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.
![[Note]](/images/2024.1.0_8669/?source=Help&id=note.png) | Note |
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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 want 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. |
To enable HDR in your project, enable it for one or more busses. HDR can be enabled on any bus except Auxiliary Busses and Master Audio Busses.
If you enable HDR on a bus, this is inherited by its children, and the option to enable HDR becomes unavailable on the child busses.
To enable HDR on an Audio Bus:
Open the Property Editor for the Audio Bus.
In the Property Editor, enable HDR. If it's not present, then HDR isn't available on this bus.
The volume of all sound structures routed to the HDR bus, or to one of its children, is affected by the HDR bus. Sounds that are not routed to the HDR bus are not, but can coexist with sounds routed to the HDR bus within your sound design.
As mentioned previously, sounds at the top of the HDR window will come out at 0 dBFS. Use the bus volume slider of the HDR bus to scale down the output of the HDR system before mixing it with other non-HDR sections of your project.
To have sound objects in the Actor-Mixer Hierarchy or Interactive Music Hierarchy use the HDR system, you need to route these objects to the HDR bus.
To route a sound to an HDR bus:
Inspect the sound object to show its content.
In the Property Editor, go to the Output bus group.
Select the HDR bus as the Output bus.
The Voice Monitor view displays the volume of voices and their envelope (if available).
To understand and debug the HDR system:
Open the Voice Monitor view.
Drag your HDR bus into the view's context.
Set the Mode to either Bus input or Bus output. Bus input mode displays voice levels in decibels as seen by the HDR bus, before the HDR compression, while Bus output mode displays them at the output of the HDR bus, after HDR compression and the bus output gain. Most figures in this document are images of the "two sides" of the Voice Monitor view.
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