Official Luthiers Forum!

Over the last few weeks I have had some VERY interesting phone conversations with different luthiers, some of them participants of the OLF. Somehow, the following discussion always seems to arise, so I thought I would post my thoughts and then see what kinds of responses it recieves.

I am going to be real up front with my first statement: I dont buy into the tap tuning theory. There, I said it and I mean it!

We are not making bells, we are making speakers. Here is the basis for my argument boiled down to some real simple terms: You can design the best bell in the world (start reasearch here), but if you sit that bell on the ground and eliminate it's outer edge from vibrating, you have killed the bell. You can create a guitar top that rings like a bell (in effect it *is* a wooden bell) and then glue it's outer edge to a stiff guitar rim, you have just killed all of the qualities of the bell.

Dont get bent out of shape yet ! read on....

I will agree that if you take the time to tap tune your guitar by thumping it , you have most likely reduced the mass at the outer edge of the top, including the thickness of the top and also the brace thickness (and strength) at the outer edges. The mass at the center of the top has more or less remained constant, agreeing that minor variations are accepted.  I think that we should be studying more of the reponse of the acoustical speaker technology (begin reasearch here) and adjusting our efforts on how to project sound as though we were building a speaker rather than a bell.

At some point in the making of a speaker, you have a center core that is fairly rigid at the point where vibration is induced. In the case of the speaker, this is achieved with electromechanical means.... in the case of the stringed instrument, this is achieved  with the vibration of the strings. Same principal, different source of vibration.

In the cone of the speaker, you can adjust the rigidness of the cone in concentric circles and greatly affect the frequency response of the final product.  Some manufacturers achieve this by placing rigid concentric circles at different distances from the main center of the cone, some make the cone material thicker at the center and progressively thinner toward the outside. Almost all manufacturers of speakers have gone to a rubberized material on the outer edges of their speakers which greatly increases their ability to move air even in the smallest of speakers. The placement of the ridigness (is that a word?) shows its effects by adjusting the frequency response of the speaker.

If you apply this thinking to the guitar, the top bracing of the guitar's braces will affect the overall frequency response. By scalloping the braces, you have created rigid spots in the top's  vibrating surface (or cone of the speaker) and you have shaped the tonal qualities of the sound by trimming some frequencies while allowing others to remain. I will agree that bell manufacturer's also use this thinking to shape the harmonics of their bells by grinding certain spots on their bells.

We can adjust the frequency response of the guitar (or bell or speaker) by moving the rigid spots in the surface of the top of the guitar. This will apply to parabolic bracing as well because we can move the point of arc of the brace inward and outward toward the outer edges to "clip" certain tonal qualities and harmonics.

Am I saying that tap tuned guitars dont sound better?

No! by all means this may be achieving the same result as just thinking of the guitar as a speaker rather than a bell. Some of the same principals apply to each.

When I decide on a guitar top's thickness, I am thinking of it as the "cone" of a speaker (not a bell). I will decide on how much reflex I will allow by adjusting this thickness. After that decision has been made, I can then adjust the bracing into a pattern that will allow for maximum rigidity at the core and then further adjust the harmonics of the guitar's top by deciding on the spots of rigidity to shape it's harmonics and frequency responses.

Either way you go about thinking of how you shape your top, the end result may be similar. What I am trying to say here is that we are not making bells. We are working in a very limited range of the human ear's ability to hear and we are projecting sound over a wider range of frequencies than any single bell can achieve.

One more thing. I think of Tim McKnight's "double backed" guitars somewhat like the BOSE speaker technology. By adding the second back to his guitar, he has in effect created some echo chambers of differing sizes which is allowing very minute delays of sound to transfer to the top. I applaud Tim for his innovative thinking! He now has me thinking that one could actually use this space to create a "micro chamber" similar to the BOSE speaker system. I'll have to give that some thought...I would love to be able to play and study one of  his double backed guitars.

The end result...

In no way do I mean to discredit the years of research by masters of the trade that have adopted tap tuning as their standard.  What I am saying is I dont beat on my guitars unprotected top wood trying to get it to ring like a bell. I dont want to compress the cell structure of the top's wood even in minute amounts. As you can see, the outcome of tap tuners may be similar, but my way of thinking and building guitars is a LOT different. Thanks for reading

Ken

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Reguards,

Ken H

I think that's a pretty neat way of thinking, Ken. I lack the physics expertise in acoustics and the experience in building to make a serious comment, but I do think this will stir up a lot of interesting retorts. I'm just gonna hide and watch.

