This module corresponds to section 3.5.2. (Velocity Dependent Doppler Effect) of the OpenAL Specification and Reference (version 1.1).
The Doppler Effect depends on the velocities of source and listener relative to the medium, and the propagation speed of sound in that medium. The application might want to emphasize or de-emphasize the Doppler Effect as physically accurate calculation might not give the desired results. The amount of frequency shift (pitch change) is proportional to the speed of listener and source along their line of sight.
The Doppler Effect as implemented by OpenAL is described in detail in section 3.5.2 of the OpenAL 1.1 specification. Note that effects of the medium (air, water) moving with respect to listener and source are ignored. There are two API calls global to the current context that provide control of the Doppler factor and the speed of sound. Distance and velocity units are completely independent of one another (so you could use different units for each if desired).
dopplerFactor is a simple scaling of source and listener velocities to
exaggerate or deemphasize the Doppler (pitch) shift resulting from the
dopplerFactor to a negative value will result in an
ALInvalidValue error, the command is then ignored. The default value is 1.
The implementation is free to optimize the case of
zero, as this effectively disables the effect.
speedOfSound allows the application to change the reference (propagation)
speed used in the Doppler calculation. The source and listener velocities
should be expressed in the same units as the speed of sound. Setting
speedOfSound to a negative or zero value will result in an
error, the command is ignored then. The default value is 343.3 (appropriate
for velocity units of meters and air as the propagation medium).