Quote:
Originally Posted by Herb Hunter
I note that though you say, "a lot of what Andy writes does not make sense,” you only list one item. Given that statement, it seems to me that you are obliged to list more than but one nonsensical example.
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OK Herb, I'm picking up the glove.
Here we go.
Andy wrote:
As an example, the E5 note on the high string at the 12th fret has a frequency value of about 659 Hz. An octave below is the open high E string at 329 Hz. If a guitar has a natural resonance hot spot around 320Hz, it will vibrate whenever either of these E naturals are played. But what we’ll hear is a mix of 329 or 659 and 320. That’s going to have a detrimental effect on what we get to listen to.
This is simply not true. The E5 note on the high string at the 12th fret has indeed a frequency value of about 659 Hz. But this is the
lowest frequency the string will generate. Along with the 659 Hz, it will also produce many higher harmonics, all at different amplitudes, at integer multiples of 659 Hz. The open high E string will not resonate at 329 Hz, because the string is fretted at the 12th fret. If the guitar has a natural resonance hot spot around 320Hz, it is true that it will resonate when the E5 is played, but
only for a very short time! The 659 Hz of the E5, while ringing out, will not excite the 329 Hz body resonant frequency. So you will not hear "a mix of 329 or 659 and 320Hz" like Andy stated, but only the 659Hz with its harmonics, along with a very short signal around 320 Hz, which has such a wide spectrum that it will not be perceived as a tone, but rather as a 'thud' sound.
The reason that the 320Hz only resonates for a short time, producing the 'thud' sound, is because it is a
body resonance, not a string resonance. It is excited by the attack of the string (the 'pluck', if you will). This attack will send a shock wave through the string with a very broad frequency spectrum. It basically contains all audible frequencies, which will run through the string to the bridge and to the top of the guitar. The guitar top receives all these audible frequencies at the same time, but will resonate only along with a few of them. These are the resonant frequencies, and these create the short 'thud' sound, very similar to what you hear when you tap with your finger on the bridge. It's basically the same thing.
You can easily measure the resonant frequencies of your guitar. I just did it for my Taylor BTO GC Cedar/Walnut. Simply tap the top, and measure the frequency content with a simple spectrum analyser app (I used an app called
Analyzer, for iPhone). It took me about 3 seconds to produce this:
As you see, there is a resonant peak at 106 Hz, something at 180, a small one at 570 and a few other very small ones. These frequencies are indeed the 'tones' you hear when you tap the top of my guitar (they're different for every guitar). This is the 'thud' sound I referred to earlier, and this is the same 'thud' sound you hear after you pluck any string. Try it out yourself: plucking a string creates a sound as if you tap the top - after which the string will ring out.
However, as you can see from the graph, the resonant peaks are quite wide. This explains why you hear a 'thud' and not a clear tone, which would happen when you tap against a wine glass:
As you see, the spectrum of a wine glass has a much narrower peak, and therefore you hear a clear tone when you tap it, just like a church bell.
So yes, if a guitar body had similar resonance characteristics as a wine glass, then Andy Powers would have a point. Every time you would pluck a string, any string, the attack would produce a clear 'pinnnggggggg' tone, not a 'thud', which would probably be dissonant with the notes you played. But as we all know, the guitar will only produce a 'thud', with a wide spectrum, which lasts only very short, so it does not interfere with the intonation of the guitar, nor does it produce 'warbly' tones or 'fights'. It simply does not happen.
Another erroneous statement by Andy is:
"...since guitar strings do not introduce pure sine wave signals, but multi-frequency content ... When you sum these things together, you end up with a surface that resembles wrinkling tin foil...".
I know where he is going to, but he is mistaken. A vibrating guitar top does not look like wrinkling tin foil, but always vibrates in quite an orderly fashion, even for the high frequencies. Its behaviour is identical to the vibrational modes of a guitar string (albeit in 2 dimensions). Just like a guitar top, a vibrating guitar string also contains a plethora of different frequencies, all happening at the same time at various wavelengths. However, the vibrating string does not look like a wiggly chaos of all kind of different waves happening at the same time (the one-dimensional version of wrinkling tin foil): it essentially looks like a simple and orderly
polygon all of the time. You can see this below in a computer simulation I wrote a while ago, which simulates a vibrating guitar string and the sound it produces depending on where it is picked:
It is true that the polygon contains many kinds of different frequencies, but they all add up to make just a simple polygon. A guitar top does something similar, albeit in two dimensions.
However, the most disturbing point about Andy's story, in my opinion, is the fact that he only tells
that V-bracing solves all the alleged intonation issues, but he never explains
how it does such a thing. Why would V-bracing systematically shift all resonance frequencies towards positions where they are "more in tune" with the strings? How can you reproduce this effect on mass-produced factory guitars? If it works so reliably, there must be a very fundamental principle underneath it. But Andy fails to make this clear - in fact he does not even mention it. I think that is disturbing.