Sound pressure is measured in decibels (dB), which is a logarithmic unit used to express the intensity or power ratio of sound. The formula to calculate sound pressure level (SPL) in dB is:
SPL (dB) = 20 * log10(P / Pref)
Where P is the sound pressure being measured, and Pref is the reference sound pressure. The commonly used reference sound pressure in acoustics is 20 micropascals (μPa), which is the approximate threshold of human hearing at a frequency of 1,000 Hz.
The perception of loudness in human hearing is not directly proportional to sound pressure level in decibels. Instead, loudness perception follows a nonlinear relationship with changes in sound pressure. In general, the perceived loudness of sound increases logarithmically as the sound pressure level increases.
In terms of acoustic engineering, a doubling of sound pressure results in an increase of approximately 6 dB. This means that an increase of 10 dB is perceived as roughly double the loudness to the human ear. However, it's important to note that the perceived loudness of a sound is also influenced by factors such as frequency content, duration, and the characteristics of the human auditory system.
To account for the way humans perceive sound, different weighting filters are applied to sound measurements to create frequency-weighted decibel scales, such as the A-weighting (dBA), which is commonly used to approximate the response of the human ear to different frequencies. This allows for a more accurate representation of the sound's loudness as perceived by humans in various contexts, such as environmental noise assessments or occupational noise exposure evaluations.