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Sonnie wrote:
 Yep... I think Andrew gets the bone on this one. Excellent answer, although I have no idea if he's telling the truth, but it sounds really good.  Just picking at you Andrew. |
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DS-21 wrote:
A few things. First, all tweeters are "horn" tweeters, in the sense that all of them are loaded on a waveguide of some sort. (A flush baffle is just a 180deg waveguide.) Some of them are designed to improve power response (and thus imaging) by providing constant directivity over a given area, with a smooth rolloff of treble energy past that point. (Many coaxes, such as the KEF Uni-Q and Tannoy Dual Concentric, attempt constant directivity by using the cone profile as a CD waveguide.) Others (e.g. the Tractix flare used by Klipsch and others) does not attempt constant directivity. |
While technically you are correct every tweeter is a "horn" this is not the common terminology most are familiar with and I was answering with respect to the OPs terminology.
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DS-21 wrote:
A 180deg waveguide is basically hopeless, and leads to all kinds of problems in the midband power response, because it does not control the dispersion of the tweeter at the bottom of its passband at all. |
I am not sure if I understand your thought process here. With proper integration of drivers (the midrange and the tweeter) this is not an issue at all. *Clarification* What I mean by this is: With the proper crossover settings along side a midrange and tweeter with similar on and off axis responses this issue can be overcome. I am fully aware of this issue due to my current testing in which I am doing exactly this. With a mock baffle set up I have encountered no such issues.
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DS-21 wrote:
So generally, I think it's safe to say that the best imaging - especially the best imaging over a zone in a room, as opposed to a spot. - comes from constant directivity waveguides, with other waveguides being various levels of inferior to a good CD waveguide. |
In terms of the loudspeaker itself it is off-axis response that creates imaging and the sense of a sweet spot. Simple physics dictates that for the largest sweet spot greater off-axis dispersion is required and for this to be pleasurable this response need to mimic the axial response.
Of course the room plays a hugely important role in this situation, but again the OP asked about which type of tweeter which is why my emphasis lay on the loudspeaker not the room.
As far as waveguide versus off-axis dispersion they are two different but interrelated things. A waveguide does exactly what the name implies where the horizontal and in this case less importantly vertical response are "guided." While this can limit a tweeter's off-axis dispersion characteristics it does not entirely control the linearity of these responses which is why this distinction is important. Waveguides can control actual dispersion, but the dispersion itself dictates imaging so I guess in a sense both the waveguide and the tweeters resulting dispersion characteristics will be indicative of imaging/sweet spot sensation. If one is looking for a way to quantify this data off-axis response will be the most important data to collect and analyze.