Air Band Enhancer

Two complementary brightness tools in one render. The air shelf is the classic mastering move: a wide high shelf (8/10/12/15 kHz corners, up to +9 dB — the same corner positions as the Mäag EQ4 Air Band selector) that lifts what is already there. The exciter is what a shelf cannot do: following the original 1979 Aphex Aural Exciter block diagram, everything below a tuneable 1.5–4 kHz corner is removed with a 2nd-order Butterworth high-pass, the remainder is pushed 9 dB into an asymmetric biased-tanh soft clipper (val(x) expressions evaluated per channel), so the surviving tones sprout new octave and fifth harmonics an octave or more above the original content, and the distorted band is re-blended with the dry signal at a low mix level (0–60%) exactly like the Aphex Mix control. Because the harmonics ride on the music’s own high-frequency energy, the effect stays level-balanced instead of just getting louder — a limiter guards the summed output anyway. Before and after the render the tool measures the air band energy around the shelf corner and the harmonic growth in the 8.5–11.5 kHz zone with an FFT probe and reports both deltas, so you can see the shelf lift and the synthesized harmonics separately. If there is essentially no content above the exciter tune point (below −75 dBFS), the input is rejected instead of producing hiss-shaped nothing.