I get that. I guess I was just confused as to why your first thought was to use a speaker to move the photo diode as opposed to an Arduino with a stepper motor.
Sawtooth, not a square wave. Hook the speaker up to some kind of frame, glue a small hinge to the cone, hinge to an arm that's anchored relatively close to the speaker, diode at the far end of the arm in a tube so the light is directional, instant scanner :)