What it is
The energy difference between the first harmonic (H1) and the second harmonic (H2) of the spectrum, in dB.
It is a thickness gauge for the vocal-fold vibration pattern: the more fully the fold body (TA-dominant) vibrates, the stronger the low harmonics — H1 > H2 (a positive difference, thicker); when only the thin edge layer (CT-dominant) vibrates, the higher harmonics gain relatively — H1 < H2 (thinner).
Where it comes from
After the FFT, simply measure the dB gap between the first and second harmonic peaks.
It is insensitive to recording level (a relative measure) but fairly sensitive to the microphone’s low-frequency response.
How to read it
High values read thick (chest quality, a modal leaning); low values read thin (head voice, a falsetto leaning). High notes naturally thin out; if the low range also stays thin, the voice sounds frail. The TA/CT Balance chart draws it as a continuous curve.