Harmonics Question for Luthiers (Full Version)

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bahen -> Harmonics Question for Luthiers (May 26 2006 6:34:10)

This is probably a question you luthiers (/luthier enthusiasts) are better equipped to answer: what does a guitar's harmonics tell you about the guitar itself?

First, let me perhaps clarify what I understand harmonics to be - it's the very high-pitched ting you hear when you place your finger at the 7th or 12th fret, for example, but do not press down completely, but do hit the string. Ting. I noticed I can do this at the 3rd fret, 7th, 12th, 15th (I think), and even dead-centre of the sound-hole. Am I being clear? You slight press the string with your left hand, then hit it with the right.

What do these positions tell you about the frequency of the guitar? What do they tell you about the guitar in general? I have a blanca and it has very, very loud harmonics.

Any informed comments are welcomed. I'm very curious.

- bahen



Escribano -> RE: Harmonics Question for Luthiers (May 26 2006 8:20:39)

Hit the harmonics at the 12th fret and compare with the fretted note below, a good way to check any deformation in the neck, though I think this problem is more common in steel and electric guitars. Any comments on this?

I use harmonics for tuning i.e. 5th position harmonic on one string against 7th position on the next (except 3rd to 2nd string, which I fret).

I like this method as the harmonics sustain and allow me to twiddle the tuner with my left hand, free of fretting. It also checks the accuracy of the frets to each other as a poor setup/construction would mean this tuning will never work against the frets.



TANúñez -> RE: Harmonics Question for Luthiers (May 26 2006 14:26:28)

When you strike a guitar string, the note that you hear is called the 'fundamental'. It's the loudest note created, but along with it you are also hearing 'harmonics'. These are subsidiary tones that accompany the fundamental, and are responsible for making each instrument sound the way it does.

Guitar 'harmonics' are created when you lightly touch the string with your finger at specific positions and then pluck the string. This causes both sections of the string, to the right and left of the spot you're touching, to vibrate simultaneously, giving a bell like quality to the note. What you are doing is removing the fundamental and only hearing the harmonics.

There are three points along the string : the fifth, seventh and twelfth frets ... directly above the fret-wire. At these points, the string is divided exactly into fourths, thirds and halves respectively. The sound comes from these perfect fractions of string ringing together. If you try it anywhere else on the string, you hear a dead sound without any ring.

The notes created are not always the same notes as the fretted note. At the 12th fret, they are the same note, and in fact playing one after the other is a good way to see if the intonation on your instrument is right: both fretted note and harmonic should be identical in pitch. If they're not, there could be a problem with the bridge.

At the 7th fret, the harmonic is one octave higher than the fretted note. At the 5th fret, the harmonic is the 'fifth' of the fretted note, but 2 octaves higher. In other words, on the E string, the fretted note at the 5th fret is an A note, but the harmonic played at the 5th fret is an E note two octaves higher.



bahen -> RE: Harmonics Question for Luthiers (May 26 2006 19:03:33)

Very informative - that was great. Harmonics are also present at the 16th fret, and a little bit to the right of the centre of the sound-hole.

Also, what do loud harmonics tell you about the guitar? That it's a loud guitar in general? Poor harmonics are a result of bridge problems? I have been told that I need to lower my action just a bit.



JBASHORUN -> RE: Harmonics Question for Luthiers (May 26 2006 20:00:58)

quote:

There are three points along the string : the fifth, seventh and twelfth frets ... directly above the fret-wire. At these points, the string is divided exactly into fourths, thirds and halves respectively. The sound comes from these perfect fractions of string ringing together. If you try it anywhere else on the string, you hear a dead sound without any ring.


As far as I can remember, a harmonic sound is produced at EVERY division point of the string. Eg. 1/2: 12th fret, 1/3: 7th fret, 1/4: 5th fret, 1/5: approx. 4th fret, 1/6: approx. 3rd fret, and so on.

It should be noted that you will get the same harmonic sound at each position relative to the division of the string: divide the string into quarters, and you will get the same harmonic by moving your finger a quarter of the way up the string.

