Hearing detects pressure changes, not absolute ambient pressure

Evidence

Fluctuations of pressure above (condensations) and below (rarefactions) the average pressure, arriving at the surface as sound, cause a very small increase or decrease in the number of collisions per second, and a corresponding tiny but measurable change of force on the surface. These fluctuations above and below the ambient pressure are called the pressure amplitude δP, where the total pressure is P = P0 + δP, and P0 is the ambient pressure. Usually only the amplitudes matter to us; it is changes in pressure that we hear, not the ambient pressure. We (and other animals) however are spectacularly sensitive to these changes; a pressure fluctuation of just a few parts in a billion (a few billionths of an atmosphere) is enough for us to hear if it happens fast enough. — Eric J. Heller, Why You Hear What You Hear- An…, 46

Categorization

• DEFINE: Defines pressure amplitude δP and establishes that the ear responds to changes in pressure, not ambient level.
• CANDY: The fact that we can hear pressure fluctuations of a few parts per billion is a striking, tweetable number.