Help please ref Fostex FE-88 SOL and JBL Driver OB positioning.

[itskeith]

Hello,

I’m just about to start my first two OB builds this weekend (hurrah), and am hoping for some help on a couple of what should be quick and easy questions for you guys. Your advice would be much appreciated / and allow me to get a flying start!
Cheers.
Keith

I’ve seen builds with the Full Range Driver and the Bass Driver being on the same vertical centre-line, and on other builds the Full range driver is offset to the side slightly.

Tweeter / Full Range is Fostex FE-88 SOL
Bass is JBL 12” 123a.

Baffle size will be 1200mm tall x 600mm wide, but have 2 x 1200mm x 300mm wings to add if needed. Thought it wuld be a good lace to start without having to do too much reading (a bit like the Glow In The Dark Builds).

Is there any accepted / best way of doing this: in line or offset?

Also, I can’t find any builds which show the height of the of the driver off the floor. I understand lower is good, but would you go for 100mm? 150mm etc

Any advice / help and suggestions would be much appreciated.
Cheers.
Keith

 

[itskeith]

Should I offset the tweeter from the bass driver vertical centre line?

How high to put the 12 inch driver off the floor?

Any help would be much appreciated? PLEASE!

 

[Scottmoose]

No fixed answer. It depends on your design objectives, your baffle design, on-baffle responses &c. Close positioning of the woofer to the floor should increase boundary gain a bit. Offset can help diffraction signature on-axis; doesn't nesessarily apply off axis though.

 

[scottjoplin]

Read Linkwitz Lab site. This too gives an idea of what'a happening Equivalent Baffles

There are ways to spread the diffraction frequencies and smooth the on axis response whilst keeping the off axis response more symmetrical. So centre-line mounting can work. Bare in mind an OB speaker is often only an approximation of a dipole.

 

[picowallspeaker]

Close positioning of the woofer to the floor should increase boundary gain a bit.

Close positioning of the woofer to the floor should increase boundary gain a bit.

Reading a little bit of math it seems that the integral of sines, cosines etc. which represent the radiation of the baffle+membrane, you'd obtain a larger emitting area. That would make the origin of the sound more diffuse i.e. the so called imaging would be rather...unsubstantial.
Everyone talks about imaging and focusing but IDK what it refers to, really.
Also articulation....wait there's also they sounded amazing

 

[planet10]

You want the centre spacing at less than a 1/4 wavelength at the XO point. I’d put them as close together as possible.

dave

 

[itskeith]

Thanks everyone and much appreciated.

I was hoping for the quick and easy solution to get me going, with maybe some reading up at a later date.

At the moment i'm putting some 8" full rangers into some J.E. Labs styes baffles. Maybe that'll give me something to listen too whilst i do some reading up for the Fostex / JBL's.

 

[scottjoplin]

That will work to give you an impression of OB, as you read more about the way to make a dipole work it becomes more complicated, it depends what you want from your speakers.

 

[itskeith]

Thanks Scott,

What I'd like from the speakers is:
- engaging immersive sound
- subtlety and lyrical
- big and high soundstage
- not that fussed about very heavy bass

Just starting to come round to dipoles are not that easy!
Plenty to be reading up on now thanks.

Supravox's should be finished this weekend.
Cheers.
Keith