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Backing up Soundports
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=20334		 Page 1 of 2
Author:  SteveSmith [ Mon Jan 05, 2009 11:47 am ]
Post subject:  Backing up Soundports
I'm putting my first soundport into the upper bout of a walnut OM. What are you using as a backup to strengthen the area, if anything? I was thinking of using a crossgrain patch thinned down enough to flex into position.
Author:  Michael Dale Payne [ Mon Jan 05, 2009 12:02 pm ]
Post subject:  Re: Backing up Soundports
SteveSmith wrote:
I'm putting my first soundport into the upper bout of a walnut OM. What are you using as a backup to strengthen the area, if anything? I was thinking of using a crossgrain patch thinned down enough to flex into position.


I have used cross grain patch of like wood, cross grain patch of contrasting wood but mostly I apply Bias tape saturated with slightly thinned titebond twice as long as the port and full width between the linings. If you do this when you cut out the opening the fibers of the bias tape will be frayed. I take a Bick lighter and singe them back and never have an issue with them fraying again.
Author:  SteveSmith [ Mon Jan 05, 2009 12:07 pm ]
Post subject:  Re: Backing up Soundports
Thnaks Michael, I've got bias tape reinforcements about every 5" around the sides so this would be relatively easy to do. Based on something I read at MIMF sometime ago I used HHG to attach the bias tape and then put a coat of shellac on them for protection.

When you used a cross-grain patch how did you shape it to the side? I can think of several ways but thought I'd ask.
Author:  JJ Donohue [ Mon Jan 05, 2009 12:13 pm ]
Post subject:  Re: Backing up Soundports
Steve...I've used 3 layers of .020" fiber in alternating colore B/W/B. It not only reinforces the port but it also adds a decorative feature as well.

Attachment:
Sound Port Detail 1.jpg


Attachment:
Soundport detail 2.jpg
Author:  SteveSmith [ Mon Jan 05, 2009 12:22 pm ]
Post subject:  Re: Backing up Soundports
JJ - that looks real good!
Author:  jordan aceto [ Mon Jan 05, 2009 1:35 pm ]
Post subject:  Re: Backing up Soundports
I usually use a piece of contrasting wood or fiber with the grain oriented opposite the side, then a piece of the side wood oriented like the side, pretty much like JJ Donohue does it. If the contrasting bit is shorter than the final lamination, you can feather everything together for a nice clean appearance on the inside. I also run the laminations the full height of the sides, and then run the linings over them, so as not to have a crack prone area where the laminations meet the linings. Everything gets bent on the side bender, so it fits nice. The contrasting elements look good when the port is on a curve, like the middle of the upper bout, and you make the hole with a cylinder, see JJ's picture for this effect. If the port is on a flatter area, like up by the neck or south of the waist, i think it looks better as a hole going straight through, without the contrasting lines.
Author:  Alan Carruth [ Mon Jan 05, 2009 3:02 pm ]
Post subject:  Re: Backing up Soundports
I used bias tapes on the twenty ports on the 'corker' experiment. No cracks so far.

Lately I've been using a thick circular backup piece of soft wood, often willow, that is sanded to shape and then tapered after the port is drilled. I've also been putting in a decorative surround.

To make that I start by drilling a hole the size of the port I want along the grain in a piece of the same wood I'm making the binding from, that's about 2" long. This gets mounted on the lathe using a pair of centering mandrils, and the outside is turned to a slight taper, leaving a wall thickness of no less than about 1/16". I then wrap this in a layer or two of contrasting veneer, to match the purfling.

The hole in the side is drilled after I'ver dressed the rim to height, and is made a little bigger than the small end of the turning, for obvoius reasons. I then wrap a piece of sandpaper around the turning, and use that to taper the hole, carefully, so as not to enlarge it to much or round off the corners. The taper can then be glued in, trimmed off, and smoothed up.