There are certain points (such as at the 5th, 7th and 12th fret) where the harmonics are more obvious than others. But if the room is quiet enough, and you listen closely, you can actually hear the harmonic sound at other positions too.



That said, I think the number of possible harmonics isn't infinite... but depends on the number of octaves the instrument has.

As for what the harmonics tell us about the quality of a guitar, I'm not quite sure. Although you can check the intonation of a string by comparing the note at the 12th fret to its harmonic in the same position. If the two don't match, then you need to adjust the intonation accordingly... which is easier said than done on acoustic guitars!

Man, I wish I had payed more attention during those acoustics lectures!

Jb



Armando -> RE: Harmonics Question for Luthiers (May 26 2006 20:23:24)

Hi bahen

The basic things about harmonics have been already correctly explained.
Some luthiers put a lot of effort into the tuning of the components of their guitars in order to have the completed guitar responding on a certain pitch. They think that this is essential in order to build a certain caracter into the instrument. some luthiers use a lot of laboratory equipment such as strobe tuners and signal gernerators etc.. in order to achieve the desired pitches. This scientific approach (also known as left brain lutherie) allows to tune a soundboard to an exact pitch. The back of the guitar is often tuned to a pitch close to that of the top in order to have them coupling with each other. This will produce a louder sound especially around the 3rd top resonance. Of course the harmonics of the 3rd top resonance will also respond louder than other notes.
In general i think that loud harmonics are indicating a good instrument. I haven't seen a bad guitar with loud harmonics so far.

Armando



bahen -> RE: Harmonics Question for Luthiers (May 26 2006 20:27:26)

And how do you compare harmonics across blancas? Across negras? How would you compare harmonics between the two?



Armando -> RE: Harmonics Question for Luthiers (May 26 2006 20:34:39)

O.k. tricky question

I didn't mind about that so far, maybe because i think that it doesn't matter whether we talk about a blanca or a negra.



JBASHORUN -> RE: Harmonics Question for Luthiers (May 26 2006 21:00:20)

quote:

And how do you compare harmonics across blancas? Across negras? How would you compare harmonics between the two?


As far as I know, harmonics should be pretty similar on a blanca and negra. But thats just one person's theory, as I was told the ribs and back of a guitar affect the sound by 1% of the total. But if this percentage is higher, you might well expect to see a stronger response with the more dense woods.

I was told that the main factors influencing the harmonics are the nut, saddle, strings and soundboard. Denser nut and saddle materials like bone, etc will give a harder barrier to reflect the waveforms off (and transmit string vibrations), and produce a stronger output than less dense materials- this is why some materials are preferred for nuts and saddles. Also issues like string length, thickness, tension, etc.

Armando is correct about soundboards being tuned to different frequencies, but many luthiers do not actually have the scientific equipment to manipulate this whilst making guitars.

I think you can check the approximate frequency by cutting a round piece of cardboard shaped like your soundhole. Then pluck each open string seperately whilst covering the soundhole and then removing the cardboard (open/closed/open closed/etc.). The string where the contrast between the loudness and quiteness is greatest is closest to the frequency. you can then do the same thing whilst working your way up the first few frets of this string, as you might find it a position or two further up.

Luthiers: correct me if I got any of that mixed up, after all, I'm still quite new to this.


Jb



Armando -> RE: Harmonics Question for Luthiers (May 27 2006 19:48:42)

JBASHORUN

I think what you are talking about will measure the cavity resonance (also called helmholtz resonance) The helmholtz resonance needs a hole like the hole of a bottle to exist. If the hole is covered, the helmholtz resonance will desappear. The helmholtz resonance will have an influence mainly on the basses and lower mids of the guitar but not directly on the harmonics.



JBASHORUN -> RE: Harmonics Question for Luthiers (May 28 2006 12:50:44)

quote:

I think what you are talking about will measure the cavity resonance (also called helmholtz resonance) The helmholtz resonance needs a hole like the hole of a bottle to exist. If the hole is covered, the helmholtz resonance will desappear. The helmholtz resonance will have an influence mainly on the basses and lower mids of the guitar but not directly on the harmonics.


Thanks for the correction, Armando! You are right, I remember now, it was the Helmholtz resonance I was thinking of.

Jb



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