What is the difference between Spanish guitar and acoustic guitar?
[...]Who knew? Read the whole thing for more details. I think the nylon strings on a Spanish/Classical guitar would be easier on the fingers. I find the metal strings on a regular acoustic guitar rather painful.
Both "Spanish" guitars and acoustic guitars are acoustic instruments, generally made of tone woods, usually consisting of spruce or cedar tops, mahogany or rosewood backs (or often cypress for Flamenco), and many other varieties.
Both usually have a range of scale (playing length of the strings) from about 609.6mm (24") to about 650mm (25.6"). Let's not dwell on the other similarities since they're obvious from the picture.
String Material: Well the strings of course. Steel or other metals for acoustic, nylon for Spanish. One cannot simply put steel strings on a classical or nylon strings on an acoustic (see why in String Tension below).
Wider neck on Spanish: Most acoustic guitars have a neck width at the nut (where the neck meets the head) of about 42mm (approx 1-11/16") to about 45mm (approx 1-3/4"). Classical and flamenco guitars are closer to 2" wide (approx 49-52mm). This may seem trivial, but it makes a significant difference.
Neck to Body: In most modern acoustics, the neck meets the body on the 14th fret. Most Spanish guitars, it is on the 12th fret. Therefore the bridge (body end of the strings) is set back farther from the sound hole on most classical and flamencos (you can see this on the picture).
String Tension: Acoustic guitars must be built stronger, because the tension of the metal strings is approximately twice that of nylon. This is done with bracing. Any acoustic guitar top must be thin enough to resonate, but so thin that the top alone could not hold it together against the string tension. The bracing adds strength with a goal of minimal damping of resonance. Bracing patterns vary widely, but most Spanish guitars use "fan bracing" and most acoustics use "X bracing." [...]
This short video explains the differences well:
The video that starts playing right after this one has a young guy explaining some more differences, which is interesting. Watch that one too if you want to know more. (or open it here.)