How to Soundproof: Acoustic Foam Does Not Block Sound

For instance, some speakers used to use foam as the speaker grille cover years ago. If foam is that great at blocking sound, then why do that? Or the old earphones on Walkman’s from back in the day (I’m showing my age here). They had foam right over the head phone to make it more comfortable for the listener. The foam in both of those examples was not blocking the sound in any way. The sound just poured right through.

The point I’m trying to make is that putting a few squares of 2” thick foam here and there on a partition wall in an apartment will not keep someone from hearing the other guy’s TV and sub woofer at 3 AM. Even covering the wall 100% with 2” thick foam is not going to, to the extent of the person’s expectations, stop that sound from traveling right through the wall. Acoustical foam is porous and does not block sound simply because it is porous which allows sound to pass through.

Now, a lot of Audiophiles and people very familiar with the nuances of sound will say: “Well if you have a relatively small confined space with a given large sound source the waves will build up and potentially amplify certain frequencies due to modal responses of the shape of container—and therefore adding absorption to the inside of said confined space will indeed reduce overall dB from escaping into the environment,” but that’s not the point , I say.

It’s not that I’m trying to ruin anyone’s day here—I would love to have foam that could stop sound from going through walls. I would sell a ton of it. But physics is physics. We are not going to sell anything to someone that has zero chance of meeting a customer’s expectations.

This title says it all, basically. I explain that acoustic foam does not block sound about 5 times a day to prospective customers wanting to know how to soundproof. The question invariably comes back, “Why not? What the heck is it used for?”

Absorbing Sound And Blocking Sound

Here’s the deal, absorbing sound and blocking sound are two totally different things. Sound absorbers prevent sound from continuing to bounce and echo through out a space. This is done by disrupting the sounds path and converting the energy to heat. This is often done by using fluffy and porous materials. Blocking sound requires materials that are heavy, dense and thick. These materials are often used in multi-layered assemblies so that sound will not penetrate or pass through. Soundproofing a wall requires the structure itself to be modified.

I will explain further with some analogies. The color white “reflects” all light, right? And the color Black “absorbs” all light. What we perceive as white is simply just all colors of light mixed together, and what we perceive as black is the absence of all color. Shine a flashlight at a bright piece of tissue paper and you will register a great deal of reflectivity. Shine the flashlight at a dark black colored piece of tissue paper and you will register very little reflectivity.

However, being that it’s tissue paper, you put either the white or black paper up against the light and use a rubber band to totally cover the end of the flashlight with it, and you will register nearly the same amount of luminosity traveling through both colors of tissue paper. Maybe a tiny little less with the black…it’s not a perfect analogy. Splitting hairs aside though, the point is that sound does the same thing: It reflects off certain surfaces and it’s absorbed by other surfaces, in much the same way white reflects light and black absorbs light.

When you look at recording studios that have all this fancy foam all over the place, don’t make the assumption that it’s the foam that is blocking sound from going through the wall. It’s not. They have added layers of mass and caulk and isolation to that wall first to block the sound from traveling through, then added the foam to reduce echoes in the room for various reasons.