'Ribbons' that are driven by audio loudness and tuned/draped for mask avatars

To enable, use developer->experimental->string hair
Huh.  Yeah, you're right.  _loudness is used unprotected in Hair::simulate() but there is no race condition there.  _loudness is used in Hair::render() but is wrapped by an if() check that bypasses when _loudness is NaN.

Ok.