Often, folks think that by adding some acoustic treatment to the walls, you can stop sound from traveling from one room to another. This misconception is in fact not true. This is not to say that sound absorption panels will not help, but it is important to note that to stop sound, you must stop all types of vibration from traveling between the two adjacent rooms. The challenge is that vibrations travel through the air - as in sound waves - and if sufficient vibration energy is developed, the energy can travel through just about any material including sheetrock walls, ventilation ducts, through and around doors and even concrete.

How do you stop vibration energy? You don't. What you do is contain it. For instance, if sound travels through the air, it makes sense that we have to stop air flow. This means that you have to stop the air from escaping under and around doors by applying a good insulating air seal. But this is only part of the challenge. As previously mentioned, if sufficient energy is produced, it will cause objects to vibrate and the sound will pass right through. The heavier the object, the more energy will be required to make it vibrate.

To stop sound traveling through walls, there is usually a four step approach:

Step 1: Increase the mass

This is easily done with sheetrock by adding a second layer. The more mass you add; the more energy will be required to get the wall to vibrate. In fact, each time you double the mass you will reduce the sound transmission by -5dB. This is why for years; studios used lead to line the inside of their walls. Today, soft rubber-like loaded vinyl has replaced lead. Loaded vinyl is often the best choice when trying to stop sound from traveling through drop-ceiling tiles into the plenum and out such as in office areas.

Step 2: Decoupling the transmission lines

Vibrations need some form of object or molecular structure to pass along their energy. In space, there is no sound because there are not enough air molecules to sustain the vibration. Bang on a typical sheetrock wall and you will certainly hear the noise on the other side. By eliminating (decoupling) as many connections between the two wall surfaces, you will eliminate the energy transmission. Another method of achieving similar results is by using resilient channel to mount the second coating of sheetrock. Resilient channel is a metal strip that is available at most hardware stores. It is designed to float the second wall on top of the first like a spring, leaving an air gap. It works like this: the first wall is set into vibration from the sound generated in the room. Because the first wall vibrates without touching the second, the sound that passes through in between the two walls must now set the second wall into vibration.

Step 3: Building your offset stud wall

This is easy; Instead of using typical 2"x4" plates, use wider 2"x6" plates and run a series of studs on both sides of the plates in a zigzag. Mount your sheetrock panels on both sides and you are set to go! Each wall has its own set of studs and both walls are disconnected from each other except at the floor and ceiling. You can further increase the efficiency of this system by mounting two different masses on each layer such as two sheets of ½" sheetrock for the first wall and two sheets ¾" sheetrock on the second. Because the two wall masses will be different, you will have created an impedance mismatch and they will no longer vibrate sympathetically. The wider the air gap between the two walls - the better. By filling the inside of the wall with soft pink insulation you will dampen the wall and improve the performance. For extreme sound containment, you can add the loaded vinyl. Do not stretch it, leave it hang naturally. This will act like a limp mass and will further assist in containing sound.

Note: Sound also travels through floors and ceilings! Resilient channel on the ceiling is a good bet and for 'full-on' studio construction, floating the floor using rubber isolation mounts or pouring a thin coat of concrete to add mass will help.

Step 4: Reducing the vibrations in the room

Once the room has been built, you can now add acoustical panels inside the room to help absorb the energy and reduce the demands on your newly constructed off-set stud walls. Absorptive panels work by transforming sound energy into heat as the sound penetrates the soft material.

 

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One of the questions we often get is: 'how do I stop the sound escaping my room from disturbing someone else?' This can include situations like a project studio under a bedroom, a home theatre next to the kitchen, two adjacent apartments or even containing the sound between a boardroom and a joining office. Sound is sound. However the louder the sound or the more bass frequencies you have to contend with, the more challenging it will be to control!

The first solution is easy: Turn down the volume! Sorry - obviously this is not the solution you want to hear. Acoustically speaking, you have to reduce the sound that is bouncing around your room. Rule #1: A good place to start is to control the sound inside the room itself by mounting Broadway absorptive panels on your walls in the room. Although this will not stop sound from escaping; it will help attenuate the sound inside the room. This makes it easier to contain and will make your room sound better at the same time!

To develop a working solution, we must first understand the problem. Sound is vibration. Big vibrations - either in amplitude (loudness) or in width (bass) - require more energy to develop and therefore require more effort to control. Sound vibrations also travel through pretty much anything. It is important to note that the higher the pitch (frequency) the more directional the sound will be. Bottom line: If it can vibrate… sound will pass.

