Official Luthiers Forum!

That's the abridged version, of course!

As mentioned in another current thread here, tap testing tops gives you the same data as deflection testing, just more quickly. So you can compute long, cross and shear modulus and use those numbers in another equation (derived by a guy who was testing wood for aeroplanes during WW2) to figure out the thickness you need to reduce it to to get a consistent performance irrespective of the material properties. So the process takes one of the great imponderables (how thick should I work this piece to?) out of the game. I use this approach because I use CF in most of my bracing, so I don't like to mess with the bracing once it is installed. There's then a suite of trimming processes that I use during the build so that I end up with the modal resonances I want. It's the modal resonances that essentially define the sound of the guitar.

I did a whole lot of correlation testing between tap sound as I hear it and log. dec. (damping or "Q') and found that I could rank wood quite successfully by ear with respect to log. dec., even with rough sawn wood. But it's very much harder to get a good handle on stiffness just by feeling the wood and as Al C. and I are both wont to say, people think they are much better at it than they actually are. I know I can't do it very well just by feel, because thickness is such a large part of the equation. So when I have the luxury of selecting wood personally (not enough top tonewood suppliers here to allow that!) I look for good quarter and low long grain run-out (which has been correlated to damping (Schleske, 1990 iirc)), listen to the tap tone and avoid those with obvious defects. Then out of that pile I select on cosmetics, confident that I can get a decent guitar out of anything from the pile. To test for the elastic properties you have to work the wood, however you test, so you have to buy it first, generally on a no returns basis. So I still get wood with a variety of material properties but all that is compensated for when I get to measure its properties. The difference between really good wood and not quite so good results in a small difference in top panel mass and so a not quite so responsive guitar. But when the factories set the bar as low as they do, it's very hard for most people to pick the differences between my guitars compared to the difference between one of mine and a factory instrument.

Building classicals is harder because the bar is so much higher due to the prevalence of luthier built instruments. So you need your best wood and a following breeze to get into the higher echelon.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

In my limited experience, good tap tone on a top is more a function of the thickness of that piece of wood than anything else.

Perhaps if all the pieces you are grading are the same thickness and size then taptone might tell a story.

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Joe Beaver
Maker of Sawdust

Jim-

I actually had extensive conversations with R&D on the topics of top and side selection as well as tap tuning. The cosmetic factors tend to be strong indicators of mechanical and acoustical properties. Wood science backs up the reasons for this. While valuable information can be gained from measuring modulus and weight ratios, it does not appear to be necessary. For fun I did check the tap tones while selecting the wood for my guitar but they were effectively indistinguishable. I like to think I have a good set of ears. During my time at Martin I took part in several playing tests. I was also accepted to some of the top conservatories in the country along with healthy scholarship offerings. Not trying to toot my own horn, just trying to qualify my credentials. The best back and side sets for rosewood were invariably the most dense and oily. Part of the theory there has to do with maximized mineral and oil content.

Martin also experimented with tap tuning during the mid to late 90's and found that the scalloping process they've used for so long (and takes all of 10 to 15 minutes in experienced hands) sounded just as good through hundreds of playing tests. I personally chalk up the minor tweaks that might make a 1% difference to the overall tone of an instrument as insignificant (although I'm by no means disrespecting luthiers who take the time to tweak every last aspect to perfection.) The reason is that fingers and playing technique contribute more to tone than subtle tweaks. Anecdotally, I found this to be very true from my time in music school. After recitals it was common for people to ask me who built my classical and what it cost... All I was working with at the time was an Alvarez Yairi that cost all of $1000. A lot of snob appeal goes on in the world of stringed instruments (more so than any other type I've encountered.) I happen to be a very firm believer that once a certain minimum level of quality is achieved, the rest is due to the performer.

Hugh: Not sure what you are saying here. Wonder if you could expand?
Tom

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A person who has never made a mistake has never made anything!!!

Tom I'm talking about me as a supplier. If I'm building I certainly would tap.

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Beautiful and unusual tone woods at a reasonable price.
http://www.rctonewoods.com/RCT_Store
The Zootman
1109 Military Rd.
Kenmore, NY 14217
(716) 874-1498

Bob: You are ever the gentleman even with brash folks like myself...! Always thankful for your input.
Tom

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A person who has never made a mistake has never made anything!!!

Tom: I was referring to the process of tap tuning, which is a systematic process of shaping braces based on acoustical response, patterns formed by light powders placed on the top, etc. It's not uncommon for high end luthiers, since it's the kind of attention to detail one would expect from such an instrument. In factories it's not widely practiced because the difference it makes compared to the additional time required does not justify the cost. Does that answer your question?

And just to be clear for those relatively new to this stuff, modal tuning, which involves tapping a closed guitar box, is very different from "tap tuning" which usually involves free components; and tapping panels to determine their elastic properties using vibration equations is different from both. They should never be confused with each other.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Hugh: Roger,OK I have it now. After reading your reply I went back and read the post I questioned. Seems quite obvious what you are saying now. The old brain not working the best. Thanks for reply.
Tom

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A person who has never made a mistake has never made anything!!!

Wow this topic blew up. Lots of great info. I will admit to tapping my plates so much that my wife thinks im nuts. I say to her listen to it ring! At some point all the tapping in the world wont tell the final sound but its a decent place to start. Thanks for the informative discussion guys. Zootman thx for the top.

If there's interest (most likely) I dig up a link to several scientific studies of empirical methods for wood selection. If I can get permission from the author, I have several additional articles that are not publicly available. They even go into simple methods by which acoustical properties can be determined and used as criteria for matching. It's great stuff and I hope I can secure necessary permissions to share them with everyone here.

Always interested........!!
Tom

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A person who has never made a mistake has never made anything!!!

The terms 'sap streak' and 'sap pocket' are bogus. A pitch streak appears as a dark yellowish line, usually within one or two growth rings. It generally runs through the wood at an oblique angle, so it will appear higher or lower on each side of a bookmatched top. Pitch streaks are cosmetic in nature, but finishing problems do occur. A coat of shellac is usually all that is required to address the issue.
A pitch pocket is a void that is filled with pitch. Here is a large one:



Unlike pitch streaks, pitch pockets represent a structural weakness, although they are always contained in one growth ring and typically are less than 1" long. They also run parallel with the wood fibers, and cutting away 1/4" to 1/2" of wood usually eliminates them.

I graded the red spruce tops that I cut, and the criteria are mainly cosmetic. That is because careful cutting will ensure good quarter and minimal runout. When building guitars, I do tap the raw wood, but I get just as much or more information from judging the density and cross gran stiffness, simply by handling the top.

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John