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.
Dense materials carry the sound! I lived in a high rise with concrete subfloors and you would not believe how sound carried! You wouldn’t hear talking or TV but any tapping, banging, or similar carried like crazy! When we had a toddler across the hall, his dropping toys on their wood floors sounded like golf balls bouncing on my head.
The two main components to soundproofing/sound isolation are density and separation.
In professional studios, the recording booth is generally acoustically isolated by separating the room from its surrounding structure by building a ‘room within a room’ using two sets of parallel walls with the inner walls standing on a floating floor platform which is then generally sitting on something like dense neoprene spacers. And then the walls are hung with heavy drywall, sometimes two layers.
All of this serves to minimize the transfer of mechanical energy between the inner room and the larger structure by minimizing contact and using materials that minimize energy transfer where there must be contact.
The foam products you’re thinking of are for sound deadening/acoustic tuning which isn’t about isolation but about changing the ‘sound’ of the room — that is, largely how much echoing and reverberation there is inside of the room. Since foam has many tiny surfaces which all point in more or less random directions, it helps the sound to disperse in random directions instead of harshly reflecting off of the flat, hard walls.
It’s basically the same difference you notice between an empty room with wood floors and a carpeted room with soft furniture in it. The room sounds better and more ‘dead’ with a couch in it. But it doesn’t change how well you can hear the truck that’s going by outside.
You use foam when you want to have a space that is quiet. This is because foam is good at absorbing sound energy and stopping it from bouncing around the room.
You use dense materials when you want to stop the sound from one area from going into another area (i.e. a “sound barrier”). They tend to be ‘reflected’ sound back so the ‘loud’ side of the barrier will get even louder.
Sound can travel through solid objects it even in some cases is a lot easier, what sound finds difficult is travelling from one medium to another and back again, so going from solid to air to solid again and again muffles the sound.
If you want to have a room where the sounds outside can’t reach inside; or viceversa the inside sounds can’t escape, you need **dense materials**, usually solid brick walls, and sometimes a double wall, with an air gap between. Also, heavy doors with good seals, and sometimes even double doors. I’ve seen them in recording studios and the like.
The foam you see, is used to prevent a lot of echo and reverberation inside the room. If you build a soundproof room, made of concrete or brick, and you don’t have anything in its walls, you’ll get a lot of echo, like you hear in an empty apartment, a subterranean parking lot, etc. You need the foam to prevent the sound from bouncing in the walls, reducing echo. The foam in itself doesn’t soundproof anything (you can check this holding a piece of foam in front of your face and talking to someone, they can hear you just fine. If you put in front of your face a piece of concrete or a piece of brick wall, they will have a hard time hearing you…), but it makes the inside of the soundproof room a good environment to record.
Sounds travels best through dense materials, the less dense, the less sound. Sound travels much faster in water than air, for example. Sound also doesn’t travel well through the water/air transition. From above it’s hard to hear anything below, and vice versa.
So we take a very light foamy material full of air holes. The sound goes through the material, then through air, then material, then air, and so on. This is pretty good at deadening it.
The foam like material is absorbent as opposed to reflective or prone to vibration. The foam doesn’t reverberate in the same way solid objects do. Think of yourself yelling in a parking lot for example. All concrete, and quite loud yeah?
Well if you want a simple image of materials, then at small scale, they are like bouncy balls connected via springs.
If you have a solid chunk of material, the sound vibrations can spread straight through it.
Foam is mostly empty space and the material in it is thin zig-zagging pillars. Thus the vibration has to take looong detour, and in the direction changes it bounces back and forth spreading out and the sharp back and forth wobble pattern becomes more spread out and less and less recognizable.
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