In part one of this mission to try in cover all the main points of soundproofing a control room, we talked about room resonances, the frequency of resonance of a room, parallel walls with proper room sizes and the associated volumes. In Part II, we will discuss more issues that must be addressed starting with a term called a sound lock, doors, weather striping and HVAC noise. What is a sound lock?

Sound Lock

Remember in the movies, especially Aliens, where Weaver was always blowing creatures through a device called an air lock. An air lock provided a barrier between two atmospheres. We had the atmosphere of the ship and then we had outer space. A sound lock is similar to the air lock. A sound lock is a barrier between the actual studio or control room and the out side world. The sound lock is a corridor or hallway that has been acoustically treated to absorb lots of energy. When the outside world door is opened and someone comes in and closes the door behind them, they are now in the sound lock chamber. Now, they open the control room door and leave behind about 10 – 15 db. of noise level. Outside noise is like dirt on your shoe. It should be left at the door and not brought in.

To accomplish this large rate and level of low, middle, and high frequencies, one must use a combination of different materials. First, we will need to address low frequency issues. We will need low frequency absorbers (diaphragmatic) that are built into the existing walls. We can use the existing wall to assist us in getting the correct rates and levels necessary. Foams are good for middle and high frequencies on walls insides. One must use a minimum of 4″ thickness to achieve the rates and levels needed to reduce the levels inside the sound lock.

Acoustical Doors

It is too bad we need doors to access our control rooms. I wish we could just migrate through some time change chamber and just be in the control room. Doors have edges that move and are expected to touch the room’s surface and provide a seal. They do not most of the time and the continuous back and forth movement the door goes through just opening and closing causes wear and surface to surface separation. There are some solutions we can consider. None are without issues.

Weight is our friend when it comes to control room door construction. Mass is our building material requirement. We need a mass of at least 7.0 pounds per square foot to produce the necessary densities to stop sound energy from going through. We will use a series of different materials sandwiched together with vibration reducing compounds between each material layer. Place this approximately 6″ thick door, 300 pounds, on a bearing recessed into the floor and top door frame. Weatherstrip the inside frame. Secure a large handle to both sides and people will think they are opening a bank vault door to get into and out of the control room.

HVAC

HVAC stands for heating, ventilating, air conditioning. We have a blower or fan pushing hot or cool air through a series of metal, rectangular shaped, vent duct and this air is moving. We all know from Part I, how a rectangular room exhibits certain resonances because of its size. A rectangular vent acts the same way with similar resonances just at different frequency ranges. Air movement causes vibration and vibration can produce noise. We do not want this wind noise coming through the duct work and out the vent into our control room. We do not want our air vent turned into a speaker of noise, especially if we have numerous air vents. Just what any engineer wants, a series of mono, noise producing, “speakers” beaming high frequency energy into the control room and then in the mix. We must lower the air flow to less than 340 cubic feet per minute.

We can line our vents with sound absorbing material. It needs to be at least two inches thick. An open celled foam would work. They are inexpensive and easy to apply. Our goal is sound attenuation. We want to reduce energy levels within the duct work by an average of 30db. Make sure you place foam over the air vent itself. This is a special type of speaker grill cloth.

Soundproofing our floors requires that we employ barrier technology. We need to put a barrier between the noise coming from other sources outside our room. We also need to put a barrier between the noise we are generating from inside our rooms and the other rooms that adjoin our room.

Put Up A Barrier

Barrier technology is all about using different materials with different densities arranged in a constrained layer, mass damped, arrangement. This methodology involves using the density of the material coupled with other materials arranged and assembled in a way that reduces vibration. Barrier technology is all about minimizing vibrations. Sound energy creates vibrations in our room boundary surfaces and that vibration of our wall, ceiling, and floor surfaces creates noise indirectly and in some cases directly. Wall, ceiling, or floor surfaces can go diaphragmatic or move in response to sound energy, especially low frequency energy. When the wall or floor goes diaphragmatic, it moves like a speaker cone and can produce “sound” of its own. Our goal is to make the barrier or floor as rigid as possible, but flexible enough to move without making any “noise” of its own.

Noise Level Number

Our first goal is to determine what our actual noise problem is. It is important that we put a number to the noise. The number tells us how big our problem is and how big of a solution we need to use to make the numbers work. One can measure the noise levels in their room with a simple device called a sound pressure level meter. Radio Shack has one for around $60 that will work. Make sure you ask for a dB meter and not a sound pressure meter. They use the unit of measure as the name for the device.

Measure all sound pressure levels throughout the day. Turn off all noise generating devices inside your room and record the number. Do this throughout the day or night, probably for every day during the week. This number will be our number that shows what the sound pressure level is when our room is not the cause of the noise. If possible, go into the next room and perform the same series of measurements with all noise generating sources turned on in our room. This number will tell us what noise we are generating that will enter other rooms.

Standard Building Materials

Once we have our “noise” number, we can then decide what type and how much of barrier technology we need to use. If our noise issues are not severe, we can begin with a standard sheet of plywood. Next, we put a piece of drywall on top of the plywood. Lets use a 3/4″ piece of plywood and a 1/2″ piece of drywall. By varying the thickness of the drywall and plywood, we use mass damping technology to slow down the vibrations that are being transmitted through the materials. We can next add another piece of plywood to the second drywall piece. When we attach the plywood and drywall together, we should cross seam each plywood piece. Cross seaming means that we do not run the plywood pieces the same way or direction. If the grain on the top piece is positioned vertically, then we run the grain on the second piece the opposite direction. This process will aid in the reduction of vibrations. Each layer of plywood and drywall should be glued and screwed with screws 12″ on center.

Floor Floating

Once we have our floor assembled, we do not want to set it upon the existing floor. If we set it upon existing floor, we will have some noise reduction, simply because we have added more mass. However, this will not provide us with the maximum sound isolation. We need to physically separate our new floor from our existing floor. We have to mechanically decouple our new floor from our existing floor with at least a 5″ air space. A 6″ air space is even better. We “float” our floor by placing it on “hockey pucks”. Well, they are really not hockey pucks but vibrational isolating feet that look like hockey pucks. We will place them on 24″ centers throughout the total floor space and then place our new drywall/plywood floor on top the isolators. We must be careful not to contact the sidewalls of our room with our new floor. We should leave about a 1/4″ air space around the edge of our new floor. That 1/4″ space can be filled with a rubber isolation strip or acoustical sealant. This rubber strip or acoustic sealant provides another material type for vibrations to have to go through. The more materials for vibrations to go through, the better.

To sound proof our floor, we must first measure the noise level we have to control. From there, we use different materials with different densities arranged in certain ways to maximize vibrational control. Gluing and screwing the materials together adds the necessary rigidity for vibration control. We then separate our new barrier from the existing floor through the use of vibrational isolating feet with an appropriate air space created by the isolators.