Why doesn't it work?


Why:   Sound deadening board is too rigid to be a vibration isolator, too lightweight to be a good barrier, and too solid to be a porous sound absorber.  In almost all cases where it appropriate to add more material, a barrier layer such as plywood or gypsum board will be more effective.


Sound deadening board is often placed on an inner stud face inside a double-stud wall.  But dividing up the interior air cavity will cause the two smaller cavities to resonate together, and reduce the overall sound rating from STC 60 down to STC 46.  Sound transmitting through the STC 46 wall is nearly 1 times as loud. Putting layers of any material (such as plywood, gypsum board, Celotex, etc) on the inside surfaces of the wall will reduce the STC rating dramatically.




Why:   Triple glazed windows are usually symmetrical, creating two small identical air cavities using three panes of the same thickness.  The glass and air spaces resonate sympathetically, passing certain frequencies easily through the entire window assembly. The same effect happens in a double-stud wall that gets divided by internal layers.

Window sound ratings can be improved by: using the largest practical airspace between two panes, and using dissimilar panes (different glass thicknesses, laminated and regular glass, etc). It is also critical to maintain an airtight perimeter seal. A thin vinyl interlayer added between two panes of glass does not count as a third pane.  The thin layer is there to improve R-values, and is not massive enough to affect the resonances within the cavity.


Why:   The batts are too porous and light to act as a sound barrier, and the ceiling tile is already absorptive.  The batts simply add some additional absorption. While a tiny amount of extra energy is absorbed, most of the sound filters through the batts virtually unimpeded.


When the wall stops at the suspended ceiling grid (referred to as a ceiling height wall) it is nearly impossible to get room-to-room sound ratings greater than STC 35, regardless of the wall construction. That is true even if the wall was thick concrete!   Penetrating the ceiling grid by four or six inches does not help either.  As long as there is an open plenum above the wall, STC 35 is the best possible result when using acoustical tile.



Why:   It is nearly impossible to avoid rigid contact with the underlying layer.  In a typical wall or floor-ceiling, even a half-dozen contact points are enough to completely negate the acoustical value of the resilient channels.  The small air gap also couples the two layers together, limiting any potential benefit.  The only meaningful solution is to remove one layer of gypsum board, put RC channels on the studs, then add new gypsum board -- preferably two new GWB layers. As a rule, we discourage the use of resilient channels whenever possible, because there are many ways to install them incorrectly, and most contractors have no idea what is right.



There are many myths and misapplications of acoustical materials.  Attack the problem with the right type of material.  The most common error is trying to use an absorber where a barrier is needed.  There are really only three things can be done to sound waves:

    - absorption  (absorber)          fiber glass, foam, heavy curtains, padded seats
    - reflection    (barrier)               plywood, glass, masonry, metal, gypsum board
    - scattering   (diffuser)             convex shapes and angles that break up reflections


Absorbers must be porous to work.  The sound waves have to interact with the "nooks and crannies" of the material.  Non-porous or closed-cell materials such as Styrofoam or "blue board" have no useful sound absorbing qualities. How do you tell if a material will be sound absorbing? Put it in water. If it soaks up moisture, it will also soak up sound. To work, absorbers must not be covered my impermeable materials such as heavy vinyl upholstery or sheet rock. The quick test: if you can breathe through a swatch of the covering, it will not hurt the absorptive performance of the underlying fiberglass.

Barriers reflect most of the sound back, preventing its transmission to the other side.  Barriers must be both reasonably massive (1-2 psf minimum) and impermeable. Think heavy and watertight.

Diffusers scatter the incoming sound waves back in a random fashion, rather than allowing a clean reflection such as from a hard flat surface.   A specular (clear image) reflection occurs with a polished mirror;  a diffuse (light/dark) reflection occurs from a matte surface like a piece of aluminum foil.  Diffusers are usually used in an acoustically critical room such as a studio or concert hall to prevent noticeable reflections or echoes that would degrade the listening environment.  Diffusers have little effect on overall sound levels, and are not terribly useful for noise control purposes.