Sound absorption and sound isolation are confounded but behave inversely in materials
Evidence
The first thing to note is that just because a material is a good absorber of sound doesn’t mean that it is a good isolator of sound. In fact most absorbing materials are terrible at sound isolation. This is because, in the sound isolation case, we are interested in the amount of sound that travels through a structure rather than the amount that is absorbed by it, as shown in Figure 6.54. A poor value of sound isolation would be around 20 dB yet it corresponds to only one hundredth of the sound being transmitted. A good absorber with an absorption coefficient of 0.9 would let one tenth of the sound through, which corresponds to a sound isolation of only 10 dB! As we are more interested in sound isolations of 40 dB as a minimum, absorp tion is clearly not the answer. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2159
Categorization
- DEFINE: Clarifies the technical distinction between absorption coefficient and transmission-based isolation, which sound engineers and ordinary listeners commonly conflate.
- CANDY: The punchy numbers — a 0.9 absorber gives only 10 dB isolation — make a quotable dinner-party fact about why acoustic foam can’t block noise.
- XREF: connects to: Good sound absorption does not imply good sound isolation — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2112; Absorbing materials are nearly worthless at isolating sound; Surface porosity, not mass, determines how much sound walls absorb — Eric J. Heller, Why You Hear What You Hear- An…, 2440
Broader Connections
- Absorbing materials are nearly worthless at isolating sound
#exemplifies(0.88) - Good sound absorption does not imply good sound isolation
#exemplifies(0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2112 - Surface porosity, not mass, determines how much sound walls absorb
#qualifies(0.75) — Eric J. Heller, Why You Hear What You Hear- An…, 2440