I don't really care about how much sound is reflected: I want to know how much is transmitted! And NRC says nothing about that. Instead, there's another measure (STC. sound transmission class) that describes what I care about.
(Image credit: WorkingWalls workingwalls.com/nrc-and-stc-wh)
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Acoustic foams, carpets, tapestries, and so on all do a good job of reducing sound reflection in a room. This is useful if you're trying to record since it reduces the amount of reflection that the mic will pick up. It's also useful in echo-y spaces like gyms.
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None of those are good at reducing the amount of transmitted sound, though. The main thing that materials with high STC ratings have in common is that they're heavy: lots of mass to move means that lots of sound energy is expended moving the heavy thing, so less gets through.
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(There are a few techniques that go beyond that, like special glues that convert sound energy into heat, but in broad strokes, a high STC means that it's heavy.)
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I'm pretty sure that the standard advice for reducing noise in a bedroom should be "hang mass-loaded vinyl" (basically just vinyl impregnated with metals to make it heavy), not "put up tapestries". Unfortunately hanging heavy things is more difficult, expensive, and destructive.
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I am glad I noticed this, but it kinda worries me that so much standard advice (e.g., various web listicles, Reddit posts, etc.) is so confused, especially since figuring this out only required high school-level mechanics.
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And of course it worries me that I hadn't noticed this distinction before, and just sort of absorbed the advice that was "in the air" about what soundproofing is. It's kinda embarrassing that I'd never thought this through.
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One funny conclusion: this explains why sticking your head into a pillow to block out someone using a leafblower at 7:30am doesn't usually work.
A pillow probably has a pretty good NRC, but a low STC!
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Mostly I think that "soundproofing" and "sound absorption" are bad words that might mean two different things.
Straw proposal: materials like acoustic foam "block echos" while materials like concrete and mass-loaded vinyl "are hard for sound to get through".
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Glad it helped! For a door I’d probably try adding a sweep/seal first, since sound is good at refracting through any gap. Something like: nesoundproofingstore.com/Door-Kits_c_10
If that’s not enough, it shouldn’t take too much mass-loaded vinyl to improve on a hollow-core door.