Those frequencies of light which are outside the range of human perception, i.e. infrared, ultraviolet, x-ray light... There is a world that exists outside of what you can touch and see. Music is an experience that falls into that realm beyond the visible world. It is something you feel, rather than observe or perceive. Just as instruments must be used to allow our eyes to observe infrared and ultraviolet light, so must instruments be used in portraying emotion to our hearts.