This is a bit of extra work, but it looks finished. Somehow when you just drill a hole it loks like you just drilled a hole..... ;)
Author:  SteveSmith [ Mon Jan 05, 2009 3:09 pm ]
Post subject:  Re: Backing up Soundports
Alan, that's a great technique. Thanks for sharing. Your 'tapered circular binding' would then hide the soft wood or whatever else is used for reinforcement - cool!
Author:  douglas ingram [ Mon Jan 05, 2009 7:22 pm ]
Post subject:  Re: Backing up Soundports
I lay in two layers of 6oz fiberglass set in epoxy. It takes just a few minutes and is really strong for the cross grain, small section areas that I have to deal with.
Author:  cphanna [ Mon Jan 05, 2009 7:26 pm ]
Post subject:  Re: Backing up Soundports
Steve, I'm glad you posted this question. I've been curious about this for a long time. The replies have been very interesting. Thanks.
Patrick
Author:  Pete Brown [ Mon Jan 05, 2009 10:27 pm ]
Post subject:  Re: Backing up Soundports
Like JJ Donohue, I use three layers of contrasting veneer, with the grain running perpendicular to the sides. The glued up veneers fit between the linings and, because the sides taper towards the head block, can be shaped so as to wedge tightly between them, giving a near perfect fit.

Because the glued up veneers are quite flexible, I use a scrap of walnut bent to a slightly smaller radius than the curvature of the sides as a caul when gluing the veneers to the side - experiment with the curvature of the caul to get the clamping pressure you want. I've glued strips of spruce to the ends of the walnut caul to stiffen it at either end. I find a couple of spring clamps over the ends of the caul provide adequate clamping pressure to the veneer laminates as the glue sets.
Attachment:
SoundportClamp.jpg


The photo shows the side braces in place which of course were added after the veneers were attached.

Cheers
Pete
Author:  Jeffrey L. Suits [ Tue Jan 06, 2009 8:55 am ]
Post subject:  Re: Backing up Soundports
SteveSmith wrote:
I'm putting my first soundport into the upper bout of a walnut OM. What are you using as a backup to strengthen the area, if anything? I was thinking of using a crossgrain patch thinned down enough to flex into position.


Thin hobby plywood.
Author:  Dave Higham [ Mon Jan 12, 2009 3:08 pm ]
Post subject:  Re: Backing up Soundports
I, like Steve, was just about to do my first soundport so this thread was well timed and I decided to follow Alan's method. Of course, if I'd thought about it a bit more in advance, I'd have done it before installing the linings and glueing the back on [headinwall] .(As the French say, "Why make it easy when you can make it difficult?") Still, I managed it and I'm quite pleased with the results.
Author:  SteveSmith [ Tue Jan 13, 2009 8:22 am ]
Post subject:  Re: Backing up Soundports
I sure appreciate all the comments in this thread. I've backed up my soundhole area much like JJ's but I used maple veneer laminated with some ebony thinned enough to be flexible - about .060". I then installed some spruce reinforcements. I'm not using a circular or elliptical hole, but am trying for something somewhat in the shape of a cross. I'm testing several patterns now and should have this done this week sometime.

Dave - that looks real good but I can't help noticing you've used double linings and was wondering what the reason was for that?
Author:  SteveSmith [ Sun Jan 18, 2009 3:16 pm ]
Post subject:  Re: Backing up Soundports
Here's where I ended up along with some shots of how I did it. I wanted something with some detail in it so decided to bevel it like JJ's so the laminate lines would show.
Attachment:
BeveledSoundport.JPG

The backing with the spruce reinforcement strips. Unfortunately I cracked the lining when I glued in the backing because my cauls were not shaped quite right.
Attachment:
SoundportInside.JPG

I don't know how everyone else does it but I wanted a cross-shaped soundhole so I drew this on the computer, printed it out and used thinned Titebond to glue it in place. Then I routed it with a 1/8" bit in my laminate trimmer.
Attachment:
SoundportPattern.JPG

If you don't have a roll of sticky sandpaper you need to get one although I used to do the same thing by gluing sandpaper to a stick or using double-stick tape. Using the sticky sandpaper is just ridiculously easy. I use it to make all sorts of little sanding blocks for shaping things like this double-ended one I did for the soundport.
Attachment:
SandingStick.JPG
Author:  JusSumguy [ Sun Jan 18, 2009 4:37 pm ]
Post subject:  Re: Backing up Soundports
Image

Very inspiring. I really like this...

Are they functional? If so, what do they add? Do you leave them open, or back them with black cloth, or.....?


Image - Phil
Author:  Hesh [ Sun Jan 18, 2009 5:38 pm ]
Post subject:  Re: Backing up Soundports
Phil my friend sound ports are left open, typically, and act as a monitor for the player. They do indeed work very well.

Steve buddy I really like what you have done too - good going! [:Y:] [clap] [clap] [clap] [clap]
Author:  Dave Higham [ Sun Jan 18, 2009 6:09 pm ]
Post subject:  Re: Backing up Soundports
Steve, my soundport is on an ABG with quite deep sides. I thought it might be good to make the sides as rigid as possible and the double reverse kerfed linings do just that. Then I went for overkill and put 2 layers of veneer between them and inside them. They sure are rigid!

