MSC,

Pay the price because energy is built up?

     My point is that the acoustic chamber of an acoustic guitar amplifies sound.
     It does this by, as you say, by building up energy in a certain local (the acoustic chamber).

It is "amplification" - but it is not *net* energy amplification.

     
      First... you can't have it both ways.

      Second ...sound waves 1000 times larger come from the Chamber (than from the string) & also sound out longer.
                       and you think there is no amplification of Sound?

                                             I have one question for you.
                         "How tall would you be if you were 1000 times taller?"

Sincerely,
                 The Observer

P.S. It makes me a little nuts when it appears
               I'm only person who thinks the acoustic chamber of an acoustic guitar amplifies sound...
                                 so forgive me if I sound a bit defensive. ;o)


             

...

When a guitar string is plucked, a finite amount of energy is imparted to that string.  That energy is then dissipated in several ways.

Mainly, you notice that the string will vibrate at it's natural frequency.  This vibration creates sound waves in the air surrounding the string that radiate outward, similar to the way light waves radiate from a light bulb.  Only a small portion of the entire amount of energy initial imparted to the string is imparted to each sound wave (each vibration cycle).  And only a fraction of each sound wave (radiating 360 degrees from the string) is going to reach your ear.  The further from the string you are, the smaller still the amount of each sound wave that your ear will receive.  This is why the sound is less loud the further away you are, exactly how a light appears dimmer the further from it you are.

In an acoustic guitar a large portion of the sound waves facing the acoustic chamber interact with it in several ways, localizing it, and therefore not dissipating into the air further away.  First, the sound waves may reflect and bounce around in the chamber, and ultimately be re-directed towards the front of the guitar through the opening as constructive interference.  Secondly, the sound waves cause the wood of the acoustic chamber, and especially the top sounding board, to vibrate at the same and harmonics of the same natural frequency, again, localizing the sound energy rather than letting it dissipate into the surrounding air.  The vibration of the sounding board also constructively reinforces the vibration of the string both acoustically (through the air) and physically (through the bridge).

The second, and often overlooked, way that energy is dissipated from the string is by energy transfer from the vibrating string through the bridge, directly to the sounding board of the acoustic chamber.  That energy is also localized and vibrates the wood of the sounding board adding to the constructive reinforcing interference and subsequent increase in heard volume of the struck note.

These are all reasons why the size, shape, thickness, and material choices for the acoustic chamber, bridge, and especially the sounding board are important to achieve an acoustic guitar with particular "tone" as well as volume.  It all has to do with how much of the energy from the initial plucking of a string is imparted to these constructive interference sound wave producing elements.  Any energy lost to dampening materials or through the air not directed to your ear is energy not perceived by you as sound.

An electric guitar lacks most all of the constructive interference sound wave producing elements of the acoustic guitar.  Instead, it is designed to keep the energy in the string itself.  These strings are thinner, stiffer, and looser.  They are designed to vibrate with a smaller amplitude than their thicker, softer, tighter acoustic counterparts.  This causes less of the energy caused by plucking to be imparted to the air as sound, for sound is just a loss since it is not the method by which energy is transferred to electric guitar pickups.  The electric guitar string is also often ended at a heavy nut and bridge that are designed to keep the energy in the string, not transmit it to a sounding board.  You can notice this effect as more natural sustain in electric guitars with heavier nuts and bridges/tailstocks.  Likewise, the frets on the electric are also usually much heavier than on an acoustic so as to transfer less energy to the wood of the neck and keep it in the string.

The electric guitar pickup includes a magnetic field that resists the vibration of the string and unfortunately dampens it somewhat.  This additional loss mechanism is not present on the acoustic guitar.

If you string an acoustic guitar with the thinner, looser strings of an electric, and dampen the constructive  interference sound creating properties of the acoustic chamber (mainly the sounding board) by attaching a heavy weight, thickening it, etc, it's volume will decrease immensely.   Eliminate the hole so that sound cannot enter or reflect back from the chamber and I believe you will be down to the volume of a similarly gauge string electric.

Likewise, take a well built electric with a heavy (brass) nut and bridge through an amp turned up to the same volume as the best acoustic, and I guarantee you it will ring the notes for longer.  The electric guitar strings can vibrate for a longer time since the initial energy is not transferred to other acoustic elements directly.  They are just not optimized (like the body is not optimized) for producing sound directly.

M.

Well Observer, if you want to be an asshole, that is your prerogative...

I said nothing about nodes.

I also did NOT mean the electric would ring longer if in feedback with the amp.

