wool on the baffle?

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Well, I think it IS ROCKET SCIENCE or at least COMPUTERS which is the same thing to my simple mind...it says so here:
The compact speaker is the result of exhaustive research and the latest technology in computer assisted optimization.

An externally hosted image should be here but it was not working when we last tested it.


That's sure a dumb looking speaker. (Just my opinion....) :D

Now I found some confusing and unreadable stuff by some guy called Carl, which I have knocked into shape on one page so that at least we get the idea. Curve 2 is with the thick felt, Curve 4 is without and you can ignore the rest:

An externally hosted image should be here but it was not working when we last tested it.


So what's going on here? :confused:
 
Commendable job of not rising to the bait, Mr. SpeakerDoctor! That Spica has some nice features. ;)

I'm starting to form a notion that the felt makes the tweeter behave as if there is no baffle at all. More of a point source, and less beaming. The 6db fall in output makes sense. Why square cutout, and not round or elliptical? Dunno...will sleep on it. :cool:
 
Commendable job of not rising to the bait, Mr. SpeakerDoctor! That Spica has some nice features. ;)

I'm starting to form a notion that the felt makes the tweeter behave as if there is no baffle at all. More of a point source, and less beaming. The 6db fall in output makes sense. Why square cutout, and not round or elliptical? Dunno...will sleep on it. :cool:

You can ask John Bau about the cut out. He's still around. He does respond at the yahoo groups spica web site.
I forgot to mention that the SPica imaging rivals that of electrostatic panel speakers.
 
This article has good graphs of the difference with/without various layers of felt.

Diffraction Doesn't Have to be a Problem

Note that he finds that even natural wool felt has some reflective properties and that it shouldn't be too thick or too close to the tweeter. His best results have the thickness building up (1/4" then 1/2") around the tweeter. For the same reason, the cutout shouldn't be circular and centered on the tweeter.

David S.
 
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This article has good graphs of the difference with/without various layers of felt.

Diffraction Doesn't Have to be a Problem

Note that he finds that even natural wool felt has some reflective properties and that it shouldn't be too thick or too close to the tweeter. His best results have the thickness building up (1/4" then 1/2") around the tweeter. For the same reason, the cutout shouldn't be circular and centered on the tweeter.

David S.

David R's study was referenced back in post #3. It's a very good study as well. I suspect system7's feeling of floundering may have to do with the empirical nature of managing diffraction with felt; as opposed to having a computer model that predicts the outcome.
 
David R's study was referenced back in post #3. It's a very good study as well. I suspect system7's feeling of floundering may have to do with the empirical nature of managing diffraction with felt; as opposed to having a computer model that predicts the outcome.

Hi Carl. Sorry, I missed that but did look through your good measurements.

As to the computer model, we can at least model the other way around and see the effects of diffraction and edges. I dabble with "the Edge" when I want to see what the potential effects are. The optimum result with surface damping or heavily radiused edges would be to revert to the 2 pi response (and a smooth 4 pi transition) and have none of the residual diffraction effects.

By the way, at PSB I had the latest LEAP software and his diffraction modeling is quite sophisticated. I remember doing a simulation of response of a driver on the box. When I moved past 90 degrees all response dissapeared. It turned out I had "turned off" diffraction modeling and without diffraction there is no response beyond the 90 degrees of the baffle.

We wouldn't have dreamed of these tools 30 years ago. We knew very well the principles involved but the thought of a PC program to give a fairly accurate response curve including diffraction effects.....

David S.
 
In my little study I was simply tryng to show the ineffectiveness of some commercial products on the market vs relatively thicker, needled wool felt. OTOH, David R did a much more comprehensive empirical study of felt's effects.
BTW, the felt I used in my study was exactly the same as David R. used. I believe mine was the SAE F11 type which, I imagine, is still available from McMaster-Carr.
 
I was wondering if it's just diffraction effects that can be seen in these measurements or if there's also a certain amount of absorption. One would need to look at off-axis data too.

Check out the link in post #3. The far right spreadsheet column has clickable off-axis data for 'no felt' and for 'felt'.

IMHO, absorbtion plays a large role when 1/2 inch or so thick wool felt is utilized. It's tangled mass of coarse fibers provides an ideal absorber. This felt could also be used to dampen internal cabinet reflections. After all, thick felt's absorbtion properties are exactly what the car builder use this stuff for.
 
Why not use a lot more porous absorption around the speaker and control directivity? While it is a massive waste of energy it doesn't have the problems associated with waveguides.

I've done some experimentation with this and it can be quite effective (though tends to be UGLY).

I regularly use acoustic absorbtion as part of my speaker designs, it's just another tool in the loudspeaker design kit. Around the edges of horns for example, it's a lossy way of damping the mouth termination without having to make a large mouth. It doesn't preserve energy the same way, but if the passband's above the horn cutoff anyway...

I liken it to an egg. Drop an egg, it breaks. Roll it down a ramp, it keeps rolling and is like a fullsize horn. Drop it onto a pad and it doesn't keep rolling but you still have your egg.

My first recommendation to many people complaining of bright sound is to felt the baffle, it suppresses the power response peak above the XO of typical cone 'n domes.

One thing I've been meaning to try is the use of acoustic absorbtion to damp the modal character of OB cancellation. Haven't tried it out yet though.
 
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Interesting "wool" application.

I stumbled upon this thread and thought I would offer up a recent discovery of some speakers I obtained some 20 years ago, but had in storage in their original boxes for perhaps the past 15. I recently put them into service connected to a Sony STR-6065 and Pioneer SX-780 to see how they would sound. I have now played them for the best part of a week for 8 to 10 hours a day listening to everything from NPR classical on some very clean local stations, to jazz on some fine Denon CDs I've had for many years. I had forgotten how surprisingly good and well balanced these speakers sound given their humble heritage. Indeed, I expect that already more than a few readers have started with the guffaws and scoffs to themselves. But no matter. I have other speakers that were several times costlier than these and are not as pleasing.

What I though may be of some interest to the readers of this thread is the application of the design and wool felt. The speakers are twin ported bass reflex with 8" twin mid and full bass woofers and a center miunted1 1/2" soft dome tweeter. The tweeter also has a large area of felt applied around the tweeter and the grill frame has what the makers refers to as an "acoustic lens" built into it. The twin ports can also be plugged with some supplied hard foam plugs, which I find seems to work well depending where I place the speakers and in what sized room.

With that, here are a few pictures of what I find to be a very nice and well balanced speaker especially given it;s humble heritage from American Acoustics which purportedly was an MTX brand. Perhaps they made some good product for a short period of time?

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