The formants are the distinctive bands of energy in a speech sound. They can be made visible in a spectrogram, which graphs time from left to right, frequency going upward, and usually intensity by the darkness.

After some study you can actually read off what people are saying from spectrograms, with fair accuracy (well, in your own native language anyway). Each sound and class of sound has its characteristic shape: fricatives like s sh th f have a wide fuzzy diffuse burst of energy, while other continuant sounds, especially vowels, have fairly well defined bands of energy at specific places. These are called formants.

The bottom formant, which may be called F0, is at the basic rate of vibration of the vocal chords: around 100 or 125 Hz for men and 200 to 250 Hz for women. This is present when the sound is voiced, as all vowels are.

Next are the two main vowel-quality formants, F1 and F2. The first formant has its lowest frequency value, around 250 Hz, when the tongue is highest in the mouth: the vowels of 'beet' and 'boot'. As the tongue descends to the (physically) lowest vowels ('Bart', 'bot'), the value of the first formant increases to a maximum of about 1000 Hz. The second formant overlaps this, but corresponds to frontness or backness in the mouth. A back vowel as in 'boot' or 'bot' starts with F2 around 850 Hz and rises to 2500 Hz as the tongue moves forward to 'bat', 'bet', 'bit', and finally 'beet'.

A vowel where the two formants overlap is called compact; where they are far apart it is called diffuse. All languages without exception have a range of vowels that differ by both height and frontness of the tongue. That is, F1 and F2 are important in all languages.

There are other formants, caused by harmonics and by other more subtle shaping of the vocal tract. F3 is associated with other phonetic features of the language. Beyond this, higher formants are more associated with individuals: they let you tell one person's speech from another's.

A diphthong is a vowel sound that moves from one vowel position to another. So the formants will change during a diphthong.

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