Your 'cross' is very nicely done. Putting a bevel with a small radius on eliptical holes in a curved surface is not as easy as it might look.
Author:  SteveSmith [ Sun Jan 18, 2009 8:32 pm ]
Post subject:  Re: Backing up Soundports
Thanks everyone for all the helpful comments, I'm really happy with the way it turned out too and already have a different idea for my next one.

Phil, I'm glad Hesh answered your question about how the soundports work as this is my first and I've never played a guitar with one ......yet ;)

Dave, I understand now, that would surely make the sides very rigid! And you're right, the bevel was a challenge; I free-handed it with the little sanding stick.
Author:  JusSumguy [ Mon Jan 19, 2009 12:13 am ]
Post subject:  Re: Backing up Soundports
Hesh wrote:
Phil my friend sound ports are left open, typically, and act as a monitor for the player. They do indeed work very well.]


Wow, if that's the case. Anybody ever think of putting some acoustic dynamics to work in there? I mean, besides the obvious... :)

I'm an audio engineer by trade, and I build studios a lot. I also build custom speaker installations. I know a bit about acoustics and resonant frequencies, and how to compliment/nullify them. If one could figure the resonant frequency of a guitar, (affix a small speaker from a tone generator to it, and meander through the frequencies until the guitar starts to vibrate. Very easy to spot.) along with the internal volume of the guitar (simple math). A port length (tube) could be installed to the port which would tune the box away from resonant conflicts. This will expand, smooth and lower, the frequency response coming from the guitar. The low end could be expanded by as much as 20 db and 80 Hz. No chit.

Anyway............ jus sayin..... [uncle]


Image - Phil
Author:  Dennis Wojcik [ Thu Feb 05, 2009 9:49 pm ]
Post subject:  Re: Backing up Soundports
Hey Steve, I really like your design for your soundport! Beautiful job on on sanding that radius on the inner edges. Must have been a painstaking process. So, did you route the oval curves out on the sides after you bent them or before? How did you do this by hand without making a mistake? Did you use a guide of some sort? Anyway, nice job! Dennis
Author:  SteveSmith [ Fri Feb 06, 2009 10:26 am ]
Post subject:  Re: Backing up Soundports
Hi Dennis - thanks for the kind comments. I routed this out after the sides were bent and the linings attached. The reverse kerf linings make the sides pretty stiff so I just clamped it into my wood-faced bench vise and went at it. If you look at the photos you'll see the pattern I glued to the sides. I just used my laminate trimmer with a 1/8" bit and did it freehand. I routed within about 1/16" of the line then cleaned it up with the sanding stick shown in the other photo with 80 grit sticky paper on it. Once I had the shape right I used a scraper to remove the paper pattern then I did the bevel, still with the 80 grit. Finally I put on some 220 grit and smoothed it all out.

The routing was a bit nerve wracking but not that bad. One could always just open a hole big enough for the sanding stick and then sand to the line. The sanding goes pretty fast since the material is so thin.
Author:  JasonM [ Fri Feb 06, 2009 2:33 pm ]
Post subject:  Re: Backing up Soundports
JusSumguy wrote:
Hesh wrote:
Phil my friend sound ports are left open, typically, and act as a monitor for the player. They do indeed work very well.]


Wow, if that's the case. Anybody ever think of putting some acoustic dynamics to work in there? I mean, besides the obvious... :)

I'm an audio engineer by trade, and I build studios a lot. I also build custom speaker installations. I know a bit about acoustics and resonant frequencies, and how to compliment/nullify them. If one could figure the resonant frequency of a guitar, (affix a small speaker from a tone generator to it, and meander through the frequencies until the guitar starts to vibrate. Very easy to spot.) along with the internal volume of the guitar (simple math). A port length (tube) could be installed to the port which would tune the box away from resonant conflicts. This will expand, smooth and lower, the frequency response coming from the guitar. The low end could be expanded by as much as 20 db and 80 Hz. No chit.