I have played and built guitars for over 25 years.  I wrote my explanation based on that experience as well as that of the various engineering classes and real life situation that I have encountered dealing with vibration and acoustics.

I suggest YOU try this experiment.  String an acoustic and an electric with the same gauge string (only one is necessary).  Mic the acoustic through a regular microphone, and run the electric through an amp (in a different room if you want).  Record each.  Set the volume of each so they are the same.  Record a single pluck.  Your acoustic will ring for a shorter time.  Why?  Because it is dissipating the energy of the pluck directly to the air and other acoustic elements of the guitar body faster than that of the electric.

An acoustic guitar is in fact louder by design.  It transmits the energy of the plucked string to sound directly.  However, an electric guitar string rings for longer.  It does NOT transmit the energy of the plucked string to sound directly, but instead tries to maintain it within the string as long as possible, at a lower amplitude.

Same energy is release by both.  And it is the same amount of energy introduced by plucking.

An acoustic guitar is designed to transfer the energy of a plucked string directly into sound.  The electric guitar is designed to transfer the energy of a plucked string to an electrical pickup: a permanent magnetic field that is wrapped by a wire coil.  When the commonly grounded electric guitar string vibrates in this magnetic field, a similar electric current is produced in the coil.  That is the method of energy transfer in the electric: string energy becomes electric current.  Only when that electric current is amplified and introduced to a speaker do you get the sound it was designed to make.

So to put it in terms you might find familiar:  An electric guitar produces 100% more current than an acoustic guitar!  Holy shit!

Go figure.

M.

     As a guitar maker you should know the bridge does not and can not move.
        Technically, it is fixed and reflects the wave on the string.

Actually that is not true.  The bridge on an acoustic guitar is not a truly ridged attachment point for the strings.  The bridge is attached to the sound board top surface of the resonance chamber.  That sound board is designed to vibrate along with the strings.  The sound board material, thickness, internal bracing, and location of the bridge are all specifically designed to convert as much of the string energy to sound as possible with the added nuance of doing so mostly with frequencies that produce a pleasing tonal quality.  In fact, a vibrating string alone can only move so much air to produce sound directly.  It is actually a fairly insignificant amount.  The flat surface of the sound board moves a few magnitudes volume more air, producing a much greater portion of the sound you hear from an acoustic guitar.  This effect, added to the complimentary interference from the Helmholtz resonator type acoustic chamber (or sound box), are the two main methods by which the initial energy imparted to the strings by plucking are converted into sound waves.

Tap the sound board or even the tuning head of a guitar and it will induce a ringing in the strings.  In fact, there are feedback devices that can be mounted to the tuning head of an electric guitar (electric transducer of some sort) that will vibrate at the frequency of the strings producing infinite sustain of the original notes (and not the harmonics favored by the amp/guitar body combo).

So yes, energy is transferred through the bridge "node" of the guitar to the sound board since it is not a true ridged mount.  It is also trasferred through the nut and frets to the neck where it is mostly lost I believe, though some may make it to the sound board or resonance chamber I guess.

If the sound board is too ridged, the sound is less, but the sustain is more, similar to a solid body electric.  If the sound board is not ridged enough, the sustain suffers but you can still have a loud guitar.  This is also how a banjo is designed.

In any of those classes did they mention Resonant Chambers?
   And how they work?
      I ask this because you need to realize that air inside the chamber is a spring
          that vibrates dependent on the size of the chamber and the volume of the hole.

   In other words... the Chamber is a Helmholtz Resonator.

I agree.  So?

Let's get to the basics... The real question is...

   Do two tuning forks (same natural frequency) ring louder than one when 1 is struck?

That is an easier example to work with!  But I disagree with your assesment of the "real question".  I believe it should be...

   Do two tuning forks (same natural frequency) produce more sound energy than the energy introduced when 1 is struck?

My answer to that is no, they do not.  They localize that energy by feeding back a portion of that sound energy between themselves rather than simply radiating it away in all direction.  This localization (or concentration) of the sound energy results in a sound that can be sensed as louder and longer.  But no extra energy is produced that I can tell.  I'd be happy to be shown otherwise.

M.

Paul,

1. Yes... the string vibrates causing air pressure waves.
     The Body of the Guitar is a Helmholtz Resonator.   

2. No... the node of the string does not vibrate.
    Everyone thinks this... but it just simply is not true.
 
       The tension of the string must remain constant for a guitar string to stay in tune,
           and the ends must remain fixed to reflect the string's wave back and forth..

