Part 2: Sound Isolation Techniques
By Fred Gilpin
If you read the Sound Advice column in the last issue and did the Transmission Loss (T.L.) measurements, you should have an idea of how much sound isolation you require for your room.
If you want to increase the sound isolation without tearing your room apart and starting from scratch, follow these steps and you can expect around 40dB of mid- and high-frequency isolation and about 20dB of low frequency isolation. These numbers will vary based on the construction of your existing room.
If your measurements showed that you require more isolation than that (especially in the low frequency range), you will need a room-in-a-room design and I highly recommend you hire a studio designer. A good designer will save you far more money than you pay them.
The walls, ceiling, floor, doors, and windows should all have the comparable T.L. values. Otherwise, your sound isolation will be limited by the weakest link.
Air-tightness of all the joints in the room (wall corners, doors, windows) is the first step to increasing sound isolation. Gaps amounting to 2-3 per cent of the wall area can reduce the transmission loss by 50 per cent. Holes the size of a pencil can be filled with acoustical sealant, while larger ones should be filled with something dense like drywall mud. Hollow-core doors might as well not be there and must be replaced with solid-core doors. Install compression seals around the edges and top of the door and a drop-down seal along the bottom to provide an air-tight seal.
Window glass made of two layers of different thickness (like 3 mm and 4 mm laminated together with a dampening interlayer) will provide isolation comparable to the solid-core doors.
The next step is to increase the mass of your walls and ceiling. A cost-effective method is to add a layer of 1/2-in. Donnacona fibreboard (also known as TenTest), then a layer of 5/8-in. gypsum wall board (GWB). Do not screw the Donnacona to the wall; only screw the GWB at 24-in. centres along the studs. On the ceiling, just attach the Donnaconna with a couple screws to hold it in place while you install the GWB. Apply a bead of Tremco acoustical sealant to all seams to ensure airtight joints. The Donnacona dampens the two layers of GWB, reducing the resonance of the wall by about an octave, which relates to an extra octave of increased low frequency sound isolation.
Another option is to glue a layer of 5/8-in. QuietRock 530 to the existing wall. When using glue, only install enough screws to hold the QuietRock in place until the glue dries, then remove the screws and fill the holes. If you decide to use Green Glue, it takes several days to cure, so don’t take the screws out too soon. Using this method, you can expect to increase T.L. over the GWB/Donnaconna about 6-8dB, but at a significantly higher cost.
Many manufacturers use STC values to compare their products. The problem with STC values is they are an average of the frequencies between 125 Hz and 4 kHz. While this is useful for speech or normal home noise, it is not sufficient for evaluating the T.L. of studio walls where broadband music is being played. You can expect about 20dB less T.L. at 100 Hz than the STC values and about 30dB less at 30 Hz.
Surface-mount all electrical boxes and switches and seal around the wires with Tremco. Cutting holes in the walls for electrical boxes eliminates the gains of creating an air-tight wall in the first place.
Drum kits and bass guitar cabinets will create a great deal of vibration in your floors that will travel under your walls. To reduce this problem, you can build isolation risers by gluing together two layers of 3/4-in. MDF (staggering the seams) and installing a grid of 3/4-in. thick closed-cell neoprene rubber on the bottom side. You want to use enough strips of rubber that it only compresses 1/8-in. with the full weight sitting on the riser. You can purchase 1-in. wide by 3/4-in. thick closed cell neoprene from a specialty supplier. Otherwise, Home Depot sells TAGO Closed-Cell (Neoprene) Sponge Rubber Tape that comes 3/4-in. wide by 3/8-in. thick. You can double this up to get the 3/4-in. thickness. To check if the closed cell neoprene you chose is the correct density, place a 3 lb. PSI load on the rubber and it should compress 10-15 per cent.
Using this technique for the entire floor can increase the T.L. for airborne sound 6-10 dB (depending on the structure of the existing floor) and will almost completely decouple the structure-borne vibration for the entire room. Where the floor meets the walls, install the same closed cell neoprene rubber compressed 1/8 to 1/4 in. to keep from transferring the vibrations into the walls. You will need to calculate how much weight you will be placing on the floor and install the neoprene strips so as to not exceed the 3 PSI load on the neoprene. If the load gets to be 8 to 10 PSI, it may over compress over time and pop the cells in the neoprene.
Air vents are going to be a problem that is beyond the scope of this article. A simple solution is to build a solid airtight plug for the vents that you can install and remove as needed.
Fred Gilpin is an acoustic consultant, designer, and the owner of FGA Electroacoustics. He has 35 years of experience and expertise designing studios and providing acoustical solutions at facilities around the world. Fred is based out of Abbotsford, BC and can be reached at[ firstname.lastname@example.org](mailto: email@example.com).