From Dan Wentz, Audio Designer:
The Descent series has always been a very immersive experience for me. Back when there were very limited resources to work with, we still found enough bandwidth to include simple but important ambient audio cues for entities such as exhaust fans, shield barriers, and interacting with lava. These were each addressed with very short, mono single channel loops. But the levels themselves were barren of most ambient atmosphere. Looking back, that was one of the things I wished we could have addressed give more time and resources. Especially in the single-player game, there are likely to be many situations of extended exploration that need to be filled in with appropriate atmosphere. What happens after you clear an area out? What’s left to keep you in the suspension of disbelief otherwise called a “game”?
Effective audio ambiance plays an important role in setting the game world atmosphere. To be most effective, it needs to be implemented in a creative way such that it is contained to specific areas, yet appears to emit from all around you when you are at its point of origin. In order to do this, we needed a way to effectively blend between two different listening perspectives for the same ambient emitter. Those perspectives are commonly referred to as 2D stereo and 3D mono. This is one area where Unity really helps because it has tools to address this exact issue.
When an ambient audio emitter is placed in the Unity engine, we have many parameters to help automate and control how it is played. In particular, the Spatial Blend parameter is the key to solving this duel perspective issue. With distance and volume graphed to X and Y respectively, we can control additional parameters based on how far you are from a sound. So at a distance of 0, you hear a 2D stereo loop. As you move away, the Spatial Blend parameter essentially collapses that stereo emitter as it attenuates (makes softer) and by the time you reach its outer most casting distance, you’re now hearing the same emitter as a mono 3D sound.
I’ve provided a sample movie to help demonstrate how this works in game with the ambient noise floor slightly louder to help demonstrate. Notice how sounds “open up” as I get closer to the center of each respective area? I believe techniques like this don’t happen by accident and it’s nice to see the guys at Unity thought to include tools to address this problem!