Originally Posted by UltraMega
HRTF is built into windows now I believe, and it was always separate from EAX, which is just reverb/echo. Games often sounded better on dedicated sound cards back then in ways beyond just the audio quality, and Creative basically had the whole sound card market to themselves for a long time so any computer with a sound card probably supported EAX, but EAX only did the reverb while the sound card did other things to make games sound better.
Advanced EAX was more than just reverb/echo, multiple environment (which includes environment filtering, environment panning and environment morphing) was the one stand-out feature...
Here's a description of multiple environment:
EAX ADVANCED HD adds Multi-Environment, which renders the sounds in multiple environments at the same time as you walk past a room with a different environment. It also incorporates Environment Morphing, which blends the sounds of two environments together as you walk from one room into another.
Doom3 I believe employed some of the multi-environment effects.
Here's a breakdown of the versions of EAX:
First, keep in mind most cards, and even most onboard sound solutions support the EAX 2.0 spec:
32 simultaneous voices processable in hardware
Material-specific reverb parameters
64 simultaneous voices processable in hardware
'Smoothing' between 3D audio environments
Direct access to all reverb parameters
New reverb engine
Beginning of the AdvancedHD Designation from new reverb engine
Real-time hardware effects
Multiple simultaneous environments
Ring modulation effects
128 simultaneous voices processable in hardware and up to 4 effects on each
EAX Voice (processing of microphone input signal)
EAX PurePath (EAX Sound effects can originate from one speaker only)
Environment FlexiFX (four available effects slots per channel)
EAX MacroFX (realistic positional effects at close range)
Environment Occlusion (sound from adjacent environments can pass through walls)