size? imagine if the sound hole was the whole top of the guitar, or if the hole was as big as a pea? i'd imagine it effects the sound, but if you ask me, they are all relativly about the same size
Simple answer, yes.

Is there a particular question you wanted answered?
kind of. most of the sound comes from the type of wood the top is made of. the soundhold just gives you more or less sound.
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The sound hole makes a huge difference. I have done a lot of experimenting with size, shape and placement over the last 5 years. It makes all the difference in the world. It not only changes the volume, but it changes the pitch, and projection and the overall tone. Just try taping some cardboard over the soundboard of your guitar. Cover half of it and see how it sounds. Then cover 3/4 of it. Finally cover all of it. It not only changes the volume, but it changes the tone, pitch, and projection.
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I kinda disagree here. Nick's simple and short answer is a lot nearer to the mark. Soundhole shape and position is among the three or four most important parameters when it comes to tone forming. The only thing is that the overall construction of a typical guitar doesn't leave much room for alternative designs and that the shape and size you'll see on 99% of all guitars is pretty much the most optimal solution. Apart from the regular f holes on archtops and some unusual designs on resonator guitars, the only really different soundhole design is the multiple holes variety on Ovations, but these probably only work well in combination with other typical Ovation features.
I had a Washburn with an oval sound hole that sounded pretty good for the price.

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Ok. Of course you are right. I've looked it up and found that not all Selmers with a D-shape soundhole were resonators. The most notable one was, though


Still I think it's true for the most part that the conventional sound hole design represents the most optimal solution. Making it bigger would decrease the free swinging area of the top and impair structural integrity, making it smaller would decrease volume and make for less bass, while making it any other shape than circular would only complicate manufacturing. That you have tested different shapes and sizes is a good thing, but my bet is that in the end you too will choose a design that is very close to what everybody else uses.