These are excerpts from MSVC manual and Crative's EAX2.0 manual, I hope
it will help you to undersand what I mean for quality:

"The Microsoft® DirectSound® application programming interface (API) is
the audio component of the DirectX™  Software Development Kit (SDK).
DirectSound provides low-latency mixing, hardware acceleration, and
direct access to the sound device. It provides this functionality while
maintaining compatibility with existing Windows®-based applications and
device drivers. 

DirectX  allows you access to the display and audio hardware while
insulating you from the specific details of that hardware. The
overriding design goal in DirectX  is speed. Instead of providing a
high-level set of functions, DirectSound provides a device-independent
interface, allowing applications to take full advantage of the
capabilities of the audio hardware. 

Perception of Sound Positions

In the real world, the perception of a sound's position in space is
influenced by a number of factors, including the following: 

•Volume. The farther an object is from the listener, the quieter it
sounds. This phenomenon is known as rolloff. 

•Arrival offset. A sound emitted by a source to the listener's right
will arrive at the listener's right ear slightly before it arrives at
the left ear. (The duration of this offset is approximately a
millisecond.) 

•Muffling. The orientation of people's ears ensures that sounds coming
from behind the listener are slightly muffled compared with sounds
coming from in front of the listener. In addition, if a sound is coming
from the listener's right, the sounds reaching the left ear will be
muffled by the mass of the listener's head. 

Although these are not the only cues people use to discern the position
of sound, they are the main ones, and they are the factors that have
been implemented in DirectSound's positioning system. When hardware that
supports 3D sound becomes generally available, other positioning cues
might be incorporated into the system, including the difference in how
high- and low-frequency sounds are muffled by the mass of the listener's
head and the reflections of sound off the listener's shoulders and
external ear parts. 

One of the most important sound-positioning cues is the apparent visual
position of the sound source. If a projectile appears as a dot in the
distance and grows to the size of an intercontinental missile before it
roars past the viewer's head, for example, the sound will appear to have
gone by the listener without much help from subtle cues. 
© 1997 Microsoft Corporation. All rights reserved. Legal Notices 

EAX 2.0 Environmental Reverberation Control 

EAX 2.0 offers a complete set of reverberation parameters that allow you
to control the intensity and delay of
reflections, to continuously adjust the environment size, and to set the
direct path and reverberation filters for
each sound source. These direct controls allow dedicated sound designers
to tweak environmental
reverberation to get precisely the aural surroundings they desire. EAX
can also make your programming work
easier by automatically taking care of these parameters for you. 

Distance Effects 

To help provide a better perception of listener-to-sound-source
distance, EAX 2.0 provides several different
modes of automatic distance-effects management. They manage attenuation
and filtering effects that change
according to the distance between the listener and a sound source. The
distance effects include: 

     A method of simulating the natural statistical behavior of
reverberation in enclosed spaces 
     A reverberation rolloff factor that allows you to customize or
modify the statistical reverberation model 
     An adjustable air absorption model that filters out high
frequencies with increasing distance. (You can use
     this to simulate moisture in the air such as high humidity or fog.) 

Occlusion and Obstruction Effects 

EAX 2.0 introduces occlusion and obstruction properties that provide
aural cues about objects and walls that
come between sound sources and the listener. 

Occlusion occurs when a wall that separates two environments comes
between source and listener. There’s
no open-air sound path for sound to go from source to listener, so the
sound source is completely muffled
because it’s transmitted through the wall. EAX occlusion properties
provide parameters that adjust wall
transmission characteristics to simulate different wall materials and
thickness. You can, for example, use
occlusion properties to make a voice or noise sound very realistically
as if it were coming from behind a door
or from outside the listener’s house. 

Obstruction occurs when source and listener are in the same room but
there’s an object directly between
them. There’s no direct sound path from source to listener, but the
reverberation comes to the listener
unaffected. The result is altered direct-path sound with unaltered
reverberation. EAX obstruction properties
can simulate sound diffraction around the obstacle or sound transmission
through the obstacle, which provide
rich aural cues about the nature of the obstruction. You can, for
example, use obstruction properties to make a
voice sound as if it were coming through a thin curtain or from behind a
large pillar. 

Source Directivity 

DirectSound already offers control of source directivity, where a sound
source is stronger in one direction than
in all others—like a trumpet, for example, that’s strongest in the
direction of the bell and attenuated to the
sides and behind the bell. DirectSound sets the directivity across all
frequencies equally, which usually not the
case in the real world. 

EAX makes source directivity sound much more natural by allowing you to
make sound sources more directive
at high frequencies than at low frequencies. This makes the sound of
directional sources such as a voice, a
loudspeaker, or a horn sound much more realistic as the listener moves
around the source—or as the source