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Dec 30, 2023 - 3:06:10 AM
Players Union Member

Helix

USA

17474 posts since 8/30/2006

Fresnel ledges inside the rim give more than one sized chamber so expansion can take place during music.

As far as I know, no maker nor manufacturer is interested in putting Fresnel ledges on the sidewall of a standard resonator.

I prefer angled or slanted sides, but I still haven't seen any amphitheater features like Fresnels.

John McEuen visited Earl Scruggs and found him on the front porch using a hatchet around the bottom edge of his Granada Rim.
That tells me he was looking for something
Shape of the bottom of the rim can vary from right angle to rounded or beveled.

Straight sides are easy to manufacture, banks shots inside the rim help with the 180 change in direction.
Which do you think are better?

Jan 1, 2024 - 5:12:19 AM
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Players Union Member

Helix

USA

17474 posts since 8/30/2006

At any rate, I'll go ahead next spring and produce one.

Jan 1, 2024 - 8:34:22 AM

678 posts since 2/21/2005

What’s a Fresnel ledge?

Jan 1, 2024 - 9:52:19 AM
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447 posts since 6/26/2011

I have heard of Fresnel lenses before but never in the context of sound reproduction. A fresnel lens has ridges cast or ground into the lens surface to create a magnifier. You get them on lighthouses or marker buoys at sea.
Very interesting idea.
Cheers from Bill.

Jan 1, 2024 - 10:41:31 AM

1633 posts since 11/10/2022

banjohangout.org/archive/311292

In 2015 Helix brought this up. There were good points made back then. Eight years later, n o Fresnel?!!!

I think the main reduction from 2015 is that a Fresnel would focus the sound better in a particular direction. Whether that is a good thing is individualistic.

Jan 2, 2024 - 12:12:24 PM

2394 posts since 2/9/2007

Seems to my admittedly less-than-well-informed brain that the "more than one sized chamber" is mostly about minimizing standing wave resonances by reducing the area of parallel surfaces.

Has anyone ever tried making the inside surfaces of the resonator (or rim) into something like a skyline diffuser?

Jan 2, 2024 - 2:59:22 PM
Players Union Member

Helix

USA

17474 posts since 8/30/2006

I didn't know anyone was listening in 2015.

The original amphitheaters in Greece still operate

A skyline diffuser is radical

Jan 2, 2024 - 3:03:29 PM

martyjoe

Ireland

505 posts since 3/24/2020

quote:
Originally posted by Dan Gellert

Seems to my admittedly less-than-well-informed brain that the "more than one sized chamber" is mostly about minimizing standing wave resonances by reducing the area of parallel surfaces.

Has anyone ever tried making the inside surfaces of the resonator (or rim) into something like a skyline diffuser?


I've been reinforcing the pie piece backs with hemp fabric incast in multiple layers of epoxy resin. I do this on both sides of the banjo back. On the inside I rough sand the surface with 80 grit paper, in fact I 80 grit the entire inside of the pot after finishing. For the next batch of pie backs I intend to just apply one coat of epoxy to the inside and lay the hemp cloth onto it and press it in to soak up the resin, then leave it as is. This will leave much rougher effect to diffuse sound waves but it shouldn't absorb any of sound. It may not be suitable for bluegrass sounding banjos or it may be great for those too, we won't know until we try it. The whole idea is to avoid the need for the banjo bolster device. 




 

Edited by - martyjoe on 01/02/2024 15:07:09

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