The ear senses air molecules (sound waves) hitting your ear from the vibration of a physical object. A tuning fork creates vibrations that have sound waves that bounce back and forth. If you plot the position of the tune, it looks sinusoidal. Sets up a pattern of rising and falling in the pressure of the air molecules that are pushed and rari ed due to the vibrations. The stimulus for audition is the changes in the pressure of the air due to some sort of vibration. You need a medium for the change of pressure to go through. You cannot produce sound in a vacuum or in space, as there is no medium. You can hear in water, the water in the medium though which the rising and falling of pressure is occurring. The sounds in water appear louder when you"re close to it, but the waves don"t travel as far as in air.