What is the softest sound that humans can perceive in microPascals?

The softest sound that humans can perceive is typically referenced as the threshold of hearing, which is around 20 microPascals (uPa) in terms of sound pressure levels. This threshold represents the lower limit of the auditory system's sensitivity under normal conditions.

Sounds at this level are often described as being the faintest noises that an average person with normal hearing can detect in a quiet environment. Understanding this threshold is crucial in fields related to hearing science, audiology, and noise pollution because it helps establish benchmarks for hearing sensitivity and auditory health.

The other options represent sound pressures that exceed this threshold, making them detectable but not as subtle as the softest perceivable sound. Thus, 20 uPa is widely accepted as the reference point for the softest sound in the context of human hearing.