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In Reply to: RE: Most of that made sense :-) posted by hollowboy on June 11, 2008 at 16:37:30
H
Sometimes the effort to make something simple makes it more complex ; ) Imagine a driver conected to one of the favorite imaginary acoustic theoretical devices: an infinitely long pipe. Unlike a horn, there is no expansion in the dimension 90 degrees to the axis of the driver, just a long pipe that will not return any reflections back to the driver from its end in our lifetimes. This pipe is symmetrical in that it has the same cross section at the begining where the driver is, as it does any where else along its length. A horn by contrast (even an infinite exponential horn) has an increasing expansion proceeding from the driver along the axis to the mouth of the horn (or infinity as it were), so it presents an asymmetrical load to the driver, which changes as the wave propagates along the horn depending on the wavelength. Higher frequencies will tend to "beam" and will be too small to be affected by the horn mouth, and will behave much the same as if you took a saw and cut the horn short. The higher the frequencies go, the smaller the cross section of the horn becomes that will have an effect on them.
Going back to the circular flat baffle example, if you glue a string to the center of the dust cap of the centrally mounted driver, you find that the string ends with the same dimension in any direction when swiveled around the edge of the baffle. This baffle will give you an equal 3 dB downpoint, relative to the driver, all the way around. Okay, plug the hole you cut for the driver in the baffle (you saved the piece of course), and cut a new hole towards the top away from the center. Placing the driver in the new position, you find that the baffle step is continously variable depending on where the reference string is swiveled relative to the center of the driver, and the acoustic loading of the baffle on the driver will be asymmetrical compared to the first configuration, and it will now have a variable 3 dB downpoint depending on the reference point from the center of the driver.
The differences between a square horn and a round one is a different, but related, issue. Voigt rationalized that the departures from the ideal tractrix curve in his square horns (where the corners of the flat walls of the horn were joined together) tended to average out when the horn was viewed as a whole (from the sound waves point of view). This is a comforting view to take if you have square mid horns (like Voigt and me), but may be less so if you have round horns. I've heard my square tractrix mid horns back to back with round Edgar "salad bowls" of very similar dimensions driven by various JBL and Altec phase plug drivers, and I really could'nt tell them apart. The various drivers we tried made much more of a difference in the sound, much to the surprise of an egotistical horn designer like me. So how important this is depends on who you are talking to, and what they are using for references.
I hope this clears (rather than muddies) the waters
Paul
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