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Last week I was asked for advice on the best acoustic treatment for doors. “What am I looking for? I need isolation to stop the noise coming into my studio and I’ve seen you say that doors can be the weakest link and so where do I start with that also I want to treat the face of the door that is inside the studio. Is foam best or can I hang a diffuser on it?

Wow! Well this is a lot of confusion in one question so let’s break it apart and see if we can help out a little bit here. Two technologies with rooms, barrier technology and sound treatment technology. Completely different sciences, some related but mostly not.

Barrier technology is the technology that keep sounds that occur outside the room from coming in. It also keeps sound in the room. If you ever want to stop sound, the quickest way to do it is to erect a barrier between you and the source. So if you’re in the next room and your friend is playing his guitar, the easiest way to block the most energy in that little example is to build a barrier between you in one room and him in the other and that’s the easiest way.

Now, how do you build a barrier? How do you install it? How much surface area do you use? All separate issues and are too involved for this discussion. So two technologies barrier and room treatment.

The door is the weakest link

The door is the weakest link in this system most of the time because think of your wall as a dam. In this dam you’re going to have areas that leak so you go along and you plug up those areas with concrete. In a dam you plug up those areas in your room around the edge of the door where the door seals against the structure because any opening will let the water or sound out.

The door is usually a solid or hollow structure and it’s thin. So just use your common sense here. The wall’s going to be four, five, six inches deep. Its thick and the door is thin. So sound energy is going to go to the thin area. It’s going to go take the path of least resistance just like water would go to the hole. In your car water leaks through a little hole in the hose. It always finds the area to get out. I guess it’s trapped and it wants to be free, I guess you could think about it that way.

The studio doors are a perfect place for sound to get out so the strength and the mass and the density of the barrier material in the door must be at least equal to the wall. Now if we’ve got the door, the mass, density and the material equal to the wall then what would be our next weakest link? The window and the window presents two issues, noise issues and reflection issues and glass sound issues inside.

So barrier technology is completely separate from room treatment technology? Do I hang a diffuser on the door? I have no idea if that would be a good place for the diffuser or not without seeing your floorplan. I have no idea if your room even needs diffusion because I have no idea what the usage is and we’re always going to get back to usage. I’m always going to pull you to the usage if you contact us to deal with this.

Room Usage Is Critical

You always need to know exactly what you’re going to do in the room and give me the highest and low pressure areas that were going to be working with in the room because that has a direct impact on how we treat your room. Usage is very critical. You can get the sound that you want for the usage in your room if you have enough space and you have the right type of acoustical treatment. We have the technology to do that. Now that said taking that technology and sticking it in to rooms that are really small where the usage doesn’t match the treatment all you do is compound the problems. There’s no solutions here you’re running up a hill you can never get to the top of and you will be running down a hill in sound quality that you will fall and collapse with.

So there are too many variables to say a diffuser on the door, absorption on the door… I just don’t have enough information at this time. One of the things that people do with doors that’s really powerful, it takes a lot of space but it’s so powerful it solves all the problems that doors create, is the door lock. It’s like an airlock. Its two doors like when you watch Star Trek and you watch these sci-fi movies and they’re in outer space. There’s always this chamber where you leave the internal parts of the ship and you go into this air lock and then you open up that seals you up off from ship.

Well that’s what you do in a studio. You build two doors with airspace between them, separately framed. So you’re going through two doors to get into the studio. Most people don’t have the room for that but its a very powerful solution. We’ve built many of those and boy they just work so great. You probably need an additional four or five feet but it’s like the entrance way, a hallway coming into a house. You come in, open the door, you hang your coat up, close the door and then open another door to walk into the house. You have these two barriers and then space. Air is a barrier. Air is a pretty good barrier if you have enough distance.

