Working with amplitude envelopes

Since the HDR system works with logical volume values set in Wwise, it is unaware of the actual amplitude of the input sounds, and therefore sees them as black boxes with constant volume throughout the duration of the sound. Most of the time, the amplitude of sounds vary. Imagine an impact sound with a transient and a decaying part. If this sound were loud enough to fix the position of the HDR window, this position would remain unnaturally constant during the whole duration of the sound. See the resulting effect on the window in Figure 16, “HDR window with a decaying impact sound without envelope”.

In Wwise, it is possible to let the HDR system peek into the black box by enabling envelope tracking. In the HDR tab of the desired sound's properties, select the Enable Envelope check box: the amplitude envelope of the audio file is then analyzed and is attached to the sound's metadata. At run-time, the HDR system uses this data to move the window appropriately, as can be seen in Figure 17, “HDR window with a decaying impact sound with envelope, playing above a softer, steady background sound”.

For the following figure, the input is on the left, the output on the right and the corresponding output wave is below.

Figure 16. HDR window with a decaying impact sound without envelope

HDR window with a decaying impact sound without envelope

Figure 17. HDR window with a decaying impact sound with envelope, playing above a softer, steady background sound

HDR window with a decaying impact sound with envelope, playing above a softer, steady background sound

Notice the lull after the impact sound in the first figure above, caused by the HDR system interpreting it as being constant.

Envelope sensitivity and manual editing

Once you enable the Envelope, you can preview the result by opening the source editor (next figure). If you are unhappy with the precision and/or number of tracked envelope points, you can change the value of the sensitivity slider in the Envelope Tracking group (see Figure 19, “HDR tab in Property Editor”). You can work with a group of sounds by enabling the envelope and setting the sensitivity on a higher-level sound structure, an actor-mixer for example. Note that the effect of the sensitivity is dependent on the original audio data, and may therefore be different for different variations of similar sounds. You can also edit the Envelope manually for each sound; set the sensitivity control so that you obtain a curve that is close to the desired result, and move, remove, or add new points.


The tracked envelope points only appear on the waveform if the sound is routed to a bus that has HDR enabled.

Figure 18. Source editor in RMS mode, with Envelope display

Source editor in RMS mode, with Envelope display

Figure 19. HDR tab in Property Editor

HDR tab in Property Editor

Region of interest: active range

It has been shown that in typical cases, sounds that have large level differences such as an impact sound that ranges from 0 dB to -96 dB should not drive the HDR window for their whole duration, but should instead do so only during a limited period of time. The rationale for this comes from the following paradox: HDR acts as a priority system based on loudness, but the softer parts of a loud sound should not be given as much importance. For example, the decaying tail of a grenade sound has very low importance compared to the transient, and thus should not cover up the impact of a shotgun, even though the former is technically louder, on an absolute scale. In this case, the region of interest of the grenade sound should be limited to its first impact. It moves the HDR window up and therefore ducks everything else. On the other hand, once in the tail and playing say, 12 dB below the peak value, it is usually not desired to continue ducking the other sounds, even if the grenade's volume minus 12 dB is still above the level of all the other sounds in the audio scene. Restricting the HDR window control to a region of interest can be regarded as a way of gracefully blending together various "tonal regions" to continue with the HDR imaging analogy.

In Wwise, the way to define the region of interest is to declare a range in decibels from the peak of the sound, the active range. When the sound's envelope drops below the active range, the sound is considered outside of its region of interest, and is not considered by the HDR system as being able to drive the HDR window. Figure 20, “Active range” illustrates this with three similar sounds having different active ranges.

Figure 20. Active range

Active range

The same decaying sound is played three times with active range set to 96 dB, 12 dB and 6 dB respectively, above a steady background sound. When the sound drops by that amount of decibels below its peak, the window stops following it and instead releases back to idle. The movement of the window distinctly affects the level of the background sound. On the other hand, the window or active range has no impact on the decaying sound itself.

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