|Microsoft DirectX 9.0|
DirectSound automatically creates Doppler shift effects for any buffer or listener that has a velocity. Effects are cumulative: if the listener and the sound buffer are both moving, the system automatically calculates the relative velocity and adjusts the Doppler effect accordingly.
In order to have realistic Doppler shift effects in your application, you must calculate the speed of any object that is moving and set the appropriate velocity for that sound source or listener. You are free to exaggerate or lessen this value in a particular case in order to create special effects. You can also globally increase or decrease Doppler shift by changing the Doppler factor.
The Doppler factor can range from DS3D_MINDOPPLERFACTOR to DS3D_MAXDOPPLERFACTOR, defined as 0.0 and 10.0 respectively. A value of 0 means no Doppler shift is applied to a sound. Every other value represents a multiple of the real-world Doppler shift.