[D OB G]

P.S. I just reread your post.  If it is only perforated on the rear, then any diffraction reduction effects would be complicated.  (you would convert to effectively having a shallow waveguide at the front- which might be a good thing- I don't know).

[Oublie]

Who is Lynn olsen have you a link to his website?

I was thinking more of open as in 50% open like the stator

[Russell Dawkins]

I'm intending to have a good play around with baffle spacing / angles to see what can and can't be achieved.

Off centre spacing along with standing waves etc all need to be looked at.

don't forget to include tapering the baffle to distribute the frequencies affected by edge cancellation/diffraction. I have seen many OB line arrays over the years where this could easily have been done, but seemed to have been over looked.

[Oublie]

Russell by tapering do you mean wider at the top than at the bottom ?

[D OB G]

Lynn Olson has a thread on DIY Audio called "Beyond the Aerial", which you might have seen.  Almost 7,000 replies!!!!!!

It has turned out to be a general barnstorming, brainstorming thread.

He only speculated about the perforations- didn't actually try it.  It is obviously not a part of the standard open-baffle repertoire.

[Russell Dawkins]

Russell by tapering do you mean wider at the top than at the bottom ?

...or wider at the bottom - or anything that varies the distance to the edge.

[Oublie]

Lynn Olson has a thread on DIY Audio called "Beyond the Aerial", which you might have seen.  Almost 7,000 replies!!!!!!

It has turned out to be a general barnstorming, brainstorming thread.

He only speculated about the perforations- didn't actually try it.  It is obviously not a part of the standard open-baffle repertoire.

Ah Yes,

I saw that thread started to read the first page but realised how big a thread it was and decided to leave it.

I've just had a read through a few pages, its a mine of information!  Load of OB stuff, i'm going to have to read some of it.  I havent seen any info about the holes yet though.

[Rudolf]

don't forget to include tapering the baffle to distribute the frequencies affected by edge cancellation/diffraction. I have seen many OB line arrays over the years where this could easily have been done, but seemed to have been over looked.

I don't think that anything has been over looked.
For that distribution of frequencies you want the driver-to-edge distance to vary as much as possible 360deg around the driver. A narrow line array does that very well. Widening the baffle anywhere along the array will only diminish that variance, because we lose the smallest distances.
One may argue that all drivers in an array are exposed to the same variance, if the baffle width does not change. But a closer look reveals that the driver in the middle of the array has a very different variance compared to the drivers at the top and bottom edges (which happen to have the same). So each driver is slightly different from its neighbours while the baffle width stays the same from bottom to top.

Took me some time to realise this for myself.

[Russell Dawkins]

Hi Rudolf.

So, do you mean by this that those Apogee ribbon tweeters that were parallel to the edge of the baffle could not have been significantly improved by a slightly non-parallel configuration?
I guess I have been suffering from a delusion all these years. So much for intuition!

Thanks for setting me straight. Now I have to re-think this whole thing!

For now, it still makes no sense to me how having a series of drivers with identical distances to the strongest diffraction zone (90 degrees to the side) would not be a problem in that the particular frequencies affected would be the same for each driver and thus amplified.

Does what you say apply to long ribbons?

I enjoyed what I could understand of your website. It looks like you have applied a great deal of energy to this subject.

cheers, Russell

[Rudolf]

So, do you mean by this that those Apogee ribbon tweeters that were parallel to the edge of the baffle could not have been significantly improved by a slightly non-parallel configuration?

Hi Russell,

you were talking about edge cancellation/diffraction and I answered to that. But it is only one side of the double bind each OB is in:
- if you widen the OB, edge diffraction gets worse (very generally spoken)
- if you narrow the OB, you lose support at the lower frequencies

So you need to make compromises all the time 

One compromise could be to have a wider baffle for the tweeters at the bottom of the array to enhance the midrange, and to narrow the baffle at the top to get the least diffraction at hear height and above.

Sounds like in the end your proposal has its merits. 

For now, it still makes no sense to me how having a series of drivers with identical distances to the strongest diffraction zone (90 degrees to the side) would not be a problem in that the particular frequencies affected would be the same for each driver and thus amplified.

Shortest distance means that diffraction issues will be limited to the highest frequencies. That is no bad compromise IMHO. And don't let your eyes fool you. Compared to the total circumference of the array it is only a very small fraction where the edge distance is really narrow. Could it be psychology which lets us concentrate our attention on that small zones - and not physics?

Does what you say apply to long ribbons?

I can't tell from practical experience, but from a theoretical viewpoint there shouldn't be much difference between a long ribbon and a line of densely spaced tweeters.

Rudolf

[Oublie]

Hi Folks,

It's been a long time but like all things in life sometimes other stuff gets in the way.

I've Changed direction considerably.  I ran 12 mids per side in open baffle for a few months but they were lacking any real low end although with quite a bit of equalisation the highs were acceptable.

I've attached pictures of my current setup

This consists of 6 mids in line array configuration mated to 2 15 inch qts 0.92 woofers per side and a single tweeter.  I have just purchased a set of linaeum tweeters which will replace the current tweeters for a fully dipole arrangements.

Sound is fantastic with flat response to 30hz and currently limited to about 95db from one speaker at the listening position (3m)










As you can see the measurements are pretty decent, there has been a bit of equalisation including quite a steep increase at 70hz to counteract a marjor suckout at 70hz not sure whats causing it though.

The spl graph is 1/3 smoothed showing an approximate range of =/- 2db :0)









yes they are very ugly but the final build will be far better looking i promise.