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HDR in Wwise can be enabled for any parent Audio Bus in the in the Master-Mixer Hierarchy. Once enabled, the Audio Bus acts as a converter between Sound Object volumes at the input of the HDR bus and full (device) scale at the output of the same bus. All sounds that are routed to it are handled relative to each other within the HDR bus, with the output of quieter sounds constantly modified according to the properties of the system.
The controls of an HDR bus are similar to that of an audio compressor. The properties of Threshold, Ratio, and Release Time are used to modify the behavior of the project-specified dynamic range window. At run-time, the authored system dynamically maps this wide range of levels to a volume range that is more suited to your sound system's output.
In real life, the audible dynamic range, defined by the loudest possible sound and the threshold of human hearing, is several times wider than the dynamic range offered by speakers at game play levels. The role of the Wwise HDR system is to collapse or "compress" this real life dynamic range into roughly 40 dB (70 dB SPL for TV/Music listening minus 30 dB SPL for the room noise level).
The process is a sort of behavioral compression. It affects your mix by making soft sounds inaudible as soon as loud sounds play, and then making them audible again when the soft sounds play alone. The relative levels of sounds between one another remain intact and add clarity to the mix by playing fewer sounds.
In prior literature, HDR audio systems are presented as having SPL values directly assigned to each individual sound. Wwise removes the notion of SPL, and instead focuses on relative mixing. Hence you will not find a SPL slider anywhere in Wwise; only relative decibel values are used. If you wish to use real-life SPL values into the system, then chose a value that will act as a reference, and perform the necessary subtraction to find the corresponding relative dB level. For example, you may decide that 100 dB SPL is your reference at 0 dB. Then a sound at 80 SPL should have its volume slider set to -20 dB, and a sound at 130 dB SPL should have its volume slider set to +30 dB.