SPR: Singer's Formant Ratio (Projection)
SPR = energy in the high band (2–4 kHz) ÷ energy in the low band (0–2 kHz). It measures whether your voice "carries".
In a trained singer's voice there is a concentrated "singer's formant" around 2.5–3.5 kHz — an acoustic "lens" created by opening the pharynx. SPR quantifies it: SPR = 10×log₁₀(high-band energy / low-band energy). The closer to 0, the brighter and more "metallic" the voice. An orchestra's energy happens to dip right at 2.5–3.5 kHz — so the singer's formant lets a voice cut through the band.
Experiment 10 · Open the Pharynx, Grow a High-Frequency Peak
Drag the "pharynx opening" slider: the pharynx (orange region) widens, an energy bump appears in the 2–4 kHz region of the spectrum (orange zone), and the SPR value updates live. Note: the pharynx is the vertical chamber behind the oral cavity (behind the tongue root, above the larynx). Opening it = the pharyngeal wall moves back + the soft palate lifts, while the oral cavity stays the same size — it is not "a bigger pharynx means a smaller mouth".
SPR = the "projection reading" of your voice. It is the core metric of the app's compression chart (40% weight). Related: singer's formant chart