BEWARE!!!
Misleading Advertising and Common Mistakes
The following are some common problems we see where products are improperly represented or applied.
Thin Cloth Sound Absorbers
Light Foam Fillers for Concrete Block
Heavy Vinyl
Resilient Channel that is not Resilient
Misuse of Resilient Channel
Cloth wall coverings about 1/8-inch thick are available. With that thickness, they can only absorb very high frequency sound effectively. Reputable manufacturers and sellers will indicate an NRC rating of 15 to 20, but most of the absorption that gives this rating is in the 2000 Hz band. Unfortunately, some disreputable or ignorant sellers advertise this product as having an NRC rating of around 60. If you get the test report, you will find this result was obtained with the cloth installed over a half-inch or 5/8-inch mineral-fiber sound absorber that is providing most of the absorption.
Light Foam Fillers for Concrete Block
Concrete block get most of their sound blockage ability from their heavy weight. Some blocks are heavier than others, providing better blockage especially at lower frequencies. The blockage ability of concrete blocks can be improved by filling cavities with sand or mortar. Lightweight fillers provide little or no benefit. One manufacturer of a light foam filler claims their product gives results equivalent to filling a block with sand. They can provide test reports showing the same STC for block walls with blocks filled with sand and filled with their foam. However, look carefully. You will see that the two tested walls weighed almost exactly the same. How did this occur? Lightweight blocks were filled with sand, and heavy blocks were filled with the light foam. The heavier block would have given the same result without the foam or sand.
Heavy materials block sound. All materials will have a frequency range where they are weak. The stiffer the material, the lower this frequency will be. Thus, a limp but heavy material is desirable. Thin sheets of lead meet this description. Due to environmental concerns about lead, heavy vinyl has become a replacement. Many companies make vinyl sheets weighing about one pound per square foot. These have wide applications in industrial noise control and a few architectural applications where one pound per square foot is sufficient. Most architectural applications require much more weight for adequate blockage, and such weight is available with materials costing much less than the vinyl. Some manufacturers are attempting sell vinyl for use in architectural walls. If a wall already weighs around 10 pounds per square foot, the extra pound added by the vinyl provides little benefit especially considering the high cost of the vinyl. The vinyl could provide one advantage. Gypsum panels have a weakness at some frequency in the range of 1500 to 4000 Hz depending on thickness of the panels. This weakness in vinyl is at a much higher frequency. Thus, the vinyl can be strong where the gypsum is weak. However, the same benefit can be obtained by simply using layers of gypsum of different thicknesses such as 5/8 inch and 3/8 inch and not gluing them together. Thus, there is no justification for the extra expense of the vinyl.
Resilient Channel that is not Resilient
When a design calls for resilient channel, the intent is that the channel be 25-gauge steel (or possibly 26-gauge) with stretched Z shape where one leg is screwed to joists or studs and gypsum is screwed to the other. Some manufacturers sell channel that is a stiff 20 gauge steel. They also sell product they call a two-leg resilient channel that is really more like a hat channel. This is not sufficiently resilient. Ideally, the channel also should have slots about 3 inches long with solid sections about an inch long in the web. Slots should be aligned with studs or joists. When installed on vertical walls, the leg attached to the studs should be at the bottom.
When a wall or floor-ceiling does not adequately block sound, an easy solution is often desired. A common mistake is to think that this can be achieved by simply adding resilient channel to an existing surface and then adding a layer of gypsum drywall leaving a space of only ½ to 5/8 inch between two layers of gypsum. This will improve blockage at very high frequencies but can actually make blockage worse for low-vowel sounds. This happens because the small airspace creates a resonance or condition in which the layers of gypsum want to vibrate in this frequency range. (Note that a similar effect occurs in many thermal windows with small air gaps.)