Beyond hearing your own heartbeat after a while, an ultimate psycho-acoustic experiment: hearing air molecules hit your eardrums. Taken from Master Handbook of

Beyond hearing your own heartbeat after a while, an ultimate psycho-acoustic experiment: hearing air molecules hit your eardrums. Taken from Master Handbook of Acoustics 5th edition (page 39-40): The human ear cannot detect sounds softer than the motion of air particles on the eardrum. This is the threshold of hearing. There would be no reason to have ears more sensitive, because any lower-level sound would be drowned by the air-particle noise. This means that the ultimate sensitivity of our hearing just matches the softest sounds possible in an air medium.