One of the most valuable helps for a singer is some form of feedback. Whether it is from a teachers’s suggestions, an audio recording of the voice or any other form of feedback, it is the feedback that is the primary way a singer learns to know how to create an acceptable sound. And that is learned because the feedback informs the singer of how they sound to the listener.
Feedback can take many, many forms.
Sense of resonance within the vocal instrument such as feelings of sound within the head, sinus, throat, collar bone, chest, forehead, mask etc. etc.
Audio recordings of the singing using quality recording equipment and quality playback equipment.
Visualization of the sound using spectrometric imagery combined with simultaneous audio such as is available with VoceVista and other spectrometric computer applications.
Projection of wide spectrum sound into the vocal tract allowing the singer to adjust the vocal tract to achieve maximum resonance without the use of the vocal folds as is available with Burton Coffin’s “Vowel Mirror”
The key to each of those listed is that none of them removes any of the basic or residual harmonics of the sound produced by the vocal folds. And it is the richness of the vocal fold sound, called “phonated sound” that is the key to the foundation of a quality singing voice .
Restricting or funneling the sound from the vocal tract back into the ear via equipment designed to do this is counter-productive because it removes the natural ear response to vocal sound. Using ear plugs to reduce the intensity of the sound, while necessary in most rock music, is basically denying the sonic organization of the ear/voice system to function as intended. A proof of this is that most singers automatically sing out of tune when using such ear damping devices as ear plugs
Any of the most successful feedback systems mention above will improve ones singing quality and it does so without any kind of dependency on the system used. This is so because each of these systems are nothing more that an focused form of our natural listening response.