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| Re: directivity [message #18099 is a reply to message #18098] |
Fri, 12 August 2005 20:09   |
Earl Geddes
Messages: 220
Registered: May 2009
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I did not read the Toole paper as carefully as you seem to, but I would have to say that there is not enough data to make the conclusions that you say Floyd made. In fact, my experince, and some unpublished data, imply the opposite. But either way your original comments implied that Toole claimed a "preference" for wider directivity, but now it seems that it is only a difference in subjective effect. Perhaps I should reread the paper in more detail. I do appreciate the reference to the horn mouth paper. This is much like what I use in my text. I found some slightly different results and Johansen admits that it is known that the wavefronts do not adhere to the walls in many horns, but his analysis assumes that they do. This is a weak point in the paper. In my text I do not assume this adhernce since I do the calculations using higher order modes which accounts for this effect. I would thus claim a slightly higher accuracy to my analysis than Johansens. But I really am indebted to you for a coincidence that happened after I reviewed the Johansen paper. The next article in that journal is about using woven cloth as a HT screen, which I had also recently concluded to be supperior to a commercial screen for all the same reasons as in the paper. My new screen cost me $10 (bed sheet) and replaced an inferior $1200 perforated vynal screen! Can you beat that!
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| Re: directivity [message #18101 is a reply to message #18100] |
Sat, 13 August 2005 20:13 |
Earl Geddes
Messages: 220
Registered: May 2009
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It is true that at the low end most of the wave propagated down a waveguide will be the pure longitudinal wave because all of the other modes are below cutoff. The higher order modes are dissipated away from the throat depending on distance. These modes are known as evinescent waves and decay exponentially with distance. In a short waveguide these modes have not decayed sufficiently at the mouth so as to be ignored. I have long been aware of this issue, but have never had a chance to study it in any detail. I suspect that this is a reason why short waveguides don't behave as anticipated. If the waveguide is folded or bent at all, then the higher order modes are created again at each bend and we get the problem starting a new. The thing about the screen that gets my attention is how much unnecessary hype there is now in Home Theater. Its the new "Monster Cable". Buy a multi thousand dollar screen if you want, I'll buy a bed sheet and a lot of DVD's instead. Nice talking with you.
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