Often it is believed that the pitched sounds (like piano,organ,choir,etc.) for a single note have a frequency, it's actually harmonics and nothing more. Many people try to synthesize a sound using an exact frequency+harmonics and observe that the result sounds too “artificial”. They might try to modify the harmonic content, add a vibrato, tremolo, but even that doesn't sound “warm” enough. The reason is that the natural sounds don't produce an exact periodic; their sounds are quasi-periodic. Please notice that not all quasi-periodic sounds are “warm” or pleasant.