The smallest sound wave that can be detected by the human ear is generally considered to be around 20 microPascals in pressure amplitude. This threshold is known as the absolute threshold of hearing and represents the minimum sound intensity that the average human ear can perceive under ideal conditions.
The unit used to measure sound pressure is the Pascal (Pa), and the threshold of hearing corresponds to a pressure of approximately 20 μPa (microPascals). It is important to note that this threshold can vary slightly between individuals and can be influenced by factors such as age, hearing ability, and background noise levels.
Furthermore, the minimum audible sound level can vary across different frequencies. Human hearing is more sensitive to sounds in the frequency range of around 2,000 to 5,000 Hz, while sounds at very low frequencies or very high frequencies may need to be of higher intensity to be detected.
Keep in mind that the human ear has an incredible dynamic range and can perceive sounds spanning from the threshold of hearing to levels as high as around 120-130 decibels, which is the threshold of pain.