An interesting "budget" tweeter from SB Acoustics for hard dome fans...

[JonMarsh]

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...

[JonMarsh]

What about the phase shield? Or without it?

Being the curious George sort of person that I am, and as easy as it is to do with this part, removing the phase shield and investigating seemed like an easy step to take.





The on axis response looked roughly as I expected, with the upper breakup mode unmasked.




Maybe this ceramic/aluminum composite thing works to some degree; it's certainly not looking like a typical Seas aluminum dome tweeter breakup mode.


The polar plots might be interesting, as this is where hard domes generally fall down on the off axis response when they don't use a trick like a phase shield...






This is actually pretty fair looking; possibly the shallow depression that the dome is in relative to the rest of the mounting plate has some benefits. Of course, only being a 26mm dome instead of a 29 or 30mm may be part of this too.


Just for fun I re-ran the distortion plots...





Here the impact of resonance amplification of distortion products is quite clear for both the 2nd and 3rd harmonic- does that indicate that for the phase shield measurements they're being obscured somewhat by things that the phase shield is doing?


One last plot, in the waveguide...






Interesting... and suggesting that suppressing the effect of the break up resonance would be a good thing to do, if possible, at least in the case of the 2nd harmonic.

But overall I'd say it's a pretty nice tweeter for the money... worth considering for a system design.

[ergo]

Quite promising looking plots indeed and price is very affordable.
Jon, would you have an image with that tweeter without from panel too? I assume you removed front panel to attach it to WG?
I've been thinking about next build to take on after the OLA-WG project I'm doing for a friend. It will be an attempt to do a low budget design for a change as the last Minerva and Wavecor Ardent have been pretty much high end price wise from my perspective. Something like this in a WG might be quite usable for the ideas I have in mind.

[Dave Bullet]

Jon - can you please tell me if its simply a case of unscrewing the 4 screws holding on the faceplate, to remove on the SB26CDC / ADC? Is there anything else attached? (apart from I'm guessing the phase shield)?