How does sticking weirdly shaped foam blocks on the walls of a room make noise hard to hear from inside and out?
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they also use sheets of plaster/gyprock with row upon row of small holes drilled right through them.. I asked the sound tech about it and he said it’s about breaking up the soundwave, because otherwise walls can just act like a drum
What we call sound are small air pressure variations. Upon hitting a hard flat surface, these pressure variations are reflected back (usually with some loss). Foam blocks etc are designed to absorb some of these pressure variations into motion of the foam block material which eventually disperses as heat. The irregular surface also doesn’t reflect the sound wave very effectively. (think of a mirror reflecting a flash light beam vs a painted surface that disperses light)
Sound waves propagate (travel, in other words) at around 340m/s in most room conditions. So the added absorption very quickly reduces the power (loudness) of the sound wave as it goes back and forth reflected multiple times in the room.
It’s kind of like wavebreakers made from concrete blocks. It breaks up the sound waves, reflecting them back onto themselves.
I also can move slightly, so the sound waves will expend energy moving them. Kind of like how a stone dropping on a mattress will sound less than dropping it on the floor.
So the weird pointy shape of the blocks and soundproof rooms is 100% designed to absorb all sounds.
If you stand in an empty room, you can hear the echo of your voice. Basically, the sound you make bounces on the wall back into your ears. Exactly like light bounces on a mirror allowing you to see things that are at an angle compared to you.
Now if your wall is flat, a lot of the reflected sound will reach you. If you change the angle of the wall, for example, by adding many tiny pyramids to create a lot of angles, the reflected sound will not bounce back towards you, but towards the rest of the pyramids. If you also make these pyramids in foam, most of the sound is absorbed by the foam and the only limited reflected sounds are reflected towards more foam, and you now have a soundproof room.
Soundwaves travel in relatively straight lines (each “part” of the wave at least). The waves hit a spongy material at an angle. Some of the energy is absorbed, there’s a bit reflected at a weird angle to the spongy material in the next block, it absorbs the rest.
The most simple answer is they don’t soundproof a room. As others have said in more technical detail, they stop sound reflecting back, so from inside a room they make it very “dead” (think the opposite of a large empty echoey warehouse or something) so you can only hear what is coming from the speakers. This is important when mixing or mastering recorded music.
Acoustic foam/panels, and certainly more low-budget options like egg boxes do not stop sound leaking out of the room however. To sound-proof a room you need thick dense material and ideally a floating “room within a room” design where the room is totally isolated and all air gaps are separated from the outside of the room. It’s difficult, and expensive, to do well. Simply sticking stuff on the walls is going to make no difference at all.
First off, it’s not soundproofing, it’s deadening. Rooms with hard surfaces have tiny echoes, the more you can break them up, the cleaner the sound will be. Those reflected sounds interact and cancel out frequencies, so a room can sound un-natural, and recording or mixing music in it can be problematic.
The room where I edit video and audio is kind-of-deadened. Rug on the floor, foam blocks in the corners, a large and deep frame on the back wall filled with very dense foam (Owens Corning 703 IIRC), and frames hanging a few inches off the ceiling, and stuck to the walls, with more of that foam inside them. If you’re talking as you enter the room, you can notice how much the sound changes – it’s more “dead” and voices sound fuller. A “civilian” (not into this stuff) may not notice, but point it out and they’ll say “wow, it did feel weird to walk in here”. Stick a drummer in there and things like cymbal harshness will be better controlled, but – everyone in the house is gonna KNOW there’s a drummer playing.
Some studios use diffusers, which are blocks of solid wood that look like a cityscape. Those are designed and tuned to break up reflections without deadening everything, and can make rooms “sound good” vs. removing all the sonic character from a room. High-end audio designers understand how to design and make them, I don’t use anything like that.
Sound *proofing*, on the other hand – it’s mass and isolation. Lay down a concrete pad, build a cinder block wall and fill the cells with concrete, pour a slab roof with steel reinforcing, use a steel door with a dense foam core, and seal any gaps – not much sound is getting out of there. You can go even further, float a floor above the concrete with rubber isolaters, and build a frame and drywall room on top of that, where nothing touches the walls, and low-end vibrations won’t make it out. Cheaper alternatives are to use double layers of drywall in a room with staggered seams, and use 2×6 header and base plates; then stagger the 2×4 drywall framing so the drywall only touches the common framing at the top and bottom. Fill that wall with dense foam for even more deadening. This is a good way to build apartments for more quiet between units, but it won’t be enough isolation to do something like record a rock drummer without the neighbors hearing/feeling it. Treble and midtones are easier to block than bass sounds, which are huge soundwaves.
Imagine throwing a ball at the flat, hard, wall. The ball is your voice. The ball will bounce off the wall and back to you.
Now imagine those foamy pyramid things on the wall. If you throw a ball at it, it will not bounce back at you.
This might be enough to convince you, but you might also ask: “but it’s just the foamy-ness that is stopping the ball from bouncing back, not the shape, so why is the shape a pyramid?”
Answer: Imagine the pyramid things are now made of hard materials like concrete. If you throw a ball at the pyramid things, when it hits a gap, it will bounce in-between the gaps and never back to you.
So both the foamy-ness and the shape of the pyramids help sound not bounce back at you.
Those are acoustic panels: https://resonics.co.uk/what-acoustic-panels-do/
https://www.soundonsound.com/sound-advice/beginners-guide-acoustic-treatment
Sound is vibrations. Those absorb sound and cause bounced vibrations to scatter. In a regular room if you make a loud sound, you’ll hear it reverberate. If you put up blankets and pillows and other soft materials on the wall (an inexpensive substitute) they absorb the sound.
My understanding is that they are not so much for sound proofing but to reduce reflections (echo).
Sound proofing a room requires a great deal more work, the room needs to be air tight (with proper ventilation), walls need to be constructed with heavy materials and ideally, a double wall with airspace between – and no hard connections between the two.
Doors need to be heavy, with good seals, and windows need to have multiple panes, but not parallel to each other.
In other words, a whole lot of work goes into actually making a sound proof room. Sticking foam on the walls doesn’t do it.
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