Anyway............ jus sayin..... [uncle]


Image - Phil



Could you explain more about how this works and what it would look like? Not sure a speaker application would work the same on a guitar, but I'd like to understand how this works for a speaker. I know the resonant frequencies of my guitars. The internal volume would be a little more than simple math, since you have linings, bracing, neck block and heel block to subtract from the internal volume. I'm assuming this factors into how long to make the tube? If so, the tube could be a two piece adjustable telescopic design that could be tuned by lengthening to a sweet spot? Once the correct length was determined, a solid tube could be put in place to prevent the buzz that would no doubt come from a two piece tube. All that hinging on the assumption that it would do something desirable for a guitar. Would be fun to play with it.
Author:  Alan Carruth [ Fri Feb 06, 2009 2:42 pm ]
Post subject:  Re: Backing up Soundports
Phil asked:
"Anybody ever think of putting some acoustic dynamics to work in there? I mean, besides the obvious..."

I spent a fair amount of time a few years ago measuring the response of a guitar with multiple sound ports in different locations that could be closed, and have returned to the project several times since to clear up some points. I summarised some of my findings in an article in 'American Lutherie' magazine a while back. Let's say that it's not at all a simple setup.

"If one could figure the resonant frequency of a guitar, (affix a small speaker from a tone generator to it, and meander through the frequencies until the guitar starts to vibrate. Very easy to spot.) along with the internal volume of the guitar (simple math). A port length (tube) could be installed to the port which would tune the box away from resonant conflicts. "

Luthiers do tune the 'main air' and 'main top' pitches to certain relationships, or, rather, much of that is taken care of for us in the standard designs.

One of the big issues is that with speaker design you're looking for a 'flat' response, or as close as possible, and you can afford to sacrifice some efficiency to get it. As far as I know, any flat-response speaker setup has lots of damping; in a bass reflex system the acoustic damping of the cone itself is high, and they add a lot of damping in the cabinet to flatten out the response of the Helmholtz mode. With the acoustic guitar we're stuck with a very low-power driver, and need to get as much sound as we possibly can from it, so added damping is out. We're not all that interested in 'flat' response, either: it wouldn't sound like a guitar.

From what I understand, t seems as though you get the best response from a guitar when it has lots of resonances with fairly low damping. You do have to be careful to get the proper relationship between the various strong low-end resonances, such as the 'main top', 'main back', 'main air' (Helmholtz-type), and the 'first body mode (AKA the 'neck mode), to avoid problems of the 'wolf' type. Once you get up into the mid-range, say, above about 500-600 Hz, there is so much going on that you're in a 'resonance continuum', and the problem looks more like room acoustics than a speaker cab. An odd-shaped room, too, and one with walls that flex! It keeps it interesting.
Author:  JusSumguy [ Tue Feb 17, 2009 4:15 pm ]
Post subject:  Re: Backing up Soundports
JasonM wrote:
Could you explain more about how this works and what it would look like? Not sure a speaker application would work the same on a guitar, but I'd like to understand how this works for a speaker. I know the resonant frequencies of my guitars. The internal volume would be a little more than simple math, since you have linings, bracing, neck block and heel block to subtract from the internal volume. I'm assuming this factors into how long to make the tube? If so, the tube could be a two piece adjustable telescopic design that could be tuned by lengthening to a sweet spot?


Yes, as simple as that.

Quote:
Once the correct length was determined, a solid tube could be put in place to prevent the buzz that would no doubt come from a two piece tube. All that hinging on the assumption that it would do something desirable for a guitar. Would be fun to play with it.


The tube is a very important part of the equation. Specifically, the tubes length and diameter.

The sound (air/water) enclosed in the box is no different than a bass reflex speaker. Specifically, what we're talking about here is the air. And how it flows within the box. Air, in an enclosed environment, acts exactly like water. Just knowing that, allows you to visualize what's going on inside the box.

The ribs do more than reinforce, and tune the soundboard. They keep the air from encountering parallel surfaces. When this happens, the sound (air) just bounces back and forth. Thus reinforcing the frequency it represents. Re; Make that frequency louder. By encountering the braces on it's propagational path, it causes the air to break up (change directions). This is good, and bad. It's good because we don't have comb filtering and reinforced frequencies. It's bad, because the eddies and flow changes slow the air down. Delivering less volume and frequency response. Tuning it is like hiring a traffic cop for air flow. It allows the air to set itself up in a uniform flow pattern. This alone will increase the loudness of the box.

By tuning the box to it's own resonant frequency, it frees up the volume inside the box to reproduce (do it's job) with much less resistance. Re: eddies and flow problems. This allows the air (water) to move in a complimentary flow pattern. Thus freeing up the soundboard to extend it's frequency response. Dramatically.

And yes... every bit of this is easily found with trial and error if the correct tools aren't available.

Hope this helps. I'll be glad to answer any more question.


Image - Phil
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