It's interesting to see the difference in string vibration and the maths and physics behind it. However, I would like to hear the difference between Helmhotz and double slipping surface sound. (I can tell that Tracy's daughter Amy in Coronation Street often slips up but I'd like to know what's what.) Also, are there examples where surface sound (and raucous noise) is deliberately played and actually taken advantage of, thereby enriching the melody?