Super Sensitive octave strings are specially made to sound an octave lower on a normal violin: the tension isn't significantly greater than with normal strings. They are not cheap! The sound doesn't necessarily project very well, though, and you're best off with a larger-bodied instrument for the added resonance. A friend of mine has a Maggini copy strung an octave low, and it works fine. Relatively quiet, but gives you a pretty good jaw massage. Of course, you can buy purpose-built instruments; Tim Phillips makes nice ones, but you'd be better off experimenting first; they too are of course not cheap.