This page provides an overview of the fundamental acoustic concepts related to Spatial Audio:

• Diffraction
• Transmission
• Obstruction and Occlusion
• Room Coupling

Diffraction

Diffraction occurs when a sound wave strikes a small obstacle, or the edge of a large obstacle or opening, and bends around it. It represents sound that propagates through openings (portals) and towards the sides, which means that a listener does not need to be directly in front of the opening to hear it. Diffraction is important in games because it informs players about paths that exist between them and sound emitters. The following figure is a sound field plot of a plane wave that comes from the top right and hits 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 top right is the Reflection Region, where reflection with the surface occurs and is mixed with the incident wave, which results in the jagged pattern. The lower right is the Shadow Region, where diffraction is significant. The figure is an approximation. In reality, 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.

Consider the edge to be a point source, with amplitude that decreases 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 with two of its APIs. Refer to Diffraction to learn how to use Rooms and Portals to model portal diffraction, and Using the Geometry API for Simulating Diffraction and Transmission to learn how to use geometry 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 transmission loss is the proportion of that energy that is dissipated by the obstacle. Transmission is distinct from absorption, which describes the proportion of energy dissipated by a reflected sound wave.

The interactions that occur at the interface of two media can be complex. The ratio of reflected to absorbed energy is defined by the properties of the surface of a material, whereas transmitted energy and transmission loss are related to the size, shape, and density of an obstacle.

When sound waves encounter obstacles made of dense materials, such as concrete, the proportion of energy that reaches the listener through transmission can be quite small compared to diffraction, particularly when there are openings nearby. However, if there are no openings nearby, or if the obstacle is made of less dense material, such as wood or glass, transmission is significant and is important to simulate.

Obstruction and Occlusion

Obstruction and occlusion are conditions that exist when something blocks the space between a sound source and a listener. Obstruction refers to a partial block, and occlusion refers to a complete block. When using Spatial Audio, we recommend that you focus on using geometry with diffraction and transmission to achieve the desired effect. However, it is possible to set obstruction and occlusion values as well, either in combination with Spatial Audio or without it. For more information on obstruction and occlusion, see Obstruction and Occlusion with Game-defined Auxiliary Sends.

Room Coupling

After sufficient time, a sound emitter produces a diffuse field that depends on the acoustic properties of the surrounding environment. In games, this is typically implemented through reverb effects with parameters that are adjusted to represent the associated environments. Diffuse fields also travel 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 typically model this by feeding the output of the reverb of a room into the reverb of another room.

Summary: Putting It All Together

Obstruction, occlusion, diffraction, and transmission are closely related to one another and interact in important ways in the context of Spatial Audio.

In summary:

• Obstruction occurs when an object or portal partially blocks the area between a sound source and a listener.
• Occlusion occurs when something completely blocks the listener from the sound source.
• Diffraction refers to sound waves that pass around an obstacle.
• Transmission refers to sound waves that pass through an obstacle.
• Therefore, in a situation where obstruction occurs, both transmission and diffraction are relevant because some sound waves bend around the obstacle and some pass through it. When occlusion occurs, only transmission is relevant.