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Wwise SDK 2021.1.14
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The following paragraphs provide a quick overview of the fundamental acoustic concepts related to Spatial Audio:
Diffraction
Diffraction occurs when a sound wave strikes a small obstacle, or when it strikes the edge of a large obstacle or opening, and bends around it. It represents sound that propagates through openings (portals) and towards the sides, meaning that a listener does not need to be directly in front of the opening to hear it. Diffraction is usually very important in games because it gives a hint to players about paths that exist between sound emitters and them. The following figure is a sound field plot of a plane wave coming from the top right and hitting a finite surface (the black line) that starts in the center of the figure. The perturbation caused by this edge is called diffraction. The region on the left is the View Region, where the plane wave passes through unaltered. The region on the top right is the Reflection Region, where reflection with the surface occurs and is mixed with the incident wave, resulting in this jagged pattern. The region on the lower right is the Shadow Region, where diffraction plays a significant role. This figure is just a coarse approximation; in real life the field is continuous at the region boundaries, and edge diffraction occurs in the View Region as well, although it is generally negligible compared to the incident wave itself.
We see that the edge can be considered as a point source, with amplitude decreasing with distance. Also, the amplitude of higher frequencies decreases faster than that of lower frequencies, which means that it can be adequately modeled with a low-pass filter. Wwise Spatial Audio models diffraction via two of its APIs. Refer to Rooms and Portals' Diffraction to understand how Rooms and Portals lets you model portal diffraction, and to Using the Geometry API for Simulating Diffraction and Transmission to understand how geometry may be used to model diffraction of emitters and their early reflections.
Transmission
Sound transmission is another relevant acoustic phenomenon that is modeled within Wwise Spatial Audio. Transmission describes sound energy passing through an obstacle, and the term transmission loss describes the proportion of that energy that is dissipated by the obstacle. This is not to be confused with absorption, which describes the proportion of energy dissipated by a reflected sound wave. While the interactions that occur at the interface of two media can be quite complex, the ratio of reflected vs. absorbed energy can be thought of as being defined by the properties of the surface of a material, whereas transmitted energy vs. transmission loss are related to the size, shape and density of an obstacle.
When dealing with obstacles made of a dense material, such as concrete, the proportion of energy that reaches the listener through transmission can be quite small when compared to diffraction, particularly when there are openings nearby. However, if no such openings exists, or if the obstacle is made of less dense material, such as wood or glass, the contribution of transmission becomes significant and is important to simulate.
Room Coupling
After sufficient time, a sound emitter produces a diffuse field that depends on the acoustic properties of the environment it is in. In games, this is typically implemented using reverb effects with parameters that are tweaked to represent the environment with which they are associated. Diffuse fields also make their way across openings and through walls until they reach the listener, where they excite the listener's environment. Room coupling refers to the transfer of acoustic energy, also known as reverberation, from one environment or room to another. Games typicaly model this by feeding the output of the reverb of a room into the reverb of another room.
Obstruction and Occlusion
Obstruction represents a broad range of acoustic phenomena, and refers to anything that happens when a sound wave strikes an obstacle. Occlusion is similar but implies that sound cannot find its way around an obstacle. The Wwise sound engine lets games set Obstruction and Occlusion values on game objects, which are mapped to a global set of volume, low-pass filter, and high-pass filter curves. The difference between the two is that Obstruction affects only the dry/direct signal between an Actor-Mixer or bus and its output bus, whereas Occlusion also affects the auxiliary sends. Obstruction, therefore, better emulates obstruction by obstacles when emitter and listener are in the same room, whereas Occlusion is better to model transmission through closed walls.
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