Sound Definitions

Sound effects : 3D audio effects

3D audio effects are a group of sound effects that attempt to widen the stereo image produced by two loudspeakers or to create the illusion of sound sources placed behind, above or below the listener.

There are several types of 3D audio effects:

• Those that only widen the stereo image by modifying phase information.
• Those that can place sounds outside the stereo basis.
• Those that include a complete 3D simulation.

Stereo widening

Widening of the stereo image can be achieved by manipulating the relationship of the side signal S and the center signal C: C = (L + R)(1 / 2);S = (L − R)(1 / 2). A positive part of the side signal S is now fed into the left channel and a part with its phase inverted to the right channel. Some boom boxes feature such a process.

Placement of sounds outside the stereo basis

By manipulating parts of the sound according to psycho acoustic findings in phase and sound, it is possible to create sounds beyond the stereo basis. Effects from QSound Labs have been used on albums from Sting and Madonna in the beginning of the 1990s.

Complete 3D simulation

The 3D simulation is the most advanced group of 3D audio effects. Using head-related transfer functions and reverberation, the changes of sound on its way from the source (including reflections from walls and floors) to the listener's ear can be simulated. These effects include localization of sound sources behind, above and below the listener.

Some 3D simulation also convert binaural recordings to stereo recordings.

3D simulation effects emerged in the 1990s and are rarely used today, mainly because they were extremely expensive in the 1990s and the algorithms never made it into the mass market. Another problem is that the way sound is played back through speakers at the listener cannot be predicted - each listener setup is different (different room, different loudspeakers, different listening position). Some 3D simulation effects work reasonably well in a 180° circle, but localization of sound sources behind, above or below the listener through speakers is rarely convincing. When using headphones, stunning effects can be created however.

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