• A heretical tip from a heretic.
  
• Bill Fitzmaurice Folding Geometry

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.

• Wave–particle duality
  
• Particle velocity
  
• Wave velocity

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?

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.

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....

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....

DM

• Illustrations of reflections in ducts

• Monolith horn article, page 1
  
• Monolith horn article, page 2
  
• Monolith horn article, page 3
  
• Monolith horn article, page 4
  
• Monolith horn article, page 5
  
• Monolith horn article, page 6
  
• Monolith horn article, page 7