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Old 3rd December 2007, 09:07 PM   #61
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Default Re: Re: Re: One other consideration

Quote:
Originally posted by dlr

I saw that post, but didn't reply as I have only nominal experience with waveguides. The results are interesting, but I have little direct experience on which I can make direct comments.

A waveguide is supposed to increase sensitivity in the front hemi-sphere at the expense of the rear hemi-sphere (below step) and increase it in a cone (if a circular opening) defined by the waveguide geometry. As Geddes does it he's trying, as I understand it, to create constant directivity down to the point where diffraction diminishes the directivity and the dispersion starts to increase.

If you look at the post closely, the horn effect is only below 4KHz gradually rising to + 6dB at 1500Hz. All horns, in reality, only work over a bandpass and in this case it's about 1-4KHz, above and below there are cut-offs. So what is the advantage of a waveguide fitted to a 'conventional' tweeter? There are several, improved power handling when the crossover is adjusted to take advantage, hence lower distortion, better acoustic transition as the air impedance difference is less (caused by difference in cone/dome area meeting air). But horn loading is not one of them as there is no increase in sensitivity above 4KHz.

This means the ON axis response is not boosted either, BUT the response OFF axis is. So there is one kind of advantage below the crossover (lower distortion, higher power handling) and another above it (better dispersion). But we are not achieving increased system sensitivity as is the usual reason for using a horn.

Bottom line, horns can be used for a variety of reasons/purposes.


Quote:
Originally posted by dlr

Any baffle (or waveguide) that presents a non-flat barrier is mostly about diffraction, I suppose, though some of it must be reflections inside a waveguide, not diffraction. Interesting topic in any case.

Where do you draw the line between diffraction and boundary effects, or non-flat barrier. You can get over-academic about it, but in real life you deal with it as you have to deal with other things. Use microphones, decent equipment, observe and learn. That way we can get a handle on it. I can tell you that even the great John Dunlavy, who knew a thing or two on this subject, would use observation and then the academic side of things are guided by what we observe. Classic learning process. But my observation is this: The boundary effect of the waveguide changes the OFF axis response, which means the acoustic impedance above 1KHz stays largely the same (no boost), but OFF axis as we get closer to the boundary, the pressure intensifies as it hits the boundary, the acoustic transfer impedance is changed leading to a rise in sensitivity. Note that this creates a back pressure effect on the part of the the tweeter's radiating area that contributes to that OFF axis response. Call that a transformer effect, if you like.

Any comments ? Quite welcome.

So I now carefully observe the effect of waveguides (as I DO use them) both ON and OFF axis, as well as flatness of response, what I need to do to achieve the best results. Not alone here.

FINALLY, waveguides allow us to line up the tweeter in time relative to midrange and hence a great tool to achieve the desired time adjustment I need. OK, makes front panels more difficult to make. You could using sloping, but then you loose the potential advantages of waveguide. But if I use a waveguide when mating to a 5" driver, or 6" or 8" - that changes the depth of the waveguide. As the prerequisite is the time factor, I cannot control the depth of the waveguide as it is determined by the other driver(s). Whatever it turns out to be, then I must make it work.

You are right, it's an interesting topic, especially when you can use a horn for non-horn reasons.

Joe R.
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