See that big round or oval hole on an acoustic guitar? It's the soundhole, and a lot of sound passes out of it. If you're familiar with a subwoofer, you'll find something similar. Soundholes can be round, oval, or other shapes (some are like the f-shaped holes on violins, cellos, and basses). The size of the soundhole, in combination with the size of an acoustic guitar's interior, can really make a difference in the tone. Some electric guitars have soundholes, and although they can affect the tone, they're usually cosmetic, just looking good.