(Photo: Sussie Ahlburg)

Feb. 23 (Bloomberg) -- ``It's like a velvet cushion with a sharp edge,'' says soprano Susan Bullock when asked to describe her voice. ``Sort of warm and enveloping, but with a glint of steel.''

She needs that steely edge. Bullock is one of a small number of sopranos who can sing the crazy-lady roles of Brunnhilde, Isolde and Elektra. These challenging characters demand huge reserves of vocal power and emotional stamina. Performers who can sing them as well as Bullock are rare.