Sound intensity and pressure levels match except where reflections alter arriving waves

Evidence

However, whereas the sound intensity level is the power density from a sound source at the measurement point, the sound pressure level is the sum of the sound pressure waves at the measurement point. If there is only a single pressure wave from the sound source at the measurement point, that is, there are no extra pressure waves due to reflections, the sound pressure level and the sound intensity level are approximately equivalent: SIL ≈ SPL. This will be the case for sound waves in the atmosphere well away from any reflecting surfaces. It will not be true when there are additional pressure waves due to reflections, as might arise in any room or if the acoustic impedance changes. However, changes in level for both SIL and SPL will be the equivalent, because if the sound intensity increases then the sound pressure at a point will also increase by the same proportion. This will be true so long as nothing alters the number and proportions of the sound pressure waves arriving at the point at which the sound pressure is measured. Thus, a 10 dB change in SIL will result in a 10 dB change in SPL. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 210

Categorization

• DEFINE: Clarifies the boundary condition under which SIL equals SPL and when they diverge — room reflections being the key disambiguator.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 210