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Permalink Reply by Wichita Sam on January 11, 2013 at 5:22am

After 5 years and 400+ builds these are "informed" opinions.....

 

Factors most important in the loudness (disregarding other properties, in building everything is compromise) of a cbg....

 

1. the area of a vibrating surface (size in square inches or cm's or whatever matter!)  (the larger the surface the louder)

2. tension force of strings (through bridge) on vibrating surface (higher the tension, the louder)

3. thickness and type of wood (or whatever it's made of) in vibrating surface (thinner, harder, denser wood louder)

4. location of bridge relative to center of vibrating surface (closer to middle the louder)

5. location of sound hole ( put it somewhere other than the vibrating surface, it just cuts into area and defeats #1.

 

Everything else is either for looks or has an impact on one of these factors or is discussed because it impacts some other factor such as bass response.  Of course there are limitations to the extent that #'s 1-3 can be taken.  But if loudness of an acoustic instrument is important to you, start with this list.

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Permalink Reply by David McQueen on January 11, 2013 at 5:58am

I would add to that:

Regarding 2: Tension force of the strings is

(a) directly related to the angle of the strings on the bridge for a floating bridge. The greater the angle the more downward force on the bridge to transfer energy. To get a higher bridge while keeping the action on the fret board, the neck has to be back angled from the body. I just use a cedar shim as a saw guide over the length of the box. It works because I am using a separate tail piece instead of running the neck through the back of the box. I am getting about 1.5 degrees that way.

(b) directly related to your scale length. To get to tune on a 25.4" scale, the strings have more tension than in using a 24" scale. That is why Gibson uses a 24" scale - less tension - easier to play.

(c) In using a big box with the bridge near the middle, plan on a tail piece to bring the end of the strings closer to the bridge, thus increasing the attack angle of the strings on the bridge at the tail piece. 

#6: For a floating bridge, the lighter and more rigid the bridge, the more energy is transferred to the top of the guitar. If you want examples of ultra light bridges, do some reading on Selmer Macaferri guitars from the 30's. They were Europe's answer to arch tops from the States, which no one in Europe could afford.  They had a high hollowed out ebony bridge to be as rigid and light as possible and keep the string tension high.  They had a back angle of the neck to the body of up to 5 degrees.

#7. The depth of your box and the thickness of your top will also have an effect at the lower end. It does not take that much energy to make a loud high note. It takes a ton of energy to get a loud bass note. Part of using a thin top is to drop the resonance frequency of the top to facilitate bass notes. To get the same effect with hardwood you have to have a thinner top than with softwood - hence spruce top guitars. 

#8 Your bridge and nut materials will make a difference. Bone or Corian versus plastic. 

HTH

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Permalink Reply by David McQueen on January 11, 2013 at 10:46am

More on my #6 above. There is a video posted Gypsy Swing Gitar Trio where one fellow is playing a Selmer style guitar and the other is playing a resophonic. The guy on the Selmer is playing the lead and has no trouble punching past the reso playing rhythm (Plus it is a really nice video).

That bridge/saddle is a typical Selmer style. If you were to pull it off, it is hollowed out underneath and the bridge and saddle are all in one.

http://www.cigarboxnation.com/video/gypsy-swing-gitar-trio

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Permalink Reply by Bluesheart on January 11, 2013 at 11:50am

I think more volume in the box will help but I feel that the thickness of the soundboard has more to do with volume than anything. A thin top will vibrate more and be louder, a thick one will be quieter. Those boxes in the picture are thick cardboard tops if I'm not wrong but the bottom is nice  thin plywood so may make a better soundboard.

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