There is another exercise. Fix the ears on the pranava dhvani ('OM' sound) that emanates from a river, ocean or wind. It will be heard like 'bhum' or a long 'OM'. Train your ears to hear this sound. Do this exercise at 4 a.m., or at 9 p.m. When the noise of the town has died down. Keep your ears keen. Differentiate the sounds of various kinds of birds, beasts, children, factories, motor cars, aeroplanes, cycles, yells, snores, sobbing, crying, laughing, mocking, joking, etc. The mind may be there, the instrument may be perfect, but if the mind is not functioning perfectly, you cannot see or hear properly. Perception is possible only when the mind is linked with the external instrument - ear, eye, etc..