I've heard people who have tried to assume a "folk voice," and in addition to sounding phonier than a three-dollar bill, they frequently wind up with severe vocal problems (chronic laryngitis, permanent hoarseness, loss of range, and a host of other vocal infirmities). Just sing in your natural voice. That's all there is too it.

If you have a natural vibrato, then just let it happen. Don't worry about it. If you don't, then fine. No problem. Sing in your comfortable range no matter where it is. Otherwise, you're asking for trouble down the line. Jean Ritchie sings in her natural voice. So does Margaret Barry. And Jeannie Robertson. And Frank Proffitt. And Lead Belly.

Why make hard work of it?

Like I said above, heed what Alice says. She knows what she's talking about.

Don Firth