In Wwise, users need to select a bus to act as a converter between HDR levels and full (device) scale. The input levels that this HDR bus consumes are the logical levels that you set in Wwise. Thus, sounds routed to an HDR bus can have their volume set far beyond 0 dB. The only thing that counts is their position within the HDR window, which is placed dynamically by the HDR bus according to what is playing. The HDR bus therefore acts as a logical limiter/compressor by exposing controls that are similar to those of an audio limiter/compressor. It has ballistics (infinitesimal attack, user-defined release) to control how the HDR window slides in time. It also has a threshold, which can be seen as the lowest possible position of the HDR window.
|Usage of dB SPL
In prior literature, HDR audio systems often express input side volume levels in terms of Sound Pressure Levels units (dB SPL). dB SPL are a measure in decibels whose reference (0 dB SPL) corresponds to the threshold of human hearing. The notion of SPL does not exist in Wwise because it adds unnecessary complexity, pollutes the interface, and does not make the system more usable. Instead, the input side reference is left arbitrary, and it is up to you to define it. If you wish to work in dB SPL, you can set the volume of sound structures to positive dB SPL values directly. On the other hand, the default range of volume sliders in Wwise goes up to +12 dB only, so it might be more practical to choose another reference, and make the necessary subtraction to find the corresponding relative dB level from the desired SPL value. For example, you may decide that 100 dB SPL is your reference at 0 dB. Then a sound at 80 dB SPL needs to have its volume slider set to -20 dB, a sound at 130 dB SPL needs to have its volume slider set to +30 dB, and so on. You also need to set the HDR bus threshold accordingly.