Sound pressure level and intensity level coincide only without reflecting surfaces

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

Category: Causal Relationships

• XREF: Connects to the reader’s earlier noise work — One Square Inch of Silence and the difference between measuring what a meter captures vs. what reaches the ear in reflective environments.
• DEFINE: Clarifies the conceptual distinction between pressure-level and intensity-level measurements and when they diverge.

Related

• Sound pressure level is the standard because it matches hearing (similarity 0.86)
• Sound intensity level and sound pressure level are roughly equivalent (similarity 0.85)
• SIL and SPL always change in proportion even where their absolute values differ (similarity 0.85)
• Sound level meters measure pressure, from which intensity is derived (similarity 0.83)
• Reported sound levels almost always refer to pressure, matching human hearing (similarity 0.83)
• Sound pressure and intensity are distinct quantities interconvertible by a known relationship (similarity 0.83)
• Sound power and intensity levels are unaffected by source correlation, unlike pressure level (similarity 0.83)
• Sound level multiplications become decibel additions in reflections (similarity 0.82)
• Sound level convention defaults to pressure level because it is measurable and matches hearing. (similarity 0.82)
• Sound power level measures a source independent of surrounding acoustic context (similarity 0.82)
• Intermediate separations cause loudness to follow intensity addition instead of pressure addition. (similarity 0.81)
• Loudness depends on more than physical sound intensity alone (similarity 0.80)
• Audible pressure range spans over a million to one, forcing a logarithmic scale (similarity 0.80)
• Sound pressure amplitude falls off as the inverse of distance from a source (similarity 0.80)
• Ear sensitivity depends jointly on frequency and on sound level (similarity 0.80)