Ron

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

[QUOTE=Hodges_Guitars] ........You can design the best bell in the world, but if you sit that bell on the ground and eliminate it's outer edge from vibrating, you have killed the bell.........

Ken

[/QUOTE] OK, so don't tap tune the top alone. There are those that tap tune the top after it is mounted to the sides.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

I'm off to the pub now with my Dad, (hubby has to baysit ) but I'm gonna read this interesting thread later.

We sort of had this discussion before when talking about sound ports and sound holes, and stuff flapping in the breeze, so to speak.  I agree, and I have no basis for this, but I think it is mechanically much like a tuned speaker box and speaker.  Not exactly, but you have pressure and back pressure via the soundhole, and the sound bounces around in there and projects through the hole.  The sound port may, or may not, enhance the process depending on how it is related to the soundhole in both location and size.  The guitar box, however, is somewhat different, in that you have more than one vibrating surface that adds or subtracts from the process.  Depending on how the extra vibrating surface, the back, responds to the internal vibrations, it can either enhance or detract from certain tones and or overtones.  This will then have a direct effect on volume and evenness.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Hmmmm .... There are lots of ways to build a guitar and once we build a great one our next challenge is to figure out a method or process be able to repeat and duplicate that greatness while eliminating variation.

Tap tuning is an old world process that has been used for centuries but it is a process that is learned over time. I have been using it ever since I began building guitars but I had only a vague idea of what I was doing or what I was looking or listening for. My notebook was bulging with pages of data and random thoughts but I could never correlate what I was doing with a chisel and what I was hearing when I tapped a plate. I had some basic clues but the light at the end of the tunnel was pretty dim. It wasn't until I worked with Ervin that he open my eyes and taught me the process. Then the lights started going off and I was estatic that it all finally made sense. Is it real? You betcha it is. Can one describe the process in words easily? Nope but it is a taught skill that has been passed down through the generations. Will I teach you or anyone else how to do it? Out of respect for Ervin my suggestion is that if you want to know more, take his class. I will premise this by saying that IMO a person who has built a few dozen guitars or better yet even more, will benefit MUCH more from his class than the person who has only built a few instruments.

My "Hollow Back" design was a result of trying to build an efficient guitar for a player who played standing with the guitar strapped to his body. I had a good understanding of the important role that the back plays in the guitar [machine] but I was personally challenged to find a way to allow the back to couple with the top even when it is dmapened by the players body. It is still not a 100% perfect design but it is MUCH more efficient that my previos single braced back designs.

We can make comparrisons to bells, speakers, speaker enclosures and guitars but I have limited knowledge with the first three. I am sure Rick can talk more intelligently on this subject than many.

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tim...
http://www.mcknightguitars.com

The problem is, of course, that the guitar is a bell and a loudspeaker. We want loudspeakers to be as non-resonant as possible because we can drive them with gobs of external power. We need resonance in guitars to try to eke out as much acoustical energy as possible from the vibrating string. But too much acoustic resonance tends to make for very peaky performance, so it's a very delicate balance.

Don't go thinking it's all one thing or another.

BTW, the goal of most tap-tuning is to hear clarity, not necessarily a specific note. Yes, there is a fundamental top plate resonance, and yes, there is a Helmholz frequency of the air, and yes, they couple. But even there you can have too much of a good thing.

Al could perhaps expound on his "closing the ring mode" theory a bit. I'm not very up on it, but it would seem that it moves toward that idea of clarity in the final result.

You bring up a point that I had almost forgotten Rick.

Sometime in my years of research on tops, I came across a web page that discribed a form of tuning where the luthier used glitter sprinkled on the top to create a circular shape (sorry, but I cant remember who the luthier was). If I were to be truthful about my reasoning for tuning a top, I think this theory would probably be one of my greatest influences. He would shave braces until he could get the glitter on top of his guitars to form a circle on the lower bout. I am pretty sure that this was done after the guitar was completely built, or at least the box was built. To me, this makes a whole lot more sense than simply tapping on an unglued top to hear it ring. I dont use the glitter test, but I do use some of the principals of this method to tune my tops.

