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3D audio effects are a group of sound effects that attempt to widen the stereo image produced by two loudspeakers or stereo headphones, or to create the illusion of sound sources placed anywhere in 3 dimensional space, including 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.

Widening of the stereo image can be achieved by manipulating the relationship of the side signal S and the center signal C: . 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.

Another way of looking at this same effect, without extrapolating a center and side signal from the left and right signals, is to simply add the left signal, slightly attenuated and phase inverted, into the right channel and vice-versa. Taking this a step further, a small delay (20-100ms) can be added to the inverted signal before mixing it back in to the original for output, adding a slight reverberation to the effect.

By manipulating parts of the sound according to psychoacoustic 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. Similarly, the pioneering work of researchers (Sibbald et al) at EMI Central Research Labs in England in the 1980s, and later with Sonaptic, produced "3D Audio" CDs.

A sound is placed in the horizontal plane by convolving the sound with recorded head-related impulse responses.

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 technologies also convert binaural recordings to stereo recordings.

3D Positional Audio effects emerged in the 1990s in PC and Game Consoles. Although some of the weaker technologies have problems, especially in playback through speakers, the best technologies work well in all planes, even above or below the listener, through speakers and headphones.

A site with animations and theory of a system using HRTF's to create 3D Audio: ISVR Virtual Acoustics.

• A&G Soluzioni Digitali markets the X-spat boX processor, a tool for complete 3D positional audio based on a patent pending algorithm.
• Behringer markets the Edison EX1 processor, a tool for widening the stereo image.
• Duy produces the DUY Wide software plug-ins, a stereo enhancer effect, allowing you to place the sound outside physically-possible positions according to the placement of your monitors. Available for TDM, RTAS, VST, MAS and AU.
• Maven 3D.com [1]market several versions of their 3D rendering software, including Maven3D professional. [2]
• Prosoniq markets the Ambisone VST plugin.
• QSound Labs markets QTools/AX, a product that lets the user place sounds outside the stereo basis.
• Roland corporation marketed the RSS-10 that cost over 5000 DM in 1995. It currently markets the RSS-303.
• Sensaura [3], technology is widely used on sound cards and by games to interactively position sounds in 3D.
• Sonaptic www.sonaptic.com specialise in Hi-Fi 3D Audio for mobile devices such as cellphones and mobile games consoles. Sonaptic's technology allows positioning of sounds in 3D space around the listener for high quality immersive games and music.
• Spatializer Audio Laboratories marketed the Spatializer that also placed sounds beyond the stereo basis.
• SRS Labs markets both professional products and the WOW Thing, a stereo image enhancer with a built-in exciter and a bass processor.
• Waves Ltd markets the S1 Stereo Imager, also for widening the stereo image.
• Yamaha Corporation markets the YMU786 (MA-7), an audio chip for mobile phones which installs a 3D simulator.
• Holistiks [4] markets the AMPHIOTIK range of Virtual Auditory Environment processing software.