Learning to hear functionally means that the tone qualities are judged and evaluated for their intrinsic health, and as a reflection of a coordinative process, rather than for their aesthetic charm or beauty. Not every healthy sound yielded by the vocal organs is lovely to listen to, any more than all sounds judged agreeable to the hearing are functionally healthy. Purely aesthetic listening is largely the product of conditioning and cannot be depended upon. Moreover, during the process of improving a faulty coordinative response, many sounds having little or no aesthetic value can often be extremely useful to the training program. Two such cases in point are the rather crude chest register sounds that sometimes appear in the initial stages of training when working with a badly coordinated female voice, and the male falsetto. Hearing functionally rather than aesthetically often permits the teacher to get around what would otherwise be an insurmountable technical difficulty.
Reid, Cornelius L. The Free Voice. Coleman-Ross, 1965.