: What are some properties of a string instrument's sound box that can make it louder? I would like to know what some aspects of a sound box there are that I could change in order to make my instrument louder. I imagine enlarging

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Actually, changing the geometry of the sound hole is very unlikely to change the overall volume.

As far as I can think, the only way to increase the volume of an already constructed string instrument is to put thicker strings on it. The additional mass should drive the bridge a bit harder. It probably won’t make a very big difference.

To make a louder sound box you need materials with a higher strength to weight ratio so they can be cut thinner and lighter so they will vibrate more easily. This is why solid Sitka spruce is arguably the best top material for a steel string guitar: it has a very high strength to weight ratio.

Another big factor is geometry and bracing. The more frequencies the sound box will resonate on, the louder it can be and the more even the tone. The choices of woods also can have an impact here. For the same woods and scale length, a larger sound box is usually louder. But if you don’t make sure there’s some high frequency resonates then the larger box might just be deeper and therefore could sound less loud.

Making sensitive (I.e., loud) stringed instruments with rich tone is a challenging art form, and pretty much all the parameters of that are decided before anything is glued together.

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The holes in the body have almost nothing to do with the sound level. Remember, it's using the holes to project a sound wave (as, by comparison, a wind instrument's bell shape is designed for acoustic impedance matching (or mismatching) between the pressure wave and the free air outside the instrument).

You can never get more energy out of the instrument than is put in (duh), so the best you can do is to try to transfer as much of the string's vibrational energy into the soundbox as possible. Of course, if you were really successful, the string would be damped out in a few cycles -- also not desirable in general. As luser droog points out, the overall box design should avoid "hotspot" resonances which make a particular note (wavelength) project much more loudly than others.

So, as Todd W. pointed out, making quality string instruments is a very complicated art. Certainly you want the energy which is transferred via the bridge (mostly) to the body to be emitted via soundbox vibration -- which means you don't want the body to dampen the energy it has received.