Change voice Pitch?

Hi folks,
Can somebody point me to a post or site for audio help?
I have a clip where I need to lower the pitch of a woman's voice to make her sound more ominous and menacing. Well .... she supposed to be an angry witch and all.
I have tried using the Apple effects control for pitch, but I can't tell its doing anything.
Of course I am probably not implementing it correctly.
Any help and advice appreciated.

Thanks

For a basic pitch drop, select the clip in the timeline you want to change, and apply the 'AUPitch' filter.

Double click that clip on the timeline to load it into the viewer. Then select the 'filters' tab on the viewer, and change the 'pitch' slider to, say, -500.

Render and listen to the timeline again. Adjust to taste.