Home » Audio » Speaker » Rounded Bends vs. Flat Reflectors in Folded Horns
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Re: Rounded Bends vs. Flat Reflectors in Folded Horns [message #19130 is a reply to message #19129] |
Tue, 27 February 2007 00:32 |
Cuppa Joe
Messages: 103 Registered: May 2009
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Viscount |
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Yup, those very strands are the origin of my present questions. Re-reading them gives focus to the actual root source of my confusion: real-life, 3-D soundwave propagation. Looking at a simple, 2-D drawing of a single frequency, represented as a series of colliding & expanding dots, doesn't porvide a global veiwpoint of a multi-frequency wavefront as it encounters obstacles in the course of developing and leaving its source. I have the usually recommended books and reprints, but they take up long after my physics education dwindles. (Some of the math is Sanskrit to me. OK, much of it, already!) So, first Q: Is a sound a particle or a wave? Or, both? The two concepts seem to get interchanged according to the theme of a discussion. Question 2 depends upon how we explore the first Q.
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Re: The Movement of Waves and Particles [message #19135 is a reply to message #19131] |
Wed, 28 February 2007 23:21 |
Cuppa Joe
Messages: 103 Registered: May 2009
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Viscount |
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Thanks for the links, Wayne. I hadn't thought to look there! I already had the impression that you'd say sound was both particle and wave, but I wasn't sure whether one property overshadowed the other in application or theoretical discussion. Both properties get equal billing, right? I'm still concerned about my bends vs. reflectors indecision. A wavelength in a horn doesn't just have length, it also has...diameter, I guess. If I want to pass a signal through a W-bin up to (at least) 1kHz via 45 degree reflectors, wouldn't the wave impacting the reflecting surface interfere with itself when it exits toward the mouth? I had a similar concern for a rounded bend in a W-bin. What is there to prevent a frequency leaving a driver's cone from simply shooting across the passage to the outer wall, as opposed to following the horn's inner curve? Wouldn't that cause cancellations as well? I'm not completely understanding how a wavelength's "width" affects its behavior inside a folded horn, especially as the frequency rises. When Bill F. was proposing that a wider radius passes a higher frequency, I think it's because a curve comes closer to approximating a straight wall as its radius increases. Is that about right? So, with a reflector, what does reflector size vs. wavelength mean? That relationship is yet unclear. I also noticed with some of John Sheerin's models showing a soundwave moving through a long 90 degree bend, that higher frequencies tended to adhere to the outer curve as they left the mouth. Could I expect that in a folded horn with rounded bends, where the higher frequencies might cling to the outermost edges of the horn's mouth? Would a reflector fix that problem?
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Re: The Movement of Waves and Particles [message #19137 is a reply to message #19135] |
Wed, 28 February 2007 23:50 |
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Wayne Parham
Messages: 18789 Registered: January 2001
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Illuminati (33rd Degree) |
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An omnidirectional sound source spreads as a sphere. When constrained, it spreads as a partial sphere. For example, sound radiating into halfspace spreads as a hemisphere. If you have an omnidirectional sound source sitting on the ground, that's the pattern it will make. If the sound source is directional, it will form a sort of 3D pie slice, a cone that represents a fraction of the spherical wavefront. Of course, things can disrupt wave propogation, causing the wavefront shape to change.Think about the wave that travels on the surface of a pool that forms when one rock is thrown in. It travels away from the point of impact as an expanding circle. Throw in two rocks, and interference causes a new pattern to form. Disrupt the wavefront with an object that reflects it, and the returning wave interacts in much the same way. The ripples in the pond let us visualize what's happening. When an object, small in relation to wavelength, is placed in the path of the moving wavefront, it does not act like a reflector. Waves will pass right by as if it weren't there. If an object is large in relation to wavelength, then it acts like a reflector. But the behavior of a reflector that is spaced nearby acts differently than one that is far away. If a reflector is near - within 1/8λ - then the reflection is in phase with the source and the reflector acts as a launch point boundary, merely constraining the radiating angle. If the reflector is further than 1/4λ away, then constructive and destructive nodes cause pockets in space where reduced amplitude nodes form. At exactly 1/4λ, a notch forms because the reflection is 1/2λ from the source, which causes cancellation. At distances beyond that, several nodes line up in 3D space. Between 1/8λ and 1/4λ is a transition region where the source and its reflection are not summing together fully as a single source, but are not cancelling each other strongly either. This explains the behavior of sound through ducts of various shapes and sizes. Sound travels around corners at low frequencies, basically passing right by. At higher frequencies, sound bounces from wall to wall. A reflected sound source acts something like another sound radiator, interacting with the source and other reflections. Remember that low frequencies have long wavelengths and high frequencies have short wavelengths, so what determines "high" or "low" frequency, "long" or "short" wavelength, is the distance to reflectors or other sound sources. You can visualize wave movement using an FEA program or using ripples in water to get an idea what it looks like in 2D space. Then just understand that the same thing you see as a circle on a 2D surface translates to a sphere in 3D space. A checkerboard pattern of nulls in 2D translates to a 3D array of nodes, like a Rubik's cube.
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Re: The Movement of Waves and Particles [message #19142 is a reply to message #19137] |
Thu, 01 March 2007 22:40 |
Cuppa Joe
Messages: 103 Registered: May 2009
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Viscount |
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Thanks, Wayne, for the patient detail of your answers! I already had a grasp of most of your response, but the earlier "bend vs. reflector" controversy left me with the gut feeling that I might be missing a piece of the puzzle. The physics about reflector size and distance from the source in relation to wavelength were new to me. Based upon your input, I'm considering a combination of both: flat reflectors close to the driver for the first 90 degrees of the W-bin, and rounded bends to make the other 90 degrees toward the mouth. If anyone's curious, I could provide some general descriptions of the 3 projects I have in mind. Two are currently on paper, shy of finishing minutia. When I feel like they're all presentable, then they will be posted for the general use of the forum members. That is, IF they're worth modifying or building! Or, maybe just a hearty chuckle....
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Re: The Movement of Waves and Particles [message #19143 is a reply to message #19142] |
Sat, 03 March 2007 23:15 |
Cuppa Joe
Messages: 103 Registered: May 2009
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Viscount |
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Another thought occurs (see how long it took): If I wanted to pass a 1kHz frequency (just for ease of calculation) through a W-horn, how narrow would the initial passage need to be before the 2nd 90-degree turn? I don't want the answer, just the method to get it, please & thank-you! I'm sure it's related to previous postings about diameter vs. wavelength, or fractions thereof. Perhaps it'll gel if I visit John Sheerin's website again.... Is anyone familiar with the old Acoustic 301 W-bin for bass guitar? (I had 2 of those coffins in my Glory Days, as well as a full Ampeg SVT system! Bwa-ha-ha!) If you pulled off the rear access panel, you found a wedge-type reflector built onto the panel itself. The Cerwin-Vega! 18" driver was flush-mounted to that inner panel 180 degrees opposite the mouth. I had thought to keep things as simple as that, with modifications and a 15" cone instead. The reflector would again be mounted onto the rear access panel, but it would be as large and deep as possible without interfering with the cone's excursion. Rounded corners are still under consideration for the other turn toward the mouth (with a twist). If the first section of the passage and the reflector were engineered just right, could they constitute a type of throat? I know, it's a wacky idea....
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