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Creating the illusion of distance & moving sources outdoors
I'm wondering the best way to go about this. What should I study/understand about acoustics? I understand that higher frequencies become less apparent with distance (because they run out of energy quicker), but is this a proportionate change? Perhaps there's software that can affect this better than I could ever do manually?
The human auditory system has only limited possibilities to determine the distance of a sound source. In the close-up-range there are some indications for distance determination, such as extreme level differences (e.g. when whispering into one ear) or specific pinna resonances in the close-up range.
The auditory system uses these clues to estimate the distance to a sound source:
• Sound spectrum : High frequencies are more quickly damped by the air than low frequencies. Therefore a distant sound source sounds more muffled than a close one, because the high frequencies are attenuated. For sound with a known spectrum (e.g. speech) the distance can be estimated roughly with the help of the perceived sound.
• Loudness: Distant sound sources have a lower loudness than close ones. This aspect can be evaluated especially for well-known sound sources (e.g. known speakers).
• Movement: Similar to the visual system there is also the phenomenon of motion parallax in acoustical perception. For a moving listener nearby sound sources are passing faster than distant sound sources.
• Reflections: In enclosed rooms two types of sound are arriving at a listener: The direct sound arrives at the listener's ears without being reflected at a wall. Reflected sound has been reflected at least one time at a wall before arriving at the listener. The ratio between direct sound and reflected sound can give an indication about the distance of the sound source.” From Wiki/Google.
Then too outside sources are completely “dry”, devoid of any reverberation (unless you want to simulate a large reflecting object. Note too that we retain the ability to localize sources down to very low frequencies outdoors, ‘tis only the “mess” inside a building that prevents us doing this.
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