Winter Build Off 2020 - Sub-woofer On A Stick

[Fat Freddy]

I'm back from a 2 week hiatus.

Before I left, I prepared a test front with a dog bowl resonator.

Dog Bowl Resonator Test

• 2groggy
• Feb 2, 2020
• 2020 winter build

I have a few other panels so I can make different test fronts with different resonator configurations. I expect to make a single cover that will work for all of them.

I wasn't happy with the neck so I started again. Here's a fresh POW (pile of wood) shot.

2nd Try At Neck POW

• 2groggy
• Feb 2, 2020
• 2020 winter build

I;m reclaiming another window sill. This one turns out to be poplar too, but with more discolouration. Maybe the zebra wood fingerboard will make the streaks stand out less.

Tomorrow, I'll pick up another truss rod and print out my fret spacing so I can catch up with some of the other builds.

2nd Try At Neck Rough Shape

• 2groggy
• Feb 2, 2020
• 2020 winter build

 

[Bruce Johnson]

I like the dog bowl resonator! That could work well as your diaphragm. It's relatively light weight and rigid, like a speaker cone. It could push some serious air.

But, like we talked about above, the diaphragm has to have some vertical travel, in order to do its job. You can't just set it on the body with the outer flange right on the wood. Like it is, all you'll get is a really high pitched clanging sound, like you were banging on the dog dish with a spoon. Not a bass at all.

What you need is a "springy ring" around the flange, between the flange and the body, so the whole dog dish can move up and down. One idea would be to use some heavy black foam weatherstripping, with the adhesive back. Stick a ring of that around the underside of the flange of the dog dish. Maybe rout a little rabbit around the hole in the body, for the foam to seat in to? But make sure that the dog dish is floating on the foam, and can't rattle against the wood. Or, instead of the foam weatherstripping, maybe a ring of rubber tubing? Or a large diameter soft silicone rubber O-ring? Again, the idea is to allow the whole dog dish to be able to bounce up and down, to pump out that low frequency sound.

 

[Will_White]

I like the dog bowl resonator! That could work well as your diaphragm. It's relatively light weight and rigid, like a speaker cone. It could push some serious air.

But, like we talked about above, the diaphragm has to have some vertical travel, in order to do its job. You can't just set it on the body with the outer flange right on the wood. Like it is, all you'll get is a really high pitched clanging sound, like you were banging on the dog dish with a spoon. Not a bass at all.

What you need is a "springy ring" around the flange, between the flange and the body, so the whole dog dish can move up and down. One idea would be to use some heavy black foam weatherstripping, with the adhesive back. Stick a ring of that around the underside of the flange of the dog dish. Maybe rout a little rabbit around the hole in the body, for the foam to seat in to? But make sure that the dog dish is floating on the foam, and can't rattle against the wood. Or, instead of the foam weatherstripping, maybe a ring of rubber tubing? Or a large diameter soft silicone rubber O-ring? Again, the idea is to allow the whole dog dish to be able to bounce up and down, to pump out that low frequency sound.

It won't rattle, the string pressure holds it down, but it also doesn't have much excursion. The foam surround might give it more travel.

 

[Jon Clegg]

The dog-bowl woofer.... I'll see myself out now.

 

[Fat Freddy]

The dog-bowl woofer.... I'll see myself out now.

 

[Bruce Johnson]

It won't rattle, the string pressure holds it down, but it also doesn't have much excursion. The foam surround might give it more travel.

Yeah, and you also have to make sure it stays centered and doesn't get into an annoying buzz against the side of the hole. You've got to have the travel, just like the speaker in your bass cabinet.

 

[Bruce Johnson]

The dog-bowl woofer.... I'll see myself out now.

And obviously, the bottom foot of the bridge needs to be carved in the shape of a bone. If you get it working right, just name this bass The Woofer.

 

[Jon Clegg]

Isn't that also what the spider in a resonator does?

 

[Jon Clegg]

And obviously, the bottom foot of the bridge needs to be carved in the shape of a bone. If you get it working right, just name this bass The Woofer.

Obviously a bone nut would be in order.

 

[Will_White]

Yeah, and you also have to make sure it stays centered and doesn't get into an annoying buzz against the side of the hole. You've got to have the travel, just like the speaker in your bass cabinet.

The string pressure keeps it centered too. I haven't tried putting foam under the edge of a resonator because I'm usually after the bright loud resonator tone, so I'm not sure how that would affect it but it's worth a shot to decouple the cone from the box and maybe give some more excursion and more bass.

 

[Bruce Johnson]

The string pressure keeps it centered too. I haven't tried putting foam under the edge of a resonator because I'm usually after the bright loud resonator tone, so I'm not sure how that would affect it but it's worth a shot to decouple the cone from the box and maybe give some more excursion and more bass.

You should still get most of the high bright resonator tone, because the dog dish itself will still be able to flex. The foam surround will make it a two-stage spring suspension. You may need to play with the stiffness of the foam surround to find a balance of highs and sub-lows. I had to mess around quite a bit with the spring system in Banjozilla. It was the same kind of thing; the high twang from the fine movement of the head, combining with the low thump of the long travel of the entire head.

 

[S-Bigbottom]

Spring suspension like on microphones?
Or will this make some crazy jingle jangle thing?

I have absolutely no experience with this. Just having fun reading about it.

