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Seen from a purely physical point of view, every note is the sum of harmonic notes or overtones. Every note is accompanied by a certain series of overtones. If this series changes while you are singing, the mood of the note also changes. The harmonic series consists of a large number of overtones, and some of them are continually dropping out or slipping. This does not matter, provided that the notes are still precise and sound in tune. Sensitive large-diaphragm studio microphones in particular (e.g. Neumann U 87) make it possible to hear these harmonic notes very precisely. Whereas in normal room environments a certain lack of precision is permitted and is hardly noticed, in the sound studio on the other hand it is possible to clearly hear that the sound is slightly out of tune – and you have a problem. Before you get confused, in the next lesson you'll learn more about "Singing on Stage" and "Singing in the Studio".