When you touch something you receive a very crude frequency impulse which records itself on that part of the physical body with which it comes into contact. This impress is interpreted in the brain as a substance of some particular composition which can be seen, but there being no sound wave it cannot be heard. Going upwards on the frequency scale from touch we come to sound which is adjacent, and at that point sound will be so low in frequency that it is almost in the touch scale. At the higher end of the sound scale the highest frequencies are almost into those of sight. When our ears fail to receive certain frequencies because these are too high the sight takes over. You may, for example, see a very dull red. Everything radiates frequencies and it is the impulses from these frequencies that register on our senses. The frequencies which impinge on our sight are so frail that even a sheet of black paper will cut them off from us, but the frequencies of sound, being coarser, will penetrate even a brick wall.