Bold works for both active and passive sonar. However, in SH3, using Bold is not always effective and frquently doesn't seem to help at all. I think that escort experience level also plays into the equation--the more experienced the crew the greater the chance they will ignore Bold. I can't prove it though.
I still use Bold though and think it sometime helps me escape. But timing is everything. Don't use it when the escort already knows where you are, they'll ignore it. I usually use it when they make a run on me and then run and turn from where I dropped it. I try to put it between me my pursuer.
And always with mixed results.
