When the tree falls it produces a series of pressure waves in the surrounding air. The ear drum converts these waves into a mechanical signal which is transmitted by 3 small bones to the fluid filled cochlea - the spiral bony canal of the inner ear. Hair cells of the cochlea are the actual receptors. Each is tuned to a particular frequency of the fl uid waves. What we call sound exists only in the mind of the perceiver.
Perception differs qualitatively from the physical properties of the stimulus.
The nervous system extracts only certain information from the natural world. We perceive fluctuations of air pressure not as pressure waves but as sounds that we hear. We perceive electromagnetic waves of different frequency as colors that we see. We perceive chemical compounds dissolved in air or water as specific smells or tastes. Sounds, colors, patterns, etc., appear to have an independent reality, yet are, in fact, constructed by the mind.