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ELI5: Why can I “hear” perfect pitch in my mind, but when I try to sing I sound like a walrus giving birth to farm equipment?(r/explainlikeimfive)
This “hearing” is called audiation. While you might believe that the pitch you are audiating is perfect, in reality it takes training and practice to really sing on pitch well (although talent can do a lot for that) as the human ear can really only hold onto a pitch like that for about 7~10 seconds. For instance, many amateur/novice singers when holding a sustained pitch will tend to go flat (the pitch gets slightly lower, enough to go out of tune) unless they are aware of this and compensate for it. There is also much pedagogy on vocal technique to make sure your singing voice produces a clear tone, much of which might not occur to one on instinct.
Edit: Many of you wish to improve your singing or wonder why you can whistle or play pitches perfectly on another instrument and yet be unable to sing pitches accurately. I am a music ed major in college currently enrolled in eartraining and voice methods classes, which is where my knowledge comes from. For tone production with singing, the main thing is air support. I don’t mean just getting louder, I mean using your diaphragm to push more and faster air through your vocal chords. As for why you may not be able to sing on pitch but whistle, it is a different form of muscle memory, of knowing how to shape the throat and lips to produce a certain pitch, and most of you probably practice whistling a lot more than you might sing in public. Talking won’t do it, as singing uses much more air and shape the mouth and throat much differently. One last thing, many pop singers use unorthodox technique to achieve their tone which is considered ‘incorrect’ as my training is in a more classical choral style. If you would like to learn more about singing technique, here is the text I use in class. I hope you guys find all this helpful!
Edit 2: The question of whether or not just anyone can learn to sing keeps popping up. The text I use expresses the belief that anyone can be taught to sing with the right amount of eartraining and the correct vocal technique in the choral sense. That’s not to say that with enough training everyone could go make a competitive audition for a musical on broadway, as some aspects of singing such as range and the actual timbre of your voice are genetic, but they could realistically learn to sing in tune with a pretty decent tone.