 

[Fat Freddy]

I've got a vague memory of someone building something similar with a bicycle innertube adapted to act as the suspension.....

That was for some crazy reggae speaker thingy if I recall correctly though.....

 

[Matt Liebenau]

I’m wondering if a large o-ring or some kind of rubber would work better. It seems like the string pressure will eventually flatten out weatherstripping foam. Possibly more sooner then later.

 

[Bruce Johnson]

Spring suspension like on microphones?
Or will this make some crazy jingle jangle thing?


View attachment 3693810

I have absolutely no experience with this. Just having fun reading about it.

That's the general mechanical idea: To support the cone/diaphragm/dog dish and allow it to move in and out in a straight line with some force determined by the strength of the springs.

However, we also want an air seal around the dog dish. We don't want air leaking around between the front and back sides of the dog dish. That would greatly reduce the efficiency of pumping air. That's why the suggestion of a circle of weatherstripping or a ring of rubber tubing. Those would serve as both a spring and a seal.

Look closely at a speaker. There's the cone, which is a rigid cone of cardboard or plastic (or aluminum ala Hartke). Around the outside of the cone is the surround, which is a ring of molded closed-cell foam rubber with waves in it. The surround is flexible, and it supports the cone and allows the cone to move in and out through some distance of travel. It functions as both the ring spring and the air seal, preventing air from leaking around between the front and back sides of the cone. The spring load of the surround is very light, because the force pushing on the coil is only the electrical load in the speaker coil pushing against the magnetic field from the speaker's magnet.

On this project, the dog dish is the cone, and the ring spring needs to be much stronger, because it's balancing against the download of the strings over the bridge. How strong the ring spring needs to be will depend on the geometry and break angle of the strings over the bridge. It has to be set up as a balance, so the dog dish is floating in the middle of its travel, when the strings are on and up to tune. Then, plucking a string will cause the dish to bounce up and down, pumping that air.

 

[Reedt2000]

What about a rubber gasket around the edge of the bowl, and an internal spring? If a long bolt was attached to the cone sticking in the body, it could pass through an internal brace. The brace could have a spring around the hole the bolt goes through, the bolt could have a washer that presses on the spring. The washer could be used to adjust how hard (or not) the bowl presses on the spring. The gasket would keep the bowl from popping out of the top.

I dont know if I explained that well, so I sketched it:

 

[2groggy]

Wow, I'm loving all of these ideas, serious or not.

The dog bowl came with a rubber ring, presumably to stop is from sliding on a hard floor. It is too thin to allow much extra movement. but it could probably be wrapped around some form of spring to seal the gap.

I'll have to mull over something like a bellville spring (cone shaped washer) or wavy washer while I build the neck.

Now I need to get the dog bone shaped bridge out of my head.

 

[Bruce Johnson]

What about a rubber gasket around the edge of the bowl, and an internal spring? If a long bolt was attached to the cone sticking in the body, it could pass through an internal brace. The brace could have a spring around the hole the bolt goes through, the bolt could have a washer that presses on the spring. The washer could be used to adjust how hard (or not) the bowl presses on the spring. The gasket would keep the bowl from popping out of the top.

I dont know if I explained that well, so I sketched it:

View attachment 3693878

Yes, that's definitely an improvement. An adjustable metal spring would help to dial in the load balance between the strings and the spring. That's what I ended up with on Banjozilla. The springs under the head have a big knob on a threaded rod. I cranked the knob, compressing the springs upward until I found the right balance.

But, the tricky part here is the gasket. It has to be air tight, have enough travel, and be good for a billion cycles. You don't want to blow a gasket in the middle of a gig!

 

[Bruce Johnson]

In case you are wondering why I seem to know a lot about this subject, I have an ongoing project here in my shop called the Acoustic Scroll Bass. It's an all acoustic version of my Scroll Basses. No pickups or electronics, all mechanical acoustic. A completely different construction and operation from a standard acoustic guitar. It has moving parts, precision bearings, and custom seals inside. I've been working on it for a long time, and I hope to have the first prototype done soon. I intend to make it into a production instrument that I can build and sell for many years. I'm not going to reveal yet exactly how it works, but I've written some about the project over on the Love Of Ampegs thread. Search Acoustic Scroll Bass and my name, and you'll find it.

This is probably the best post about it:
love of Ampeg and other Scroll Basses, Part 2

Anyway, my project is not like what you are doing here. A whole different internal arrangement. But, I've been dealing with many of the same technical issues: pumping air, spring loads, flexible seals, sound phasing, etc.

This is a fun project you have going here. Off in a different direction from mine, but with the same goal. This is the fun part of being a Luthier!

 

[2groggy]

What about a rubber gasket around the edge of the bowl, and an internal spring? If a long bolt was attached to the cone sticking in the body, it could pass through an internal brace. The brace could have a spring around the hole the bolt goes through, the bolt could have a washer that presses on the spring. The washer could be used to adjust how hard (or not) the bowl presses on the spring. The gasket would keep the bowl from popping out of the top.

I dont know if I explained that well, so I sketched it:

View attachment 3693878

Hmm, maybe three or more springs around the perimeter? The cone could be suspended below the face panel by butterfly bolts and springs. The spring tension could be adjusted simply by turning the screws.

Everything the I need is available off the shelf at any hardware store.