The Acoustic Guitar Forum

Go Back   The Acoustic Guitar Forum > General Acoustic Guitar and Amplification Discussion > Build and Repair

Reply
 
Thread Tools
  #1  
Old 12-25-2013, 05:01 PM
bond_fan bond_fan is offline
Charter Member
 
Join Date: Dec 2013
Location: California
Posts: 108
Default What Are we Tapping For?

I've read where different builders of both acoustics & electric guitars are tapping the wood for tone. I certainly tap the wood to listen for the thud one gets when there's a crack in the wood, much like a baseball player would do when he hits the knob end of the bat on home plate to tell if the bat has a crack in it; but other than tapping the wood for crack detection and the resonance of the wood are you builders tapping to achieve a certain note or frequency for each piece, back, top, sides, neck, fingerboard and if so are you trying to tune the various parts of the guitar to a certain note(s), a certain amount of bass, mid-range or treble? If so are you trying to have certain parts of the guitar have more prominent frequencies, like the neck to be bright, but the body to be have more bass?

Thanks & Merry Christmas to you all!
Reply With Quote
  #2  
Old 12-25-2013, 07:25 PM
Jim.S Jim.S is offline
Registered User
 
Join Date: Jun 2013
Location: Darwin, Australia, 12.5 degrees south of the equator
Posts: 1,220
Default

Merry Christmas Mr Bond, while I can't speak for we I can for me. I tap firstly to gauge damping, a thud is high damping and a long gong is low damping. Then I tap via a mic into a spectrum analyser but I don't think that helps your purpose much, however its main intention is to tune the tops main mode of vibration (monopole) to a specific frequency.

Jim
Reply With Quote
  #3  
Old 12-25-2013, 07:26 PM
Ned Milburn Ned Milburn is offline
Registered User
 
Join Date: Feb 2011
Location: Dartmouth, NS
Posts: 3,127
Default

Listening for the basic ring, attack, body, and sustain. Listening for pitch (not all people care about pitch, but classical guitars usually end up near G# on the top). Listening for over-tones.
__________________
----

Ned Milburn
NSDCC Master Artisan
Dartmouth, Nova Scotia
Reply With Quote
  #4  
Old 12-26-2013, 11:57 AM
bond_fan bond_fan is offline
Charter Member
 
Join Date: Dec 2013
Location: California
Posts: 108
Default

Hey Guys,

Thanks for the info.! Interesting to note one is trying to tap for a certain frequency or note like G#!

Is this a similar reason why a person would not want a pickup over a harmonic like the imaginary 24th fret area, because one doesn't get the full spectrum of harmonics and overtones on a 22 fret guitar where the neck pickup is placed under the 24th fret as opposed to a 24 fret guitar and the pickup is more under the 26th fret? IE. It's better to have it tuned to a frequency or note less likely to disrupt keys or open tunings most people play in like E.

Quote:
Ed Roman wrote: I prefer 24 frets, for a lot of reasons, the most important being, the rhythm pickup has to be installed off axis where the 24th fret would normally fall. On a 22 fret neck the pickup sits directly over the 24th fret harmonic node. Simply explained a node is a dead spot or a massive phase cancellation. The Laws Of Physics are Absolute click here.

These nodes occur on the 7th fret, 12th fret, 19th fret and 24th fret. If you place a pickup directly on a node the midrange frequencies will cancel themselves out. Make sure you avoid a guitar that the node falls on the pickup. Unless of course you want a muddy distorted rhythm sound like you would normally experience with a Les Paul or any Les Paul type guitar. Most intelligent people will agree on the fact that a Les Paul has a nice meaty lead tone but chords sound like dung.
Reply With Quote
  #5  
Old 12-26-2013, 12:52 PM
charles Tauber charles Tauber is offline
Registered User
 
Join Date: Feb 2011
Posts: 8,381
Default

Quote:
Originally Posted by bond_fan View Post
Is this a similar reason why a person would not want a pickup over a harmonic like the imaginary 24th fret area, because one doesn't get the full spectrum of harmonics and overtones on a 22 fret guitar where the neck pickup is placed under the 24th fret as opposed to a 24 fret guitar and the pickup is more under the 26th fret? IE.
I've not heard of anyone doing this for this reason, but, sure, if one likes the theory, go for it. Traditionally, electro-magnetic pickups have been put where they are physically convenient: placing them closer to the fingerboard, closer to the bridge or somewhere in-between will colour the sound of the pickup. Modern multi-mode, multi-transducer pickups obtain signals from various locations in the guitar.

