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It is important to realize that the envelope of a sound only has an effect on other sounds, and never on itself. This last characteristic is what mostly distinguishes the HDR system from an audio compressor. At any given time, the loudest sound is scaled by its peak value, but is unaffected by its own envelope, which contributes to making the HDR system sound transparent. In consequence, level differences between input sounds are not strictly preserved when envelopes are used. This has the benefit of blending sounds of varying "local loudness" together in the audio scene, as was discussed in the previous section. But you may find that the audio scene loses a bit of its realism when the HDR window moves wildly. In HDR imaging, the same situation arises when using a large amount of smaller tonal regions. Lots of examples of unrealistic HDR photos can be found on the web. We thus recommend that you proceed carefully in setting levels above the HDR threshold: do not just pluck in values blindly, but instead carefully set the level and design the envelope of your loudest sounds in order to make space for them in the mix.

Avoid enabling envelopes for nothing, as they require extra memory. Envelopes are useless for soft sounds that never go above the HDR threshold, so make sure to disable them.

Beware of envelopes that wiggle above and below the active range, as this might cause unwanted behavior. In the following figure, a long explosion sound goes in and out of the active range, provoking rapid changes of the HDR window. In this case, you might want to coarsen the approximation of the envelope and/or edit it manually. You may even want to proceed artistically and design an envelope that does not exactly correspond to the reality, by ducking the audio scene only during the initial impact of the sound, and recovering sooner during the rumbling part.

Simpler envelopes with fewer points result in less erratic amplitude changes of the audio scene, and this is often preferable.

Figure 30.21. Editing envelopes (a)

Figure 30.22. Editing envelopes (b)

Figure 30.23. Editing envelopes (c)

In (a), the envelope of an explosion sound is displayed in the source editor. The points circled in red are below the active range. During playback, the envelope goes in and out of the region of interest, resulting in rapid movements of the HDR window. The drastic effect on the volume of the background sound is very explicitly illustrated in the resulting output. To fix this problem, the envelope was edited manually in (b) by removing the two offending points. Recall that editing the explosion envelope only affects the HDR window, that is, other sounds, but not the explosion itself. For aesthetic reasons, the designer may also decide to duck other sounds only during the initial impact of the explosion, but let them take their full volume earlier, during the following rumble by editing the envelope somewhat like in (c). Interestingly, in all three examples the explosion is played back identically.