Sound pressure, sound intensity, and sound power are three important physical quantities that describe the characteristics of sound.
Sound Pressure
Sound pressure refers to the pressure change caused by the change in the density of air molecules when sound waves pass through a medium. The unit is Pascal (Pa) .
The sound pressure level decreases as the distance between the measurement point and the sound source increases; the relationship is inversely proportional. That is, if the distance between the measurement point and the sound source doubles, the sound pressure level decreases by half.
The range of sound pressure levels that the human ear can hear is approximately between 20 micropascals (µPa) and 20 Pa. Excessive exposure to sound pressure levels can damage hearing.
Sound Intensity
Sound intensity refers to the flow of sound energy per unit area. It can be understood as the average sound energy density per unit area perpendicular to the direction of sound wave propagation , and its unit is watts per square meter (W/m²) .
Sound intensity is an objective physical quantity that represents the strength of a sound. The greater the sound intensity, the stronger the sound.
Sound intensity is a vector that describes the magnitude and direction of sound energy. Sound intensity can help identify noise sources and their propagation paths. Unlike sound pressure level, sound intensity is less affected by the environment and can be measured on-site to eliminate background noise.
The relationship between sound intensity and sound pressure can be expressed by the formula: I = p²/z, where z is the acoustic impedance of the medium.
Sound Power
Sound power refers to the total sound energy radiated by a sound source per unit time , and its unit is watt (W) . Sound power can be understood as the integral of sound intensity over the entire surface area of the sound source.
Sound power is an intrinsic property of the sound source and is independent of the measurer’s location or distance. The sound power of the sound source should remain constant regardless of where it is measured.
Sound power cannot be measured directly; it must be calculated indirectly by measuring sound pressure or sound intensity. Therefore, to determine the sound power of a specific object, there are two methods to choose from: the sound pressure method and the sound intensity method .
Calculation of sound pressure and sound intensity
Furthermore, the decibel (dB) is a logarithmic unit, commonly used to represent sound pressure level (SPL) or sound intensity level (SIL), convenient for describing the loudness of sounds with a wide range of numerical variations. The decibel is not a unit of physical quantity, but rather represents the ratio between two physical quantities of the same unit .
The reference value for sound pressure level (SPL) is 20 micropascals (µPa), and the formula is SPL = 20 * log10(p/p0 ) , where p is the measured sound pressure and p0 is the reference sound pressure.
The baseline value for sound intensity level (SIL) is 10^-12 W/m², and the formula is SIL = 10 * log10(I/I0 ) , where I is the measured sound intensity and I0 is the baseline sound intensity. In a free-field sound environment, the sound pressure level and sound intensity level are equal.
Related Information
The University of Manchester – Basic Acoustics
HBK – Sound Power and Sound Pressure Explained