From Mods Repository Wiki
In the interest of helping you understand our tools and pipelines better, it may be good to see how we go about thinking about putting audio into our games. First and foremost, we needed a toolset that would help us create more realistic sounding games that support 5.1 and 7.1 natively, include more robust ambient or environmental audio support, and that have enough dynamic control over playing sounds so that we can manipulate what you hear in the same ways that movies do. To that end, we've come up with some new essential pieces of technology that are outlined here so you can mod them and hopefully do some amazing work with sound.
To get the most out of the new sound engine, you'll need to understand what these new technologies and processes do, and how they work together.
- Fully Dynamic Audio Occlusion - This is a system that filters high frequencies out of sounds that are played behind obstructions (which is actually a better name for what we're really doing here, but that's a different discussion). In other words, sounds that are blocked by props or terrain sound muffled. Longer sounds that continue to play as you round corners will be muffled only when you keep obstacles in between you and the sound source. So, as you play, you'll notice sounds constantly changing based on what you are doing.
- Environmental Audio System - This is a complete system utilized within Radiant that allows full control over a level's ambient sound. The system serves one primary purpose - make the level sound detailed and extremly positional in stereo, 5.1, or 7.1. This system completely removes the need for the old school background track...the game can now dynamically render this audio in realtime, giving you much greater control over how each area sounds and is fully customizable. The system allows you to control reverb (including custom presets), occlusion overrides, Ambient Accents, and RoomTones (if you want one). The system itself is extrodinarily simple to use, but also very powerful.
- FLUX - Flux is a system we developed that simulates reflections (sound bouncing off of terrain\objects) and shockwaves (sound eminating from loud sources---like grenades and other explosions). This system keeps sound moving constantly, instead of always remaining static. You can hear an example of this system in action by listening to the sounds of the sniper rifles in the game. The decay from the shot is no longer static...it moves throughout the environment and keeps everything sounding fluid and dynamic.
- Multi-Layering - This is less a piece of technology, and more of a process, however, we have implemented a method that allows things like gunshots to be multi-layered (some of the guns in the game have 5 or 6 layers, just for the shot). These layers can be blended by the audio engine in real-time, which allows for sounds to have distant, medium, and close range sounds that all come in and out seamlessly when you move through an environment.
- Event Based Music System - We've developed a music package system that allows you to define music states that have oneshot in, loop, and a oneshot transition out when states chage. This gives you a lot of flexibility in creating musical landscapes that are as nonlinear as your level.
- Fully Scriptable Busing System - This system allows you to define bus states that can control multiple sound files simultaneously in terms of volume for dramatic effect.
- Built in Ring Modulator DSP - Perfect for special effects.
- Low Pass Filter DSP - For both occlusion\obstruction effects and for distance filtering.
- Dynamic Channel Management System - This is a dynamic real-time channel management system that ensures that the only channels being shut off are channels that are playing the most quiet sounds. This reduces popping and noise caused when sounds are shut off improperly. It also allows us to increase the range at which you hear sounds dramatically.
- Instance Limiting - This system gives you the ability to control how many instances of a single alias are allowed to exist, and what to do in case more than the limited amount try to play. This gives you great static control over your channel useage and helps with phasing sounds.
- Randomize Type - This system was designed to keep sounds from phasing so much when there are several of the same sound playing at once. In COD games you tend to have multiple weapons firing simultaneously, most likely of the same type. Without this system, you would have little control over phasing---this system allows you to define certain aspects of a playing sound alias, so that if another of the same alias fires, you can give yourself great odds that it will not phase. No solution is going to be 100% on this, but the system allows for great flexibility. It is also used to kill the 3rd person pitch variance on 3rd person SMG's.
- Reverb Wet\Dry Curves; Reverb Max Distance - This gives you the ability to modulate wet signal reverb over distance.
Sources: Treyarch's Wiki