Why is an Acoustic Guitar so much LOUDER than an Electric Guitar?

[mondrasek]

     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.

[mondrasek]

The write up on Wikipedia on "acoustic guitar" is pretty good.

[The Observer]

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

[The Observer]

Hey Mon,

Thanks for bearing with me on this.
I can tell you are fairly knowledgeable and appreciate it.

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 also admit that Wiki and other places talk about the bridge.
     So I will be the only one who wonders if this is actually true due to the static nature of the node/bridge.
     

In other words... the Chamber is a Helmholtz Resonator. <--- The Observer

                                            I agree.  So? <---- Mondrasek

So...

       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.

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?

           I agree... a better way of posing the basic question of concern..

Then we must agree (about the simplest physics there is)...
                                                                   
                                    Louder Sounds carry more Energy than Softer Sounds.
                                                                            &
                  A Longer Sound carries more energy than a Shorter Sound (of same intensity)
                                                                       meaning
                                      A Longer and Louder Sound = More Sound Energy

And now for the aforementioned tuning fork experiment.

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

     1. Take your tuner and tune that SOB better than it has ever been tuned before... spend like 5 minutes.
         (Damp all other strings but the one being tuned during this process)

     2. Strum the High E String.

     3. Observe that the Low E and A String are vibrating (at the High E... a harmonic frequency of these strings)

     4. You can dampen the High E shortly after strumming and the Low E and A will be vibrating !
          (classic tuning fork experiment right?)

     5. Now we have 2 tuning forks (the High E String+Acoustic Chamber) & (Low E + A Strings)

     6. Strum High E String with all other strings damped... note Volume & Duration.

     7. Strum High E string with out damping (letting the Low E and A vibrate also)... note Volume & Duration.

    8. Do it about 20 times to make sure you are not imagining anything. ;o)

    9. If you are unsure.. use your guitar tuner  (at High E) to see the Louder and Longer effect.

                     You say that you would "be happy to be shown otherwise"... so I implore you to try it out.
               I know you have decent acoustic laying around.
      Anyone else is welcome to try this and see what motivated Tesla, Stan and others.

Have a Good Day,
                               The Observer

[mondrasek]

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).