How to Soundproof: Acoustic Foam Does Not Block Sound

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 subwoofer 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.

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 Walkmans 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

So if you want more information on how to soundproof and block sound, you may want to read:

Here’s the deal, absorbing sound and blocking sound are two totally different things. Sound absorbers prevent sound from continuing to bounce and echo throughout a space. This is done by disrupting the sound's 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.

While acoustic foam doesn't block sound, it remains a popular choice for improving interior acoustics. It reduces slap echo, flutter echo, and high-frequency reverberation, especially in home studios, vocal booths, and podcasting setups. Foam works best when used alongside proper sound-blocking assemblies, helping enhance speech clarity and listening experiences in treated rooms.

“PA systems are a great deal louder than a person’s voice. If the sound hits a surface that is very hard and immobile, it will bounce the sound energy at nearly 100% efficiency, which means the sound from a person's voice over a PA system is literally bouncing around the room for about 5 seconds or so. That five seconds is the Reverb Time, or RT 60, of that room. People speak in a quick succession of vowels and consonants, so if the listener is hearing 5 or 10 vowel sounds still hanging in the air with 5 or 10 consonant sounds… all you hear is garbled. Hanging Sound Baffles work because they are not 100% efficient at reflecting the sound—in fact they are nearly 100% efficient at NOT reflecting the sound. They are absorbing the sound reflections and therefore reducing the overall average Reverb Time.”

To get the most from your absorption materials, especially foam and fabric-wrapped panels, strategic placement is key. Focus on first reflection points—areas where sound bounces off surfaces directly after leaving the source. In home theaters or studios, this includes the walls beside, behind, and above speakers. For large spaces, distributed coverage on parallel walls and ceiling treatments help reduce slap echo and flutter. And always consider ceiling height when selecting panel thickness or baffle configuration

A common mistake is assuming that covering a wall with foam panels will stop noise transfer. In reality, porous materials like foam absorb ambient noise within a room but do very little to reduce transmission through structures. Blocking sound requires dense, sealed, multi-layered construction—not just surface-level treatment.

If you’re considering foam for improving room acoustics (not blocking sound), here are a few popular options:

To help differentiate between absorbing sound and blocking it, here is a simple table showing where each type is most effective:

CLIENT: “So, how do I actually go about blocking sound through a wall or ceiling or anything else?” That, unfortunately, is a much bigger discussion. It takes mass, or weight, and layers of it that are mechanically disengaged from one another, and sealed up tight 100% with caulk and putties, and…etc. There is also a previous blog entry on the subject.

Mass loaded sound barriers are roll goods used to add density to wall, floor and ceiling assemblies or wrap pipes or ducts to block sound.