Foam Does not Stop Sound from Going Through Walls

To tackle the topic of “how to soundproof”, I thought I should just go ahead and get that out of the way. It is used to stop echoes in a room. The reason you see it in soundproof recording studios is that they have already done things to the walls to block the sound from going through, then lined them with foam to stop the echo. Stopping echo is important, but if your only concern is your neighbor not hearing your subwoofer going through the wall, foam is not going to do anything at all. Wrong tool for the wrong job. Bass traps may sound like a device that traps all the bass from leaving a room, but they do not. They stop standing waves, which add a warbling noise to the bass within the room. Kind of the same situation.

How to soundproof? What stops sound? Heavy things. Layers of heavy things that are airtight. Heavy things that are airtight and dampened so they don’t vibrate or are made to be floppy in the first place so they will never vibrate, and cover 100% of a wall, ceiling, or floor.

Sound is simply just a vibration in a medium, whatever that medium is. If sound is transmitted through air, it has to vibrate a wall and induce a vibration in the air on the other side of the wall — that’s how the sound is getting “through” the wall. My job here is to educate you on what tricks of physics we can use to keep walls from inducing vibration in the air on the other side of the wall.

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

I’m full of analogies. Here is a good one:

If you have a bright flashlight and put a piece of tissue paper in front of it, what happens? You see about the same amount of light intensity through the tissue paper. The tissue paper doesn’t have enough mass to stop the light waves. This is like putting foam in front of a speaker…sound shoots right through. If you put a piece of construction paper over the flashlight, you may see some light, but less. Put two layers of paper and you see even less. Then for grins, put a small air gap between the layers of paper, you will see no light at all. Sound works the same way.

*Physics recap: Mass stops wave energy. Increase mass, decrease wave energy propagation. Layers of separated mass stop more wave energy propagation than would an equal mass of one layer.


Iso-Step Soundproofing Underlayment

Iso-Step® Soundproofing Underlayment

Common complaint of customers that I hear: “I’m trying to stop the sound coming through my ceiling. I can hear them walking around and it’s driving me nuts.”

Analogy: Two kids with some soup cans tie some string between them and then string them across the street to each other’s houses. One kid talks into the can, the other kid can hear his voice in the other can. This is called mechanical-borne sound. Sound is just vibrations in a medium. Remember, the string is denser than air and will more efficiently transmit the sound over a greater distance.

The customer’s problem is that the surface that their neighbor is walking on is mechanically connected to the sheetrock that makes up their ceiling because everything is tightly screwed to the floor joists, and/or glued tight to a concrete slab. All those mechanical connections are the string between the two soup cans in the aforementioned analogy. We have to “cut the string” to stop the noise.

The most effective way to do this is at the floor level upstairs. Use a squishy floor underlayment under the surface they are walking on like IsoStep®. The benefit here, and why it’s more effective than treating the ceiling down below, is because you are arresting the vibrations before they have a chance to work their way into any of the other building structures. Anything we do to the above floor is going to be restraining the sound from flanking down around the ceiling into the walls and whatever else in the building we don’t anticipate.


Green Glue Noiseproofing Compound - 28 oz. Tubes

Green Glue Noiseproofing Compound

That’s not to say treating the ceiling is ineffective, just harder to do. We need isolation clips, and two layers of sheet rock with a damping compound between them like Green Glue to hang from the floor joists or the concrete slab…in effect: A floating ceiling.

Read About Isolation Clip Installation (PDF)

The isolation clips are effectively “cutting the string” between what is being walked on upstairs, and what makes up your ceiling. Now your ceiling has more mass and a layer of damping compound between it. The damping compound is keeping the layers of sheet rock from vibrating.

Sound is nothing more than vibration in a medium. In this example, we have broken the mechanical connection, added mass, and dampened the mass. The ability for the sound upstairs to induce a vibration in the air on the other side of the ceiling is severely mitigated.

Recessed Lighting

Another common complaint is: “But, I want to add canned recessed lighting into my soundproof ceiling”.

NO! You will shoot yourself in the foot by doing that. There is no real way to get around this as of yet because recessed light manufacturers thus far that I know of DO NOT want to risk adding enough mass and air tightness to the backs of their light assemblies for them to be soundproof. Why? This is a major fire hazard. Heat would build up in that little cavity with nowhere to go…and that’s bad.

Why is this bad for your soundproofing? Because a 1% gap in a sound barrier will transmit 50% of the sound energy through it. If you go through all the time and expense of soundproofing your ceiling and then pop six or more holes in it, the whole thing is for nothing. The only thing stopping sound through that recessed light, is the recessed light housing, which is in most cases thin aluminum. Can you rig something to block the sound behind it? You are taking the risk of causing a fire. No matter how you do it, it won’t be as soundproof had you just given up on the idea of the recessed lighting in the first place. **A soundproof ceiling needs to be as unbroken as possible**. Full Stop.

Resilient Sound Isolation Clip RSIC-1

Resilient Sound Isolation Clip RSIC-1

Concrete Block Walls

They are not as soundproof as you think. They can be filled with sand or mortar to be made more soundproof, but if the building is completed, what you have is some 2″ thick elastic concrete with air cavities within resonating the sound. Basically, the vibrations in the air on one side can pretty easily vibrate the air on the other side of the concrete block wall.

Solution: Isolation clips and two layers of sheet rock with Green Glue between them, over 100% of the wall.


Doors are tricky. Before I worked in the noise control industry the most I thought about doors was opening and closing them. I have come to find that you could pretty much create an entire Associates Degree course on doors: the ways to hang them, and the ways to soundproof them. I’m going to try to make this as succinct as possible. Doors are the weak link in a wall, it’s where the sound has that 1% gap to be able to travel through. To soundproof a door, you have to increase the mass of the door and make it airtight. That’s about it. The frame is another matter, if it’s a hollow metal frame it needs to be packed with mineral wool, installed, and then caulked at the drywall returns. Real deal pedigreed soundproof doors are going to be in the STC 50 and STC 55 range, meaning they stop 50 dB and 55 dB respectively. These are expensive…in the $3-4,000-dollar range. If that’s what you need and have the budget, then it is what it is. If you don’t have that much sound to stop or it’s not critical enough to justify the expense, you can just swap out your door for a solid-wood-core door, purchased locally, and get one of our door seal kits.

I have tried for quite a while to verbally explain door seal kits. For such a simple thing, it’s very hard to describe one to someone who has never seen one. So, I just made a video, I highly recommend watching this if it applies to you:

As you can see, sound blocking is something of a rabbit hole that is easy to get lost in. I’m usually pretty brief in my blogs and discussions about acoustics. However, soundproofing things is not easy and requires a lot of hard-nosed examination of the situation at hand. Fortunately, at Acoustical Solutions, we have been doing this for quite a while. We rarely get a question we haven’t had to deal with in the past or haven’t successfully treated.

Thanks for reading and I welcome any feedback or questions. I hope this was helpful.

To learn more about how Acoustical Solutions can solve your noise control problems, use our contact form, call one of our Acoustical Sales Consultants at (800) 782-5742, or visit us on the web at