Residential Sound Isolation – A Primer

batting-insulation-0314We frequently get calls from owners and tenants who wish to improve the sound isolation between apartments and condominiums, particularly in older units. The call often begins with, “I want something I can spray on to the wall to soundproof it so I don’t hear my neighbor’s stereo…” or “My neighbor’s snoring keeps me awake at night.” Unfortunately, just putting something on the surface of the walls will not really improve things.

Sound absorbing materials only “color” the sound in the room the material is added to. Typically in a very absorptive room such as a bedroom, adding more absorption has little effect on things. Experience has shown there is only a 2 to 4 decibel change in sound levels between an unfurnished room and a completely furnished one.

“But what is a 2 to 4 decibel difference?” is usually the next question. In a typical setting, the sound level would need to change by 3 decibels for you to be able to hear a change. An example of this is the change in the sound level if you are listening to two loudspeakers side-by-side and then turn one off then back on.

“OK, but isn’t there some type of minimum code requirement?” The answer here is that in 1973 the State of California adopted Title 25, which required that the minimum sound isolation across party walls and floor/ceilings in hotels, motels, condominiums and any other dwelling area, be at least STC 50. This code has been adopted, amended, changed and is presently Title 24, Chapter 35 of the California Building Code. In case you are wondering, STC is the Sound Transmission Class of a partition. It is a single number rating system used to compare the sound insulating properties of different walls, floor/ceilings, doors, etc. primarily in respect to speech and small appliance noise. In a building the actual construction of a STC 50 partition is approximately equal to 50 decibels of sound isolation. The higher the STC number the greater the sound isolation properties of the partition.

So how do we reduce the noise if sound absorbing materials don’t work? If there is no “magic” material to spray on to the walls? As you might have guessed, the answer is to upgrade the wall or floor/ceiling constructions to increase the STC rating of the partition. This usually involves removing the existing gypsum board, adding batt insulation, then mounting or hanging new gypsum board on resilient channels. This construction has a rating of STC 50 versus an un-insulated wall with an STC 35 rating. An increase of 15 STC points! (Please see diagrams.) In some cases, when it is not desired to remove the existing gypsum board, a “new” wall can be furred out from the existing wall.

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“But how much sound isolation is enough?” As we have discussed in recent articles this is a “signal-to-noise” issue. The quieter the background environment where you will be listening, the greater the insulating properties required to keep loud noises out. What works in an urban setting with bus and auto noise will not work in a quiet, rural area with only wind and birds.