Speech School
Dark vs Bright
A major speech pathology common for men is overly dark phonation. The darkness or brightness of the timbre refers to how the energy is distributed around the harmonics, the brighter the timbre the more energy is distributed to higher harmonics.
The brightness of phonation is a result of vocal cord tension. Higher vocal cord tension carries more energy to higher harmonics resulting in a brighter timbre.
The dark phonation pathology can be seen by fat harmonics in your first formant. To correct this you must try to strain your larynx muscles while keeping the loudness and pitch of your phonation the same.
Most harmonic sounds have harmonics that decrease energy from the first harmonic. However, when belting one can phonate such that initial the harmonics have increasing energy such that the first format disappears.
Bright have a high second formant, where as dark vowels have a low second formant. Hence Palatal and Velar vowels are bright and Pharyngeal vowels are dark.
However, it is more complicated because some articulations of the pharyngeal vowels have a very large third formant around the same position of the second formant of the i (fleece) vowel, and so would be called bright.