Thursday, February 10, 2011

How Bad Can a 1% Air Gap Be to Noise Control?

by Harry Alter and Norman Varney 

Noise control is a two way street. You may have spent considerable expense on the design and materials of a wall, ceiling or floor system to keep noise from escaping or entering the space. However, you may not realize the impact a tiny hole can have on the entire partition's performance. Most people know that the door is the weak spot in a wall system. You can have a high sound transmission class (STC) rated wall system cut to half the rating if you don't incorporate an acoustical door, or not have one properly installed.

There are two primary means that sound energy can travel through walls, floors and ceilings; vibrations traveling through solid materials such as gypsum, sheathing, studs or joists are called structure-borne vibrations, and vibrations traveling through air, framing cavities and unsealed penetrations, seams or gaps are called air-borne vibrations. Both vibration paths play an important role in determining how well the partition assembly will reduce the transmission of sound through it, and a "systems" approach must be in its design to appropriately address the associated sound energies. A systems approach would incorporate a combination of blocking, breaking, absorbing and/or isolating the energy at the source, along its path(s) and/or at the receiver. We are covering just one of these aspects of noise control in this particular blog.

Air filtration and sound penetration through walls, ceilings and floors occur as one in the same. If air can penetrate a partition, then so can sound. In fact, it takes very little air leakage to cause significant sound leakage. For example, an opening or crack 1/100th of 1% of a total wall's surface area can reduce the sound transmission loss (TL) of a wall from 50 to 39 dB. That's an 11 dB drop in noise control performance. Likewise, a partition designed to achieve a TL of 40 dB would be reduced to approximately 30 dB (a 10 dB drop) with only 1/10th of 1% air leakage area to wall area. Note that the 10 dB drop in the poorer assembly would be perceived by the average person as twice as loud as the better assembly.

The above graph illustrates how openings and cracks can affect the TL (and subsequent STC) of a partition assembly. The horizontal axis indicates the design or desired performance of the assembly. The vertical axis indicates the resultant TL based on the % of surface area air/sound leakage. From the graph, the level of noise control performance will not increase beyond a certain level based on the size of the unsealed air gap. As a result, sealing air gaps reduces sound (noise) transmission through partitions. Less air penetration equals less sound penetration. The beauty is in the details.

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