The timbre of a sound is determined by a number of factors, including the overtones present in the sound, the attack and decay of the sound, and the resonance of the instrument or voice. In the case of the sitar and flute, the sitar produces a sound that is rich in overtones, while the flute produces a sound that is more pure and mellow. Additionally, the sitar has a longer attack and decay than the flute, and the resonance of the sitar is different from that of the flute.
As a result of these differences, the sound of a sitar and flute playing the same note can be easily distinguished from each other.