There are two main reasons, the more important of which Pieter already pointed out: the wavelength of sound waves is a million times bigger. (The speed is also different by a factor of about a million, but that doesn't change the spatial characteristics of how the waves behave as they travel from source to you, only how long that trip takes.) Sound in the human auditory range diffracts around a 10-m wall the same way light we can see diffracts around a “wall” with a height of 0.01 mm. The other, subtler reason is that we process auditory and visual signals very differently.Īll that really matters to your brain about the pressure signals coming into your ears is the spectrum-the amount of energy at each frequency. (Incidentally, in many cases scattering is more important than diffraction-when you hear a voice from the other room, much of that sound energy reaches you by bouncing off a wall or two.) So the signal can be diffracted (or bounce off irregular surfaces or whatever) without affecting your ability to extract information from it. Your eyes, on the other hand, don’t care about the spectrum-color is just some sort of average of all the constituent frequencies-but they do need light rays originating from various positions on some object you'd like to see to be traveling predictably along the lines from those positions. Otherwise no image of the object is formed.
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