That’s a real good solution for doors, very expensive but real good. There are things that that you can do with the current door that you have. Sealing them is critical. All that extra density and all the extra mass you would add to the door to create a better barrier or to block out the problematic frequencies you’re dealing with can all be negated if you don’t have the right kind of weather stripping or stripping around it to seal it. And there’s some great doors out there that have these systems already built-in and they’re framed and everything and you just drop the whole frame in place and the door is one self-contained unit.

Really expensive stuff but you can do it on your own, you just have to have a really good skill set as a carpenter. And how do you test if you’ve got a good seal? Well we’ll do a video on that. It’s a little trick that we do with compressed air. It’s kind of cool to see but we’ll talk about that in a video we’re going to do on that. We use smoke in compressed air. It’s pretty cool.

Using A Diffuser On A Control Room Door

In the above example we don’t know without looking at the room layout without analyzing the pressure levels in the room I don’t know how much of a problem the door is creating without knowing the noise levels outside of the door. I know what that ambient noise levels need to be inside the control room. I have some pretty good readings and data on that so I know how quiet the control room needs to be but I don’t know how much noise we’re dealing with from outside the control room.

Do we have a garbage truck outside the window? I don’t know, all the noise sources outside the room have to be analyzed for their pressure and their amplitude. How big of a problem are they? What is their frequency that the problem is at? Because how big, how much amplitude they have and what frequency they’re at tells us, without guessing, what kind of structure we need to build to stop them.

Every frequency and every pressure level has a particular structure that will stop it and you have to decide how much you’re going to spend to stop how much. That’s sound transmission class ratings, that’s what the whole system is about. You can build a studio and some people will actually design a sound transmission class number for you, a rating that they want you to achieve and if you don’t achieve it they’ll take legal action against you or they won’t pay you. I mean it’s serious stuff. Noise today in a crowded world that we live in is serious.

Sometimes You Have To Hire A Professional

You’ve got to take it seriously and that’s why I always recommend, when you’re dealing with barrier technology, to bring in a professional because it’s serious and it’s usually beyond the scope of most people. So that’s why I always say that people say ‘well don’t, let people build it on their own’ well I disagree. Because noise is a serious problem and the solution is a serious build, serious problem serious build serious skill set.

Most people don’t have the serious skill set. They have the problem and they understand what needs to be done but they don’t have the skill set to do it so that’s why I always say bring in a professional. Noise is the biggest problem in the world and the easiest to mess up on. You think having a little hole and a weather stripping around your studio door is a problem? You haven’t seen anything yet when you’re trying to stop the garbage truck outside at 7am from interfering with your recordings, not even close.

So I make light of it but it’s very serious. That’s why I always say “Look our engineers that I have on our barrier technology, they need numbers they need to know the strength of your problem and they need to know what frequencies it’s at”. You go to the doctor, you get a blood test. He looks at your vitamins, he looks at your minerals, your iron, all the lipids, your fats and all of the other things.

Well we do the same thing in acoustics. We have to know what the problem is in order to get you the right pill if you will to deal with the issue. Most the time it’s not a pill. Most the time its major surgery. So noise is a whole other area. You can build the best room and have the best quality sound that you ever thought possible in a room and it’s ruined because the train schedule is the same as your listening schedule.

So you’ve got to take all this stuff into consideration especially in the world today with so much noise and everything so crowded. I mean I take my pressure meter into restaurants and stuff just for the heck of it and measure things. I’ve done that every 10 years or so and levels have consistently gone up 3, 4, 5 dB. You have to have double the power to get a 3dB increase. Well how do we double the power in a restaurant? More people, so lot more people on the planet, lot more noise. Its just going to be the way it’s going to be. So a lot more care and attention we have to be paid to barrier technology.

It’s like the Simpsons’ character Doctor Nick

Whenever Homer is trying to cut corners on surgery, he goes to Doctor Nick and it always ends up in disaster. It’s kind of the same thing here. You save a bit of money at the start but in the end it’s going to cost you more. I want to go to the brain surgeon if I’ve got a brain problem. I don’t want to go to some guy in the backstreet, who is doing a sideline thing.

