It is better to promote a kindly attitude and nonjudgemental attitude towards ourselves when we are in the act of singing instead of constantly berating ourselves. Compassion for oneself and others is always a wonderful quality and gives greater energy and freedom anyway. Practically speaking if we judge ourselves as we sing, listening, we will be with the effect not the cause. It will mean that any changes we make will be too late and this will be frustrating to say the least. To put it another way the act of singing goes by very fast. If you are in the a racing car and going round the track you need to make the decision as to what to do before you get to the corner and trust it – what gear, how are you going to place yourself on the track and how fast should you go - and on to the next thing. You have to think ahead. The advantage of this attitude is that it focuses the mind on what we are doing and makes us more fully conscious of each moment.
You cannot change the sound when it is already airwaves but you can learn technique to become more and more fully conscious of the manner in which it gets there, i.e. the manner in which you sing yourself. Singing teachers have the huge responsibility of communicating the skilled use of the correct, trustworthy tools to each individual singer in a way that helps, that is practical, makes sense to them and does not throw obstacles in the path of the singer.
Great harm can befall a singer when they are impeded by being treated in a judgemental fashion. Teachers and coaches do not always realize that graceful care and concern for ones self, each other and the singer can improve the quality of a singer’s performance just on its own. It should not be forgotten that singing is very largely psychological and a balanced outlook makes finding a consciousness of balance in the ringing body of the sound a whole lot easier.
Good luck with your vocal journey