The 24th fret occurs, theoretically, at exactly 1/4 of the open string, low enough on the harmonic/overtone sequence that notes of that pitch can be heard, though I'm not sure of relevance that has to pickup placement.

The 26th fret occurs at a MUCH higher harmonic/overtone, one that probably contributes little to the overall tone, and is thus likely irrelevant to pickup placement.


Quote:
It's better to have it tuned to a frequency or note less likely to disrupt keys or open tunings most people play in like E.
Have what tuned to a frequency? The pickup and its placement?
Reply With Quote
  #6  
Old 12-26-2013, 02:37 PM
Alan Carruth Alan Carruth is offline
Registered User
 
Join Date: Feb 2011
Posts: 4,180
Default

The 24th fret is at 1/4 of the open string length; a 'node' (stationary point) for the 4th, 8th, 12th, and so on modes, and an 'antinode' (maximum excursion) for the 2nd, 6th, and so on, modes. A pickup with zero aperture (reading the motion of only a point on the string)would not 'hear' notes with a node at that point, and would pick up a lot of the antinodes.

As soon as you fret the string, the whole picture changes.

Dana Bourgeois taps for the 'feel' in different spots. When I was able to track his process once at a luthier's convention, it seemed that he was tuning the pitches of different modes of vibration of the top so that they would couple strongly with other resonances, either of the air inside or the back. Strong coupling can be sensed as a 'kick' if you leave your finger resting on the top after you tap it.

Many makers like to hear a wide variety of tones as they tap in different places. Some try for tones in a scale or ones that are harmonically related. Some advocate having all the tap tones fall between played notes, to avoid 'wolf' tones.
Reply With Quote
  #7  
Old 12-26-2013, 02:42 PM
charles Tauber charles Tauber is offline
Registered User
 
Join Date: Feb 2011
Posts: 8,381
Default

Quote:
Originally Posted by Alan Carruth View Post
The 24th fret is at 1/4 of the open string length; a 'node' (stationary point) for the 4th, 8th, 12th, and so on modes, and an 'antinode' (maximum excursion) for the 2nd, 6th, and so on, modes. A pickup with zero aperture (reading the motion of only a point on the string)would not 'hear' notes with a node at that point, and would pick up a lot of the antinodes.

As soon as you fret the string, the whole picture changes.
Interesting. In theory, then, when amplified, open strings would have one timbre and most fretted notes another, having, as far as the pickup "hears", a different make-up of overtones.
Reply With Quote
  #8  
Old 12-26-2013, 04:56 PM
murrmac123 murrmac123 is offline
Registered User
 
Join Date: Dec 2008
Location: Edinburgh, bonny Scotland
Posts: 5,197
Default

The thing I have never quite understood about tapping, is how the information derived aurally, from tapping, guides the builder's decisions about how to alter/rectify/fine tune the soundboard based on the received info.

It is a well documented fact that an experienced builder will be able to tell just by tapping that a certain amount of wood will have to be removed from such and such a brace in order to achieve the optimum tone once the soundboard is glued up and the box closed, but just how they arrive at their conclusions has always been a bit of a closed book to me ...

When it comes to fretting and setting up a guitar to get maximum playability, I follow a set procedure which guarantees the outcome every time. I don't guess. I don't speculate.

When it comes to constructing a soundboard, is there a similar set of rules which guarantees the best possible outcome every time, or ...is it just a crapshoot ?
Reply With Quote
  #9  
Old 12-26-2013, 05:50 PM
charles Tauber charles Tauber is offline
Registered User
 
Join Date: Feb 2011
Posts: 8,381
Default

Quote:
Originally Posted by murrmac123 View Post
When it comes to constructing a soundboard, is there a similar set of rules which guarantees the best possible outcome every time, or ...is it just a crapshoot ?
You have asked the question that gets to the heart of the matter.

