open baffle speakers

[david counter]

I fancy buying some Fostex drivers, open baffle seem the simplest way to mount them to start with, is this as easy as plonking the unit in a piece of mdf or are there rules to be followed to make them work properly,

[John Caswell]

Hi David,
Not really, just put them on the largest baffle that you can fit in and see what the results are like. There is no box to tune or line to damp or anything like that.
Gilbert Briggs has a design for a 15" and 10" in his "Cabinet Handbook" book which is very simple, but I guess if you try a full range device like a Fostex it will probably work very well.
As Briggs says the only objection is size.

John aka Dr John

[david counter]

thanks John,
sounds like my sort of woodwork,

[James D]

Hi David, John,

Whilst what John says will get you going it will not get you a balanced, natural sound and, in particular, a Fostex driver will not sound at its best. There is an awful lot of science to designing a balanced open baffle - just like any other speaker system. And choosing the right driver is just part of the equation. Fostex emphatically are not the right driver. Which is not to say you cannot use them on an open baffle but if you do there is some system engineering that has to be taken into account...

Briggs is about as accurate for open baffle as he is for any other speaker system design now-a-days. Interesting read though

James

[Black Stuart]

James,
as you know I have a pair of the Great Heils, which to give of their best must be mounted in 'free air', that's no problem but the woofer?

I would love to go open baffle but as I understand it, I would have to have a listening room the size of drill hall, because of the need for a huge baffle.

What's so appealing about the open baffle is the utter simplicity. I refuse to get involved in complicated cabinet designs.

I also refuse to use a closed design because I feel that will create an unnatural sound. I am seriously thinking of using an open backed cabinet. If it need be fairly big, I don't mind because I would use that as an excuse to decorate it - to make it an art work.

I am determined to seperate entirely the Heils from the woofer, this seems entirely sensible because it will allow me to tune one unit with the other by the simple virtue of moving one unit relative to the other and of course moving the bass unit in any direction I like to tune this to the room. As I shall be using an active x/over I will not have to think about passive x/over placing. It's just this damn cabinet for the woofer that is the problem.

[James D]

Simple, don't make a cabinet for the woofer :-)

I still think dipole bass would be the best solution for you with the active crossover. The Linkwitz site gives all the theory and practical implementation that you need for this sort of solution.

James

[david counter]

thanks James
damn I thought I was onto something with the Fostex, start with an open baffle and then move onto quarter wave or something else, I always fancied Voight pipes
when you say system engineering what does that entail,

[James D]

System engineering for OBs means adjusting the parameters of the driver to tweak it's response on an open baffle - essentially adjusting Qes - however this affects other parameters as well.

Adjusting any filter to optimise the driver response on the baffle.

Adjusting the baffle shape and size to control the response roll off to suit the modified driver response.

Adjusting the driver positioning on the baffle to optimise the system response.

Adjusting the baffle angle relative to the listener in both planes to optimise the listening response.

All these adjustments interact with each other so it has to be regarded as system engineering of the OB - then there is:

the effect of the amplifier/speaker interface.

the room/speaker interaction.

So lots of things to tweak

There is quite a lot on diyaudio and audioasylum and fullrange driver forums about OB design. As always TL has written lots. Linkwitz has good theory and a very pure engineering approach - however the theory probably tells less about how to optimise an OB than the present state of the art theory does about any other speaker type...

For back horns, quaterwave pipes and transmission lines theory that is pretty damn accurate check the Quaterwave site of Martin J. King.

James

[david counter]

thanks James
deep down I knew it it wouldn't be that easy,
seems I've got some research to do

[Scottmoose]

Fostex drivers will not be at their best on open baffles, certainly below, oh 200Hz or so, as they have a very limited Xmax. The F200a is probably the best of the lot for this application. You'd need active eq and cross-over to some hefty bass-units before you're going to get much happening below that point.

DON'T build the Lowther Club of Norway Voigt Pipe. To say that's an unoptimised enclosure would be a colossal understatement. It's an So=0 design, which is bad. Its vent area has clearly been set to the driver Sd which is worse. It doesn't have anything like enough cabinet volume (worst of all.) The response ripples like you wouldn't believe: huge comb filter effect. Massive holes all over the place, and below 80Hz, nothing much happens. Vile.

Look at Project 2 on www.quarter-wave.com

I've built 3 pairs of these. Best single driver speaker I know for under £1000. No kidding. Use the Fostex FE167E as a drop-in replacement for the now discontinued FE164. Actually, it works better than the earlier driver did.

Martin King is the man who cracked the mysteries of the transmission line. He was the first, but he's utterly ignored by the mainstream, who harp on about George Augsperger's calculations, which were done at a similar time, but are far inferior to Martin's work.