             In other words... if the ends of the string wiggled... the string would not vibrate as well
                depending on how much wiggle you wanted at the node.

It is very easy to test this... strum a string and feel the bridge.
          It does not vibrate and you cannot dampen the vibration from this point... the node.

You mention National/Dobro guitars but fail to mention they are Resonator Guitars.
  Meaning they use extra resonators to amplify the sound more than a regular acoustic.

Because they are called Resonator Guitars...
                                                                            One must ask what a Resonator is and how it works.

Question.... What is a Resonator?
                                                              Answer...Anything that Vibrates.
                                                                              ... A Tuning Fork
                                                                              ... A Guitar String
                                                                              ... The Air in an Acoustic Guitar Body (Resonace Chamber)
                                                                              ... The The Body of an Acoustic Guitar
                                                                              ... The Metal in the Cone of a Resonator Guitar

Question... How does a Resonator Work?
                                                                         Answer... 1. A small oscillatory driving force causes a LARGE Oscillation. (natural or harmonic frequency)
                                                                                               This is because Energy is Stored in the system.
       
                                                                                          2. Different Systems store Different amounts of energy. (Vibrationally)
                                                                                                 Example. A Large swing can store more energy than a small swing from the same driving force!

                                                                                          3. The System vibrates at an amplitude congruent to the amount of energy it can store.
                                                                                                We see/hear/feel this amplitude from said System.
                                                                                                   
                                            Resonance is a phenomenon... it exhibits non-linear characteristics.
                                                          This is why I am interested in everything about it...

                                                                                     The Observer
 

Observer,

I appreciate your enthusiasm with this subject.  I too find resonance facinating in all it's forms, especially electrical circuits.

The bridge is a tricky subject.
     I have not observed it vibrating.
         I cannot dampen the sound by pressing on the bridge.
             I am pretty sure wave energy reflects when encountering a material of different density.
                 That's why the strings vibrate... a standing wave is formed because of this reflection.

I would be surprised if you could feel the bridge vibrate.  Only a tiny amount of energy is moving through it at any given time.  To feel that would be like being able to feel you fingers tingle due to the sound waves hitting them when someone was talking.  Also, the bridge is the node as you have pointed out.  If it was a perfect node, it would not move at all and no energy could move between the strings and the sound board.  But it is not perfect and energy does travel between them.  The bridge "node" does not physically translate much, if any at all, when this happens.

I am reminded of how we play true harmonics on a guitar, ie. when you finger a string lightly above a node point but do not push it down to the fretboard and then pick.  Your finger is an imperfect node that causes some energy to reflect back while allowing some to continue through to the other side.  The result is a string that vibrates on both sides of the node.  If fingered above the 12th (middle) fret you get a true harmonic of that string, a note one octave higher.  Instead of ringing in the manner of a half sine wave, the string will ring with a full sine wave shape, with the center not moving at the point where your finger touched it and caused a node.  This is what happens at the bridge.  It does not reflect 100% of the energy, but say only 99.9%.  That .1% makes it through to the sound board which then moves some magnitudes more air as direct sound waves.

       A Resonator can Store Energy and a Large Oscillation can be produced from a small oscillation.

       A Resonator Vibrates with an amplitude congruent to the Energy it Can Store.

       A Large Resonator can store more Energy than a small resonator.

       Remarkably,    we see, feel and hear...    the Energy Stored Vibrationally.

We do not hear the stored energy.  We hear the energy as it leaves this storage and emanates out from the resonator.  So yes, the resonator can store sound energy, and release it over a shorter time than if it had not been captured with the result that it is louder.  Not longer.  Just louder.

You can test this with just 1 Acoustic Guitar in a matter of minutes !!!!

Unfortunately that is not a valid test and does not represent the two tuning fork experiment correctly.  This is again because the bridge is not a perfect node.  So all the strings are actually mechanically coupled through the bridge and sound board.  Thus they can excite each other mechanically.  The tuning fork experiment keeps them isolated except for acoustic coupling.

If you want to try this experiment on a guitar you should at at least do so on an electric where the sound board is not in play and the heavier bridge and nut will allow for much less mechanical coupling between the strings.

Edited to add:  In your experiment where you dampen the low E and A while timing the ring out of the high E, you are actually dampening the high E as well since it is mechanically coupled to the low E and A.  You might be able to get around this by actually removing the low E and A for that portion of the experiment, but this would also change the length of the high E somewhat as the neck springs back (if it does not ground out the high E to the fretboard completely).