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Now we obviously need to make sure that the bus hierarchy is split up and grouped in a sensible way. We don't want things to get messy, because it will circumvent what we're trying to do; which is keeping things nice and tidy and ready for ducking! Here is a very basic but common bus hierarchy breakdown:
So, at the top of the chain you have the Master Bus. Think of this as your volume control on your TV. If you adjust this, you adjust everything in the mix. Then you just follow the chain down from sub-bus to sub-bus. You would want to categorize similar types of sounds within the sub-busses. For example, you might want to put all physics-based sounds in a sub-bus under SFX (sound effects) and then all bullet impacts on a separate sub-bus under "Weapons," which is also under SFX.
Now in terms of ducking (specifically ducking when voiceover is occurring), you'd want to put all sounds that you do not want ducked in the same bus. As a practical example, we'll look at Lucasarts' Fracture.
Fracture was designed using the DESPAIR engine, a first generation technology engineered by Day 1 Studios. The audio engine is an amalgamation of Firelight Technologies' FMOD and Day 1's internal tool suite.
From an authoring standpoint, all of the voiceover files were brought into FMOD's Designer tool, placed in the VO_Story bus, and then had all priorities and parameters set from there.
After the audio pack files are built and ready to go we then open up Day 1 Studios cinematic tool, Outtake, and place the desired file in the soundtrack property:
As you can see, when this file triggers, a host of options are available to the user to essentially determine how the ducking will behave. In the case of this particular file, the ducker will ramp down all audio except the voice-over to the established target volume of -9 dB over the course of 500 milliseconds (or half a second). It will stay at that volume until the file ends and then it will ramp the rest of the audio mix back up to default volume over the course of 1000 milliseconds (or one full second).
By default, the ducker will engage at the beginning of each file, but there is an option to offset the start of the file to the ducker, so you could begin the audio ducking before the file begins to allow more of a preparation for the listener to really hear the intended sound.
It should be stated that ducking is a tool and technique that should be used wisely and in conjunction with other methods of mixing. If it's used too frequently or aggressively, it will become distracting for the listener. Sometimes people feel like this is an unnecessary feature in an audio engine, but given the unpredictable nature of player behavior I feel it's valuable to have every tool at the content creator's fingertips.
If used properly, it's a very powerful method of guiding what the player should be listening for and at its very root, helps tell the story of the game that you and your team have created.