Eli5 Why is the soundproofing that is used foam-like materials instead of dense materials?
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Because they’re heavier, more expensive and more difficult to install compared to light materials that are better insulators. Continuous materials also reflect sound and resonate.
Sound, like heat, is vibration. Air is REALLY bad at transferring vibration compared to most materials, it’s pretty happy to expand or compress under force and eat up some of the energy in that motion rather than carrying the energy through to the other side and the molecules are so far apart compared to a solid material that they don’t carry through very small vibrations like heat well, the molecules can move a lot more (eating energy along the way) without hitting another one to transfer it.
So foam is basically a mechanism by which you capture a bunch of little air pockets inside a material. Pocket-to-pocket transfer, especially through the materials used by foam, is really inefficient so it’s a good insulator for sound and heat. The same general idea applies anywhere you’re using fibers or sheets to create air pockets. That’s what the fluff in a blanket or puffy jacket does. Carpets and their under pads do it which makes carpeted floors reflect sound a lot less and insulate you from the sub-floor’s temps, etc…etc… Even the fiberglass insulation in walls creates a bunch of these pockets. Foam, unlike loose fibers, is really easy to sell in a sheet, cut to size and hang on a wall, so it’s a convenient insulator.
Now, a foot or two of dirt would work really well for sound, too. Particles, as opposed to a continuous material like metal, are pretty bad at transferring energy or resonating, but who wants to build a second interior wall in their sound studio or home theater and fill the gap with a bunch of dirt? Worse, you’d have to contain it with a stiffer material like drywall you already had so while the dirty would contain the sound in the room, the sound would still reflect all over the inside of the room causing unwanted resonances, unlike foam which both insulates and doesn’t reflect or resonate much.
Speaking of reflections, shape also matters. A flat panel reflects sound pretty directly, messing with what reaches our ears thanks to those echoes. An irregular surface, like the egg crate foam, scatters the reflections into smaller, individually weaker ones which run into each other and start cancelling each other out in the process. That helps reduce the “noise” introduced by the room reflecting the source sound from your speakers, instrument, voice, whatever.
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