/ have to work that one out without stripping the screw holes...

i uploaded some pics on the work wifi. so here they are, in order.











i have reinforced the baffle, and also the bottom of the throat chamber.

ill have a play tonight and see what i can come up with regarding the throat opening. ill take some pics, but be warned, (lol) that i get bored, and i had spray paint in my possession... haha (its now shiny red... haha)

[cochleus]

Originally Posted by xrk971

Cocleus suggested this glue many posts ago. Works very well apparently. I did not get that is why he said to wet the parts first.

The white variety works best, drys faster and is white colored. The brown colored gorilla glue works but is slower and is yellowish when dry. The stuff surprised me how much it expands, must be clamped (I used masking tape). Warning: Don't get this stuff on anything valuable, even yourself, and it never washes off clothing. I used acrylic caulking for filling big gaps.

[cochleus]

@klampykixx:

Wow, cool stuff man! Thanks for the pics.

Here is a pic of some of my experiments...they are waveguides where you simply set the speaker on top facing downward.

waveguides.jpg

[xrk971]

Klampykixx,
Your channel leading from the pre-chamber is pretty narrow and that will be your choke point, it won't help you to trim off the throat any wider than the channel width. You may find that running a single driver with more free flowing space may make more bass than two with all the constriction. Just a thought.... Also, is 3 watts enough to drive those units? From the looks of them, they appear to require circa 5 to 10 watts a piece, if not more. The pics look great though, and experimenting is they point with the foam core boards because if it doesn't work out it's no big deal.
Regards,
Xrk971

[testproggie]

OK, my curiosity is up. I can't find much on speaker suggestions for a Cornu spiral horn. Anybody have some recommendations on something from one of the vendors here in the states? Thanks for any advice! Oh, price limit is say $50 per driver.

Robert