HDR and distance attenuation

Distance attenuation curves can be designed as usual. However, be aware that there is a somewhat unexpected behavior caused by the HDR system. Recall that when you play a sound that is above the HDR threshold and is the loudest sound of the audio scene, it comes out of the system at 0 dBFS. If that sound is played back 50 meters away, but remains above threshold and remains the loudest one in your scene, it will still come out of the system at 0 dBFS. Consequently, you will be left with the impression that the attenuation curve does not work at closer distances. It is not the case. What happens is that the increasing input volume at closer distances is actually used to duck other sounds instead of increasing the output volume. If nothing else is playing, you will not notice this, but you need to keep it in mind when designing attenuation curves for louder sounds.

Figure 24. Effect of distance on loud sounds

Effect of distance on loud sounds

Here, the listener is steadily stepping away from a loud sound (in red). While the volume seems correct on the input side (left), the output (right) exhibits a plateau when the loud sound remains above the HDR threshold in spite of the increasing distance, giving the false impression that distance attenuation is not working. In fact, at closer distances, the volume offset exceeding the threshold affects other sounds (in blue) instead of the sound in question.

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