Organ pipes are fascinating instruments that convert continuous energy into oscillating energy thanks to a feedback loop. The exciter of the pipe transforms continuous energy into oscillating energy, while the resonator filters and restores the oscillating energy. The physics of flue pipes involve the interaction between an air jet and the acoustic vibration of the air column. The pipe acts as a reservoir and stores energy at specific frequencies called resonance frequencies. The shape of the pipe and the upstream pressure can affect the sound quality and timbre. The tremulant is a device that modulates the air flow and can affect the pitch, intensity, and timbre of the sound. In Organteq, the upstream pressure modulation is directly input into the model to generate variations in pitch, intensity, and timbre.
https://www.modartt.com/organteq_physical_modeling