While planning to build a CBG I am reading a lot of theory and I came across a MIT paper about sound hole shapes (https://news.mit.edu/2015/violin-acoustic-power-0210). The paper is for violins, but I think the same principles stand for a guitar. Apparently the more perimeter (edges) a hole is having, the louder sound. Thus the the area of the sound hole does not matter much as the air is hardly moving in the center of it. Long narrow holes takes less space than a round but adds more air speed (louder).

https://news.mit.edu/sites/default/files/styles/news_article__image_gallery/public/images/201502/MIT-Violin-Design-03.jpg?itok=AM91oNKE

Besides the obvious heavier strings, what other simple things could be done while building a CBG if you plan to play without a pickup and you want it to be loud enough for that?

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It’s true that some sizes & shapes are louder, but the timbre also changes with them? Some are louder because of the level of brightness. However, delta blues benefits from a low end growl. One of the best ways to figure out what you like, is to build them?

Hi Engblom, this is a big subject and a very interesting one. I will not go into the physics of the guitar sound and its mechanics, mainly because it gets too complicated for me. I read and I build using the information I understand. I also experiment to better understand, but as far as putting that stuff into my own words... very difficult.

What I will mention, and leave the rest to others or for you to continue your research, is that the soundhole is only one part of a number of areas to consider regarding the output of an instrument.

The areas I focus on in getting good acoustic sounds from my CBGs are as follows: air in the box/sound chamber, timbers used, especially for the top, position and size or shape of soundhole/s, thickness/responsiveness of the top, and the position of the bridge on the top.

Unlike when I build an acoustic full-size guitar I do not spend lots of time on bracing design and positioning.

Very important to me as well as volume is tone, a balanced tone.

Crap this is long enough without even touching on the subject Haha. Sorry.
Cheers Taff

Hi, in my post above I forgot to mention that the height of the bridge is also a contributor to the volume of an instrument.
Cheers Taff

Unfortunately it isn't as simple as soundhole shape and size to create a loud acoustic instrument. The size of the resonating chamber (ie the box) and the responsiveness of the box (most importantly the top) are very important. The top needs to be thin and light in order to resonate, but strong enough to resist the stresses exerted on it by the strings. The position and type of the bridge (floating or glued-on pin bridge) are also important. Also, there is a big difference in the way violins and guitar work- the continuous input of energy from the violin bow to make the string vibrates as opposed to the momentary impulse of plucking a string on a guitar have resulted in different construction methods. The main example is the soundpost in a violin, which couples the front and back plates - this isn't used on guitar because of the way the energy is put into the instrument. On a violin this causes the front and back to work together as the energy is continually put in from the bow into the strings. With a guitar, a soundpost will transfer the weak energy input of a plucked string to the back of the guitar, and it will be damped and absorbed much more quickly, causing a reduction of volume and sustain. In my view, and over 2000 guitar's worth of building, practical experience and experimentation is the way to go, rather than relying solely on theory.

My purpose with this topic was not to get a full complete guide to making a loud and perfect sounding CBG. More to gather pointers for what could contribute to a louder sound and still be fairly simple to do at the building stage.

I am interested what in your experience brings out more sound of the guitar. For example the bridge was mentioned, but what kind of bridge do in your opinion contribute to louder and better sound? For example, which one is more loud between a hardwood bridge or a "bolt and nut" bridge? How is the height affecting? How is the weight of the bridge affecting the sound?

Please give some concrete examples of what has given you better sound!

Hi, I think that Chicken bone John gave much good information, and I thought my account of the areas of importance to me would get you headed in the right direction.
As soundholes do not a guitar make, neither does any other one component n isolation. If I advise someone that the sound I get is such and such due to this component or material, they may not get the same result as me ‘cos I have only shared one part of what I do or the materials I use.

As I mentioned earlier tone is important to me, so I do not want volume at the cost of losing a balanced tone. Your question about bolts or timber use is a question of what tone you want [and other qualities], the bolt may sound louder but that could be due to the higher treble tone that the material creates. It carries better than bass tones, as already mentioned.

Concrete made me chuckle, I think I can safely say that nothing is concrete in our line of endeavor as we are working with wood, air, and metal. All these components come together in different quantities, qualities, and workshops, by different people.

It sounds like you are seeking to build good quality CBGs, and I commend you for that, but I think the best teacher experiences as Chicken Bone John alluded to. Learning for oneself the whys and what-ifs. I think you are on the right track, it's easy to build anything, but it's harder to do it well and it takes time.

Short story: Talking to a group of student guitar builders one day, their instructor was a guy who visited my workshop years earlier and asked to take photos and asked lots of questions. He ordered two guitars from me and now builds his own fine guitars. At this meeting, he told the group – “I learned a lot from Taff, but he never told me the complete story, always left me hanging”. My reason for that was I did not want him to think my way was the best or only way but go and think and research about the issue. He did.

Gee is that the time…

Oh, bridge height question, the Break angle of the strings over the bridge is important, it also “loads” the top, and creates good downward pressure aiding in better sound creation. That includes volume.
Taff

Regarding concrete, I was a bit tired and by mistake in my head I translated the Swedish word "konkreta" to "concrete". It should have been translated like "real" or "practical".

I wonder about the effect of back angle, depth of box, and internal volume

it would be interesting to score a pile of identical drop in lid boxes and make one body with interchangeable lids.

it would be interesting to experiment with a plain resonator cone, a cone with many small holes, and a cone with a few large holes.

"Regarding concrete, I was a bit tired and by mistake in my head I translated the Swedish word "konkreta" to "concrete". It should have been translated like "real" or "practical"."

No foul.  :)

As you used the word concrete as an adjective instead of a noun, it is a synonym for real.

I am sure Taff means the recipes will vary, nothing is set in stone.  ;) :)

Hi Carl, you lost me with your descriptions of the word concrete, ha-ha, but you nailed what I meant.

Hi Engblom, ok I got ya. If you look into my earlier post, the bit – The areas I focus on -, mentioned are the Real and Practical areas to investigate. Read different accounts by different builders, see what works for you and change it if it does not work for you.

Hi Mr. I C President, interesting ideas, are you going to do the top testing suggestion? if so it would be very interesting to hear back.
I do a similar thing when tuning guitar tops, that is attach the top to a solid form and record the changes as I work on the bracing. See photo…

I’m not so confident regarding the idea of holes in a resonator cone. I would imagine it would have a similar outcome to a speaker cone with a hole in it. It’s been many years since the Dopera Brothers developed the resonator guitar and if there was a benefit in having holes I feel it would be in use already. But I could be wrong, let us know what you find.
Cheers Taff

My loudest acoustic build used a box that weighed only 2 ounces, and had no sound holes. A slider. The whole box resonated well, resulting in max volume.

The 2 loudest I've built acoustically were.

A build where I substituted an old steel cake pan for the cigarbox, and use high action on a fretless build.

The loudest build I ever built used a small cigar box that was all wood, but only about 2 ounces in weight.

No sound holes.

Once the CBG was done the whole box resonated well, the the volume was noticeably louder thatn all the others. 

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