I haven't been exactly charmed by the updates on the original RS28a, so I've been looking for alternative tweeters with good characteristics and a wallet friendly price. The SB Acoustics SB26CDC-C000-4 can be found on Madisound's new product pages, but oddly not (the last time I looked recently) on their normal product pages. It is described as an Aluminum-Ceramic composite dome and from photo's it's clear it incorporates what is usually called a phase shield (which the original RS28a and the new RSS28a also have). The intent is generally improve dispersion off axis dispersion by changing the radiating area of a dome tweeter- it can also create some resonances and artifacts in the frequency response curve. Some designs, such as the original Avalon speakers, used tweeters which had phase shields that were removed. I did tests both in the original configuration and with the phase shield removed, both in direct radiation mode and in an experimental waveguide.
My testing did not have the tweeter on an infinite baffle, but on a test box that is more like what one might possibly use (it was convenient- I'd bought a couple of 0.7 cu ft Denovo test enclosures- more from that later).
I also did not take any special measured for diffraction control or felt around the tweeter. What I find happens then is that you get some stuff on axis which is literally due to the pile up form circular elements around the tweeter- these are all pretty much reflected straight forward; off axis at 15 degrees, you see a different picture...
Here I've assembled a composite polar plot out to 45 degrees
Here's a distortion plot in direct radiator mode, 15 degrees off axis...
And here's a plot with the waveguide mounting (basically a straight sided waveguide with a small diffraction taper).
More when I get back from errands...
My testing did not have the tweeter on an infinite baffle, but on a test box that is more like what one might possibly use (it was convenient- I'd bought a couple of 0.7 cu ft Denovo test enclosures- more from that later).
I also did not take any special measured for diffraction control or felt around the tweeter. What I find happens then is that you get some stuff on axis which is literally due to the pile up form circular elements around the tweeter- these are all pretty much reflected straight forward; off axis at 15 degrees, you see a different picture...
Here I've assembled a composite polar plot out to 45 degrees
Here's a distortion plot in direct radiator mode, 15 degrees off axis...
And here's a plot with the waveguide mounting (basically a straight sided waveguide with a small diffraction taper).
More when I get back from errands...
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