Human hearing operates near the physical noise floor of air itself

Evidence

At 4 kHz, which is about the frequency of the sensitivity peak, the pressure amplitude variations caused by the Brownian motion of air molecules, at room temperature and over a critical bandwidth, correspond to a sound pressure level of about −23 dB. Thus the human hearing system is close to the theoretical physical limits of sensitivity. In other words, there would be little point in being much more sensitive to sound, as all we would hear would be a “hiss” due to the thermal agitation of the air! Many studio and concert hall designers now try to design the building such that environmental noise levels are lower than the threshold of hearing and so are inaudible. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 645

Categorization

• DEEPTHINKING: This is a genuinely paradigm-shifting idea: the auditory system is not merely sensitive but physically optimal, capped by thermodynamics itself — the same Brownian motion argument that floors other sensory signaling.
• ADDENDUM: Relevant to the silence book: giving a room a noise floor below the physiological threshold of hearing is exactly the acoustic-design goal that enables true-quiet spaces like anechoic and Orfield-style chambers.
• QUESTION: The excerpt is cut mid-sentence at ‘second limit’ — worth checking whether later pages give the just-noticeable amplitude change figure for this hearing-peak frequency.
• SEED: Could angle an essay on ‘the universe hisses’ — thermal noise as a fundamental floor shared by biological and engineered sensing.