A formant is a distinguishing or meaningful frequency component of human speech. By definition, the information that humans require to distinguish between vowels can be representated purely quantitatively by the frequency content of the vowel sounds. Formants are the characteristic harmonics that identify vowels to the listener. Most of these formants are produced by tube and chamber resonance, but a few whistle tones derive from periodic collapse of Venturi effect low-pressure zones. The formant with the highest energy is called f1, the second f2 and the third f3. Most often the two first formants f1 and f2 are enough to disambiguate the vowel. Sonograms are used to visualise formants.