I know that I can easily surpass the tonal quality and volume of one of the largest guitar factories on a consistant basis, so I know I am on the right track. I do have a few tricks up my sleve on building guitars though. Some of what I know I learned from a master of the craft, some of it I got from reading volume after volume of information and then putting what I had read to good use to come up with my own "system".

I just hate to see so much emphasis placed on tap tuning, especially for new builders, because I feel it can lead them astray if they arent careful. If one just thinks of the top of a guitar as a speaker cone that has to be braced in order to keep the top form warping under the torque of the bridge pull, then it is much easier to get the final result that most people are seeking.

 

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Reguards,

Ken H

The "he" you are referring to is probably our own Alan Carruth, for whom I have the ultimate respect in his commitment to balancing a scientific approach to lutherie with a damned good set of ears.   Of all of us, Al has really done his homework in this area of lutherie.

Ken,

Interesting post. Personally I have never really understood the guitars as speakers thing - except for resonator guitars. To me they are as much percussive instruments and drums is a better "model" in my head. With a bhodran you set the skin tension and then use the tippler to get the top moving in different places and the hand on the back of the skin putting pressure in different places to set the sound you want. On a guitar you have the strings and the tops shape/profile and braces - part structural but important shapers of the tops response.

I think tap tuning can mean different things to different people. I prefer to use the phrase "voicing". Taping around the top as the top is shaped and braces put on and shaped helps me a lot to "shape" the final sound. I'm looking to hear clear ringing sustain everywhere, not tuning to any set notes. I see this as one "leading indicator" of how the top will perform as part of the complete guitar system. Yes it does get glued to rims with the edges stiffer than when you tap it free but the string pressure is way more that your finger tap and I don't think it moves what you are hearing in the "free state" into a totally different state. Your bell analogy is apt if you make guitar tops with an 8" spherical radius and glue it to the rims but not many people do.

Personally I don't worry to much about taping "compress(ing) the cell structure of the top's wood even in minute amounts" - the pressure of doing a French polish would compress the wood more I think. I just listen to the Andalucian flamenco players and Rodrigo y Gabriella to appreciate that the guitar is as much a percussive instrument.

On the subject of speaker systems and BOSE I always remember the throwaway remark Tony McManus made at one of his guitar workshops - "No highs and lows - it must be BOSE"

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

You are a VERY BRAVE MAN-open all packages carefully

Hmmm, soundboards as speaker cones, concentric rings? Now why does that sound familiar???

I would agree that any element that produces or delivers sound by means of vibration, air movement or such would have some similarity to a speaker cone construction. However I believe and have experienced that by tap tuning you can point out or determine certain areas of the soundboard that may be a little tight and thus lead the builder to make adjustments to the bracings shape, profile and thickness.

I think may be a little BOLD to come out and say "I don't buy into it" .

Also, taping the soundboard with the fleshy part of your finger does not present any threat the cell structure of the top's wood....


Yes, you can come up with a formula that could be a great way to determine soundboard thickness,brace profile and placement and have a really awesome sounding guitar without the tap tuning thing. BUT, tuning is just that.... since all woods structure,strength and flex are different even from the same species, tuning means you go over what you did and TUNE that for maximum optimization....

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

As one of those lucky enough to actually learn that 'glitter method' from Al Carruth, I'll throw in my semi-informed rant.

The glitter patterns are actually just a way of seeing where the top bends and doesn't at specific frequencies. So it's a sort of map of where the top is apparently stiff at certain frequencies.

By achieving the closed ring at a certain frequency, you know that at this particular frequency the top is behaving as a speaker. The ring of glitter is the middle of the 'flexible' part and so the interior of the ring is free to move relatively uncoupled to the outside of the ring.

Once the top is glued to the rims the -only- vibrational mode it can use is the closed ring (it can't bend 'across' the edges anymore). So, by using free plate tuning, you're making sure that the top already prefers to vibrate in the way which it will be forced to vibrate once joined to the sides.

There's a lot more to it than that, but I'll stop now so Al only has a small amount of corrections to make :)

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services