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Message: [QUOTE="Shamiso, post: 92513, member: 160"] A sound source moving sideways at constant speed can hit zero instantaneous Doppler shift at the exact point of closest approach. The source has not stopped. Its motion has simply become perpendicular to the listener for that instant, so none of its velocity is closing or opening the distance. This is the bit most fly-by explanations flatten into “fast object, big pitch bend.” Speed matters, but only the part aimed toward or away from you creates the Doppler shift at any given moment. A source can be absolutely hauling across the room and still have very little instantaneous pitch shift if most of its movement is sideways. ORBIT makes a useful practical example because its pitch movement comes from a changing propagation delay rather than a separate pitch shifter. It arrived among [B][URL='https://goldmidi.com/community/threads/ovni-audio-introduced-free-orbit-and-supernova-plug-ins.77888/']OVNI Audio's free spatial movement tools[/URL][/B], but the more interesting part is what happens when you stop treating the Speed knob as a direct “more Doppler” control. [HEADING=2]A fly-by is really a changing distance problem[/HEADING] Picture a source traveling along a straight path a few feet in front of you. Early in the pass, a decent chunk of its velocity points toward you, so the distance between you and the source shrinks quickly. Wavefronts arrive more frequently, and the pitch sits above the emitted frequency. As the source reaches the nearest part of the path, its direction of travel becomes sideways relative to you. Radial velocity falls toward zero even though total velocity stays the same. For a clean straight-line pass, the instantaneous Doppler shift crosses through the original frequency around closest approach before becoming a downward shift on the way out. Move the same path much farther away, and the geometry changes more gently. The source still travels at the same raw speed, yet its line-of-sight angle changes more slowly, so the pitch transition stretches out instead of snapping from obvious rise to obvious fall. A close pass sounds more dramatic partly because radial speed changes much faster around the nearest point. [HEADING=2]Radial velocity is the number your ears care about[/HEADING] Radial velocity is just the component of motion running along the line between source and listener. If a sound moves straight at you at 20 meters per second, nearly all 20 meters per second contributes to the shift. If it moves across your field at the same speed, only the toward-or-away component counts. This also explains why two paths with identical speed settings can sound nothing alike. One can spend most of its travel cutting across your listening position, while another spends longer charging toward you before peeling away. Same headline speed. Very different Doppler curve. The physics gets more useful once you stop thinking of the effect as a preset pitch sweep. A 2017 acoustic motion-estimation paper used the changing instantaneous frequency of a passing source to estimate its speed, source frequency, closest point of approach, and altitude. The shape of the pitch change contains information about the path, not merely a rough “moving fast” signal. Audio rendering can exploit the same relationship without bolting on an independent pitch processor. In [B][URL='https://www.sciencedirect.com/science/article/pii/S0003682X06002519']propagation-delay Doppler rendering[/URL][/B], changing the source-to-listener delay as the virtual source moves naturally creates the corresponding frequency shift. ORBIT follows this broad approach, with its manual describing Doppler as pitch emerging from a modulated propagation delay. [HEADING=2]Plugin controls can hide the geometry[/HEADING] A Speed control is still useful, obviously. Faster motion can create a larger Doppler shift because it raises the maximum possible radial velocity, but the trajectory decides how much of that speed points along the listener line at each instant. Cranking speed while keeping a broad, distant path can sound less extreme than expected. Distance controls matter for the same reason. Moving the path closer to the listener makes the line-of-sight angle swing harder around closest approach, which compresses the transition in time. Push the path farther out and the same travel speed produces a smoother, slower pitch contour. Circular and elliptical motion complicate things further because radial velocity keeps changing throughout the lap. Parts of the orbit may be moving strongly toward you, other parts mostly sideways, then later strongly away. A constant lap speed therefore does not imply a constant amount of Doppler. Your ears are not using pitch alone to decide when a source passes, either. In a controlled moving-sound experiment, amplitude change beat interaural timing and Doppler as the dominant cue for judging closest passage. A physically sensible pitch curve can still feel wrong if distance level or binaural timing stays static. For sound design, the practical move is to listen to trajectory before chasing a bigger speed value. If the fly-by feels weak, the source may simply be spending too much of its motion sideways or too far from the listener for the pitch curve you wanted. Tightening the path can change the result more decisively than making the source move faster. [/QUOTE]

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