Tonewoods on an acoustic guitar dictate the overall responsiveness of certain frequencies that are produced when the instrument is strummed or picked. As an example, our 1958 Martin D-28 has Brazilian Rosewood back and sides, which generally have a "scooped midrange" type sound, meaning the lows and highs are more pronounced and the middle frequencies less so. This type of response can compliment a singer very well, as the guitar's natural "smiley face" EQ curve leaves room within the mix for the voice which, for most of us, happens to be naturally mid-rangy.
Conversely, mahogany tends to be more mid-rangy, with the lows and highs falling off a bit--a "frowny face" if you will. Our 1937 D-18, for example, has a big robust sound that would do well in a bluegrass jam or acoustic ensemble because the abundant mids allow the guitar's voice to cut through in the mix. (Check out the video below of Tony Rice and Norman Blake--see if you can hear the difference between the 14-fret D-28 with Brazilian Rosewood that Tony is playing and the 12-fret D-18 with Mahogany that Norman is playing!)
These days, there are a plethora of different tonewoods available--yet another factor to consider when making your next guitar purchase. Stop by the shop sometime and try a few out!