The tree falls, thus making a traveling wave (an oscillation of pressure) in the atmosphere. Since this is the definition of a sound,
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Actually this is one of MANY definitions given and one that I personlly don’t like very well. The sound that a person HEARS is his own perception of the energy contained in the oscillations in the atmosphere. A totally deaf person standing beside the falling tree would hear no sound, therefore I say that if there are no ears there is no sound.
To refer to vibrations in the audible range as sound is no more valid than to refer to vibrations in the visual spectrum as sight. If I close my eyes and concentrate I can “see” things that I have not been around in years and if I go into a silent, dark place and concentrate I can “hear” things I haven’t been exposed to in years. I hope you follow my meaning, it seems a little difficult to put into words.
If a blind man strikes a match that no one else sees, does it make a light? Well...yes. Whether or not anyone's eyes intercept the photons that are produced, the light itself still exists.
Similarly, when the tree falls, the traveling wave (the sound) still exists regardless of whether or not it interacts with anyone's ears.