Start by understanding that string instruments, including guitars, historically were developed and improved by trial and error, er, I mean "empirical methods". The scientific theory regarding much of the acoustics and even mechanical structure of things largely came afterwards. And, most instrument makers, historically, likely weren't at the pinnacle of the scientific fields involved.

Historically, the process of learning the craft involved apprenticing with someone already considered a "master", that is someone already well-established and knowledgeable in the field. Ideally, the master taught the apprentice "all" that he knew, bringing the apprentice fairly quickly ( a few years) to the current state of the art. A talented apprentice would build upon what he was taught, with each generation of apprentices gradually discovering new things about the craft that improve the instrument. This includes methods of work and methods of getting instruments to sound a certain way.

These methods, historically, involved "feel" and "intuition". For example, Jose Romanillios once showed me a method he uses for determining appropriate stiffness of a top in a finished classical guitar. It involves squeezing the ends of the bridge with one's thumbs to determine how "springy" feeling it is and to watch how the top deforms as you do it. (He can also use this method to determine the thickness of the top, the placement of the braces and the height and width of the braces anywhere on the top, to within .1 mm.)

The downside to the "feel" methods is that they need to be experienced first-hand. That requires either someone to teach them to you - "See, when you do this, that happens, feel how that feels when you do that..." or, "Do you hear THAT?..." - or a lot of experience in which one makes certain correlations via one's own observations. This can't be taught well in books, videos or on-line forums.

In more recent years, considerable scientific measurement has been done on guitars and considerable science has been applied to guitar design and response - an amount unprecedented in the history of guitar making. These methods provide additional tools to achieve repeatable, specific desired results. These, potentially, take much of the "guess-work" out of doing so. They also are something that can, like the scientific method, bring an orderly, repeatable approach to guitar making. It begins to get closer to a formula in which some of the variables are known, measured and manipulated to produce a repeatable result. Potentially, it reduces some of/much of the "touchy-feely" part that was previously done by hands-on feel - something that took time, experience and, perhaps, a good teacher.

While the scientific approach is quickly gaining momentum, it is, I believe, still largely in its infancy. To date, to the best of my knowledge, the most complete material on the application of practical scientific methods is found in Trevor Gore's books. I'm continuing to work through the material he presents - not an easy read and probably largely inaccessible to those without some science background.

Obviously, many makers use varying amounts of both approaches - feel/intuition and science.

Perhaps something to keep in mind is that there are now exponentially more "guitar makers" than ever before. Many now learn the craft via books, videos and the internet. Relatively few learn the craft in a traditional apprenticeship at a master's bench. This leaves the novice maker in a situation in which they don't learn the "feel" of the craft but the science may be well beyond their ability to apply meaningfully. For those in that situation, the craft becomes largely a woodworking project. That said, it isn't really that difficult to use a time-tested design to produce a steel string guitar with a very pleasant tone, one equal to many of the better factory-made instruments.
Reply With Quote
  #10  
Old 12-26-2013, 08:34 PM
Jim.S Jim.S is offline
Registered User
 
Join Date: Jun 2013
Location: Darwin, Australia, 12.5 degrees south of the equator
Posts: 1,220
Default

Quote:
Originally Posted by murrmac123 View Post

When it comes to constructing a soundboard, is there a similar set of rules which guarantees the best possible outcome every time, or ...is it just a crapshoot ?
It is not a roll of the dice for me nowadays, if I am shooting for a 180Hz main top mode then I get it, spot on, no worries (thanks Trevor). You still have to make choices though like, do I go with 180Hz, 169Hz or 201Hz and what are the implications of those choices regarding the tone I want. Do I choose 180Hz with a stiff top and heavy bridge or 180Hz with a loose top and light bridge. Plenty more other choices to make as well to get the sound you aim to get.

Jim
Reply With Quote
  #11  
Old 12-27-2013, 01:48 AM
bond_fan bond_fan is offline
Charter Member
 
Join Date: Dec 2013
Location: California
Posts: 108
Default

Thanks for everyone's information! This thread was better than I thought it might be with lots of great info.!
Reply With Quote
Reply

  The Acoustic Guitar Forum > General Acoustic Guitar and Amplification Discussion > Build and Repair

Thread Tools





All times are GMT -6. The time now is 01:29 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright ©2000 - 2022, The Acoustic Guitar Forum
vB Ad Management by =RedTyger=