Surface porosity, not mass, determines how much sound walls absorb

Evidence

Reals walls and buildings can vibrate (with low Q and relatively high friction) at many different frequencies. Because it is relatively stiff and heavy (very high impedance), a brick wall tends to absorb much less sound energy than wall board. It also transmits less, so 99% of the energy can reflect from a painted brick wall. But an unpainted brick wall reflects typically only about 97%. Painted concrete blocks reflect about 90%; unpainted, only about 65%. The paint does not change the mass noticeably and does not affect the internal friction in the material—so why does paint increase the reflection of sound? The difference is the porosity of the surface. Tiny cracks and pores cause microturbulence and furthermore provide a larger surface area for heat transfer, both of which dampen the sound energy. Air heats up as it is compressed, and if it transfers some of that heat to nearby surfaces, it cannot rebound with the same vigor it arrived with. — Eric J. Heller, Why You Hear What You Hear- An…, 2440

Categorization

• QUESTION: Concrete blocks painted vs unpainted reflects 90% vs 65% — a huge gap for such a small treatment; worth chasing the physics of microturbulence damping further.
• SHARE: The painted vs unpainted concrete reflection numbers (90% vs 65%) are exactly the kind of table-thumper an acoustician or building scientist would love.
• XREF: connects to: Sound absorption and sound isolation are confounded but behave inversely in materials — David Howard, Jamie Angus, coustics and Psychoacoustics, 2159