Air provides an excellent transmission line for sound. When we speak with each other, air vibrations leave my mouth and arrive at your ears where your brain deciphers the vibrations and turns the vowels, consonants and pitch into words that we understand. Rule #2: you want to shut down air passageways. For instance, studios will use big heavy industrial doors with gaskets or air seals around the edges to stop sound (air vibrations) from leaking out. If you happen to have an air duct that is split and feeding two rooms, sound will pass. You will need to close one off. If your window is open and so is your neighbor's, sound will pass and you will hear their TV. Solution: Shut the window.

But sound does not just travel through air. A simple experiment proves it: (Put a large green plastic garbage bag over a friend's head and have him speak. You can be sure you will hear the yelling just fine. That is… until the air runs out. (Don't try this! Unless you have lots of friends and can spare a few) Why can you still hear the screaming? Simple - the mass of the plastic bag is so light that the sound vibrates right through the thin plastic film. Rule #3: to stop sound you need mass. To set a heavy mass into vibration like the wall in between two rooms, you need more energy. Ever notice that you can hear the stereo in an adjacent room when it is being blasted (especially the bass), but cannot hear a whisper? It therefore makes sense that if you introduce a heavy mass in between the sound you want to retain and the room you want to protect, more energy will be absorbed during the transfer. This is why the studio door we discussed above is heavy. This is also why studios will double up the amount of sheetrock or drywall when they build their walls. Mathematically speaking - double the mass and you will achieve approximately 5dB in sound reduction.

Now the tricky part. 'If all one needs to do is stop air from traveling and have a heavy wall in between the two offending rooms, why can I hear the stereo in my bedroom upstairs? The floor is heavy and there are three doors separating the sound to control air flow?' I did say this was the tricky part… this has to do with coupling or more precisely, decoupling the lines of transmission. Think of a hammer and chisel. When the hammer strikes the chisel, most of the energy is transmitted through the chisel to the wood. Herein lays the challenge: Rule #4: you have to cut the transmission lines to stop the sound from traveling through one wall into another. You have to decouple the source from the destination. This can be done in various ways.

In studios, the most common approach is to build an offset stud wall. [link] Simply put, instead of building a wall using standard 2"x4" plates and studs, you use 2"x6" plates on the top and bottom and double up the studs in a zigzag so that each side of the wall and subsequent sheetrock have their own connecting studs. When set into motion from ye old hi-fi system, the first wall will vibrate independently from the second because they are decoupled. Double up the sheetrock to increase the mass and this wall will reduce the sound level by as much as 25dB!

If you have an existing wall or ceiling and want to increase the mass without loosing floor space, a good option is to use what is known as a resilient channel to do the same type of thing. Resilient channel is basically a U shaped spacer that screws on top of your existing wall. These allow you to add a second layer of sheetrock and introduce an air space in between the two layers. What you are essentially doing is creating a 'spring' mechanism to allow the first wall to float without connecting it to the second wall. This is also done on ceilings to stop sound from traveling upwards. Studios use a similar trick when they 'float floors'. Floor joists are 'suspended' on rubber isolation blocks on which a room is built inside a room.

Another good solution is to add a limp mass into the equation. A limp mass is basically a substance that when set into motion, will not transmit energy. Kick a sand bag; what happens? Nothing. No sound except maybe a whimper from the pain. The energy has been completely absorbed by a limp mass. Good news is that a limp mass also comes in the form of a sheet of heavy barium impregnated vinyl called Barrier. [link]. Barrier weighs around 1lb or 2lbs per square foot. Studios will often suspend a layer of Barrier in between the offset stud wall to increase the wall's efficiency. We also use this trick inside the Primacoustic FullTrap and MaxTrap to improve the bass absorption. Barrier can present a good solution if you have a drop T-Bar ceiling and want to stop sound from exiting the room. All you do is layer the Barrier on top of the ceiling tiles. Try to overlap the Barrier to stop air from passing. And shut don't forget to shut the window.

Summary:

1. Reduce the noise inside the room by adding Broadway absorptive panels
2. Stop air from traveling out of the room by closing air passages
3. Add mass to the walls, ceilings and doors by doubling up the sheetrock or adding Barrier
4. Decouple the wall and ceiling surfaces by introducing an air space and a spring surface