In order to create sonic illusions through commercial reproduction systems (loudspeakers, headsets), and render virtual sound sources and environments, some acoustic phenomena has to be considered (mentioned before); in the case of headsets being used, Interaural Level and Time Differences are used, altogether with Head Related Transfer Functions (Sima, 2008); this is called Binaural Audio.
Moving outwards, using normal loudspeaker systems instead of headsets, other techniques such as Ambisonics, Wave Field Synthesis (WFS), Transaural Stereo, Vector Based Amplitude Panning (VBAP) and Virtual Microphone Control (ViMiC) have been developed to reproduce 3D audio. A description of the above mentioned techniques and comparison between them follows next.
Ambisonics is basically a microphone technique, which records sound pressure and directionality of 3D sound and then reproduces it on a B format using a number of speakers placed symmetrically around the listener, (Malham & Myatt, 1995) and (Pulkki, 2001), on a cubic array. This technique relies largely on the positioning of the loudspeakers used to reproduce the ambience and creates the so-called sweet spot, where the listening illusion is optimal.
Wave Field Synthesis is based on the work done by Professor A.J. Berkhout at the Technological University of Delft (T.U.D.) in Holland (Corteel & Caulkins, 2004). It basically takes a very large array of speakers placed on the listening room and reproduces sound through them in a synchronized fashion, such that phantom audio sources are created on places that do not correspond to the real speakers. These illusions are created based on the Huyghens’ principle (1678) and opposite to common Spatialization techniques, WFS creates the physical attributes of the rendered virtual sound sources, forgetting about the sweet spot.
Transaural Stereo is effectively the translation of Binaural Audio into a normal stereo loudspeaker system (Bauck & H. Cooper, 2002), which uses signal cancelation and interaction (crosstalk) between the speakers used, to make the sound arriving at each ear resemble the one that would do so by using the h previously described Headset Binaural Audio Technique. As it might seem clear to the reader, this method also produces a very sensitive sweet spot, making it less attractive.
Vector Based Amplitude Panning (VBAP), proposed by Ville Pulkki (Pulkki, VBAP, 2002), utilizes a maximum of 3 speakers working simultaneously playing a single mono sound. These speakers are arranged on a triangle facing the listener, and the phantom sound source can be placed within the boundaries of this triangle, the calculation of the amplitude on each speaker is computed by a vector calculation. To create a more realistic illusion, several triangles can be used surrounding the sweet spot.
The final technique I am going to describe is Virtual Microphone Control (ViMiC). This will be applied for the project purposes, and it is explored to detail on a separate section.
Further on this chapter:
22.214.171.124 COORDINATES SYSTEM
126.96.36.199 DELAY AND GAIN
188.8.131.52 SOUND ACQUIREMENT
184.108.40.206.1 BASIC FUNCTIONING
220.127.116.11.1 VIMIC MODULES
18.104.22.168.2 OUTPUT MODULES