If a Tree Falls in a Forest...How Does Sound Work?

If a tree falls in a forest and no one is around to hear it, does it make a sound?   Such a deep and troubling philosophical question regarding the nature of reality and perceived reality!  Too bad this question has become something of a cliche as the thought experiment it is meant to provoke is actually quite intriguing.

Today I'm not looking at the philosophical side of the question.  Instead I am looking to answer the question scientifically.  And the answer is no.   Well mostly no.  Okay so really the answer is two thirds "yes" and one third "no".  To understand the answer to this question we must first understand what makes a sound.

Sound is a wave.  Waves can be best described as disturbances or a transfers of energy.  An example of disturbances resulting in sound waves are the vibrating vocal chords in a human's throat agitating air between a speaker and a listener. In order for a sound wave to exist, there must be a medium for it to travel though.  In the given example, air acts as the medium through which sound travels.  Remove the medium, and the sound waves have no way to travel.  This is why there is no sound in the vacuum space.  It is also why the space battles in Star Wars are a lot less impressive when they to more accurately portray the laws of physics.

Watch a higher quality version here.

Going back to the question regarding trees in the forest, we can see we have the first two parts of a sound.  The force of the tree hitting the ground would most certainly cause disturbances resulting in sound waves, and as we can't have a forest without an atmosphere there is most assuredly a medium for the sound to travel through.  So what are we missing in this scenario?

Here's a picture detailing how the little wizards in your head make sounds.

This is where science starts to sound a lot like existentialism.  See in order for a wave to be considered a sound wave it has to be heard.  To put it simply, sound is the brain's invention for perceiving and differentiating different frequencies and pitches of sound waves or vibrations in the air.  When a tree falls it causes vibrations in the air, assuming there was a human within a range close enough, the vibrations would travel through the air hitting the eardrum of the person, which would then cause an internal bone to vibrate in a certain way.  The person's brain would then interpret the vibrations as an almighty CRASHing sound.  Without the human (or other animal) brain to interpret the vibrations caused by the falling tree are just vibrations.

Boom.  That was the sound of my mind being blown.

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