Intensity measurement in decibels and specific acoustic impedance?
Sound intensity
The sound intensity (acoustic intensity), I, of a sound wave is defined as the average rate of flow of energy through a unit area normal to the direction of wave propagation. The unit for sound intensity is watts per square metre (W/m²), which can also be expressed as joules per second per square metre (J/s/m²).
In some cases it has been stated that the loudness of the sound is determined by its sound intensity.
This is not true in the general case, for loudness and (sound) intensity are not synonymous.
The loudness of a sound is subjective, and the loudness is in all cases a sensed combined function of both sound intensity and frequency.
Acoustic impedance
The factor ρ0 × c is called the acoustic impedance Z0, and describes the conditions for sound propagation through the medium.
ρ (rho) = density, c = speed of sound, and v = sound particle velocity.
The unit for acoustic impedance Z0 is N·s/m³. As seen above, the acoustic impedance Z0 is the ratio between the sound pressure and the instantaneous particle velocity: Z0 = ρ0 × c = p / v.
The acoustic impedance is an important factor in all evaluations of sound waves, especially when comparing sound measurements in air and in water. For such evaluations, it is customary to specify the characteristic acoustic impedance Z0 as follows:
Air: Z0 = 413 N·s/m³ or Pa·s/m at the temperature of ϑ (theta) = 20°C at standard atmospheric pressure.
Water: Z0 = 1 480 000 N·s/m³ (distilled water at 20°C).
