It is important to prevent sound transfer between certain spaces. Offices need confidential privacy. Neighbors sharing common walls need to have a reasonable degree of separation for basic comfort and privacy. Special areas like movie theaters or recording studios cannot tolerate intrusive sounds from other areas.
Sound isolation is achieved by both proper wall selection and thorough attention to detail. It is not just a matter of adding extra mass to the wall. The right construction techniques must be used to get the maximum amount of separation.
Walls are rated in terms of the Sound Transmission Coefficient (STC). An STC 45 wall will stop about 45 decibels of airborne sound transfer from one space to another. However, the STC rating does not address very low frequencies. Noise insulation Class (NIC) is the field version of the STC laboratory test.
For residential projects, many state or local building codes require that walls between dwellings be rated at STC 50 or higher. The International Building Code (IBC) and the Uniform Building Code (UBC) both contain STC 50 and IIC 50 minimum requirements between dwellings. However, not all building departments adopt or enforce these sound rating requirements.
There are also requirements for footfall noise resistance for floor/ceilings expressed as the Impact Insulation Class (IIC) ratings. With carpeting, almost any floor-ceiling system will be rated at IIC 60 or better. It is very difficult to achieve IIC 50 with a hard surfaced floor, even with cushioned vinyl. This is often a problem with kitchens and bathrooms, where using carpet is not practical.
Published STC and IIC ratings vary for any given wall type. One popular single-stud / single sheet rock wall construction has test reports showing every value between STC 42 and STC 51. However, the average value is STC 46. If your project relies on one abnormally high value, the chosen wall may not really meet the minimum STC 50 code requirement.
A wall that has a laboratory tested rating of STC 50 is never expected to perform better than STC 45 in the field -- even with perfect construction technique. A five-point loss is considered normal for real world versus laboratory conditions, and is even incorporated into the building codes.
Details and quality control are critical. It does little good to choose a wall with a high sound rating, and then leave the perimeter and penetrations uncaulked. In fact, a sound leak totaling only 1% of the total area can degrade the overall sound rating by 15-20 decibels. One common example is unsealed electrical junction boxes in a party wall, especially when they occur back-to-back.
Conceptually, think of flooding the wall with water. Wherever the water leaks out, sound will also escape. These are the areas that need acoustical sealant. Acoustical sealant is non-shrinking and non-hardening, so it remains in place and pliable. There are special mastic pads available to seal electrical boxes.
Here are three common examples of sound isolation problems:
Given the problems with installation, we generally prefer either light gauge metal studs or staggered wood studs for most sound rated walls. Click here to download a one-sheet compilation of sound ratings. wall comparison chart
A layer of "sound-deadening board" or sheet rock, or a similar material is often used in the center space of a double stud wall, between the stud rows. It seems intuitive that adding an extra layer of material would improve the sound rating. However, dividing the internal air cavity changes the resonances of the wall, with a serious loss of performance. Without an open internal structure a double stud wall is typically rated at STC 60. With an inner layer, the rating drops to STC 46.
Sound-deadening board is too lightweight to be a noise barrier, too rigid to replace a resilient channel, but not porous enough to be an effective absorber. There are very few situations where sound-deadening board has ANY useful acoustical properties.