I’ve actually told clients not to treat their noise issue because they have to wait till they get the funds to do it because putting a Band-Aid on it, they’ll just waste their money and they won’t be happy with the results. I’ve told that to many people and some listen and some don’t and go on ahead and do it and then inevitably in two, three months I’ll get a call and they’ll say they’re not happy and now the expense really comes.

I was working with a client the other day and the room was forty by forty and it was framed with two by four construction. That’s only three and a half inches deep and he had a neighbor that was sixty feet away that didn’t like sound and what was the usage of his room? It was a home theater. You ready for this? It was 10 channels. So we had ten speakers, four subwoofers and a neighbor sixty feet away that is noise sensitive and living in a paper to paper house, that’s what I call it.

Two by four is paper, your house is made of paper, your room is made of paper and too many things are wrong with that. So we’re going to build a room within a room. We’re going to leave the outside shell and we’re going to use that as one of our calculations for the barrier. It ain’t going to do a lot but we’re going to use it because it’s already built. Then we’re going to build another room inside of that room that has the barrier technology that he needs to keep his neighbor happy.

So here’s a situation where the room was built without any kind of analysis at all and what are we doing? We’re building another room inside that room when it would have been better to build it originally the correct way and just build one room. Of course, now I’ll make this work for him because barrier technology involves mass, the more mass we have is usually the better. So we’ll have it two by four, framed outside wall, I’ll use that in our calculations but that’s an expensive barrier. We could achieved the same result in a barrier that you’d get in a three-and-a-half, four-inch wall in one inch of properly designed barrier.

So you’ve got to be really careful here. It’s not it’s not as easy as a lot of people think and I know because I’ve built many and I followed the standard guidelines when i first did the RND. I said ‘alright let’s take existing literature, let’s take existing acceptable practice and build them and see what’s up’. Well what’s up? They are Band-Aids on arterial cuts so that’s why I designed what I designed because I wasn’t going to have any of that stuff.

Get It Wrong And You Can Be Sued

Sometimes I’ll even turn jobs down job if people aren’t taking it seriously because there are legal implications if you get this wrong. Especially in commercial buildings. I had certain speech intelligibility numbers I had to hit inside the conference rooms that I used to build. I had certain noise levels and ambient levels I had to hit in the rooms when nobody was speaking. I had certain intelligibility, I always put that in quotes, “levels for music” that I had to hit when there were video presentations. I had certain speech intelligibility requirements I had to hit if it was a man or woman speaking.

All kinds of issues have to be addressed. It’s not just sound. It’s not that simple. It’s a wonderful thing to listen to music using, it’s not easy to get it to perform the way you want to and it’s not that easy sometimes to get the rooms in which we listen to sound, to get out of the way. I mean that’s all we’re doing with acoustical treatment. We’re just taking the room sound out of the equation, well that’s not easy. The best kind of sound you could have is to take your system and set it out in the woods somewhere where it’s really quiet, with no boundary surfaces other than the earth itself. Just the floor so to speak, no walls, no ceiling listen to your system.

Now take your system and bring it home after you spend two or three days listening and then you’ll figure out that ‘Hey! This is a good sound, this is a sound that I like’. Now come and take your system and put it in your room, completely different, you won’t like it and you shouldn’t like it. So what do we have to do as engineers and designers? We’ve got to make that small room sound like the countryside. That ain’t easy and you’ve got to do the right things in the right sequence and you’ve got to do a lot of them, because it’s completely different.

In Summary

I hope this discussion on acoustic treatment for doors didn’t meander too far from the point. I just think its important to stress how important many of the elements involved are. If you want to learn more about this subject please sign up for our free room acoustic treatment videos and ebook which provide step by step instructions on all major room acoustic issues. Get instant access by signing up now. And if you would like your room acoustic issues analysed for free by me then please fill in the form here and I will be happy to take a look for you.


Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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