Misleading Advertising and Common Mistakes
The following are some common problems we see where products are improperly represented or applied.
Thin Cloth Sound Absorbers
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.
Misuse of Resilient Channel
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.)
Where NOT to use Fiberglass Ceiling Panels
Ask what is the best ceiling panel one can buy and answer often seen is Fiberglass, products such as Armstrong Optima, USG Halcyon ClimaPlus, and Certainteed Symphony f. In one regard that answer is right. However, in many situations these are the WORST ceiling you can have. Where they excel is in not reflecting sound. However, part of the way they accomplish this is that sound passes through them very easily. Thus, if you need to block the passage of sound, you do not want these ceilings. The most common mis-application is in the ceilings of closed private offices. The fiberglass ceiling is the ideal panel for open office areas. However, when there are closed offices or conference rooms adjacent to the open offices, those closed areas must have a mineral fiber ceiling panel. Such mineral fiber ceiling also should extend for about 4 feet into adjacent open areas. All three of the manufacturers know this and produce fiberglass and mineral fiber panels with IDENTICAL appearance so they can be used in adjacent areas and still look right.
Top 10 ways to foul up a resilient channel installation
In the spirit of David Letterman’s top 10 lists, the 10 most common mistakes in the use of
resilient channel that reduce acoustical performance.
10. Overlap ends of channel several inches, resulting in squeaks.
9. Use double leg channel that is not really resilient
8. Use 20 gauge channel that is not really resilient
7. Install resilient channel in line under trusses instead of perpendicular
6. Install resilient channel upside down
5. Install resilient channel with solid part of web at joists or studs
4. Put resilient channel between two layers of gypsum
3. Use hat channel instead of resilient channel
2. Use long screws from gypsum into wood above
1. Use poor quality resilient channel just because it is labeled RC-1
Wall STC Variation
There is an unfortunate assumption that the STC of a wall is a precise and 100% repeatable result. The truth is that in cases where many test results for the same construction are available, a wide range of results are seen. With enough data a median or average expected result can be established. However, with limited data, one cannot be certain whether the result seen is truly representative or high or low. With so many new products coming available, we have limited data that could be very misleading. Fortunately, for traditional constructions, and now for some of the newer constructions, an acoustical consultant who specializes in sound isolation and does a high volume of testing ismcompiling results to get statistical data on performance. For instance, consider wall that most typically performs around STC 51 or 52 in laboratory tests and is widely used in applications where a wall of greater than STC 50 is required with an expectation of greater than ASTC 45 in the field. His data on a very large number of these walls indicates that the wall will usually achieve greater than ASTC 45 in the field, but around 12% of the time it will not, even if nothing is done wrong in the construction. In our experience, we often find architects selecting walls that would typically achieve only around STC 48 in the laboratory for these applications, based on an unrepresentative test that indicates STC 50. Such walls have a very high risk of being less than ASTC 45 in the field.
Steel Studs – Light Gauge or Load Bearing – Big Difference
We are often called in on situations where walls with steel studs have not given the performance the designers expected. The common problem is that the designers have depended on data for “steel studs” but have used load-bearing heavy gauge steel studs. There are many tables of data for “steel studs” and even some original test reports that make no mention of the gauge of steel used for the studs. In almost all these cases, the data are based on 25 gauge studs. The gauge of the steel makes a major difference in the sound blocking ability of walls when the gypsum is attached directly to a single set of studs. The flexibility of a light 25 gauge studs reduces the structural transmission of sound through the stud and the stiffness of the wall moving the stiffness resonance down in frequency. A heavy load-bearing stud will behave much like a wood stud. This difference is reduced when resilient channel is added. The difference is also minimal in double stud or staggered stud arrangements unless bracing is required between studs on each side of the wall. When such bracing is required, performance is deteriorated some with light gauge studs and strongly with heavy gauge studs.
Unrealistic Wall Test Results
The sound blockage ability and STC of a cavity wall can be improved when the gypsum layers on each side are well isolated from one another using light gauge studs, resilient channels or one of the new resilient clip systems. However, when only a single layer is used on each side the low-frequency performance is poor and the STC is often controlled solely by the performance at 125 Hz. That means the STC can vary widely from test to test of the same wall dependent on the 125 Hz result that typically varies widely. The low frequency performance can be improved even with the light weight if the air space is large enough. However, we are concerned that some suppliers are publicizing test results with small air spaces and a single layer of gypsum on each side with STC results that are not realistically representative of what can be normally expected. We have also noticed that a major supplier of gypsum is now publishing the highest test result they can find for wall designs rather than typical results. Users are cautioned to be careful about results that look too good to be true. The result may have been achieved in one test, but one test does not verify normal expectations.
Laboratory Test Reports Can Be Erroneous
We once reviewed a laboratory report on a type of glue that appeared to indicate miraculous performance in preventing impact sounds when used on a concrete slab with no ceiling below. After sending an email to the lab that did the test we got a quick return phone call after they had reviewed the situation. They had somehow accidentally failed to mention that a well isolated ceiling was installed under the slab during the test. The manufacturer of the glue product was honestly unaware of the very misleading claims made about his product as a result.
Condos – Many Problems Out There
Many condominiums are built without good guidance on acoustical issues and this is creating serious problems. The most serious is floor-ceilings that do not meet the basic building code requirements for IIC rating. Such problems are extremely difficult to fix after the building is constructed as impact isolation must be designed into the heart of the floor structure. Tearing into structure after a unit is occupied means relocating people. Getting the materials in and out can be very difficult. Raising a floor to add materials creates problems with doors and counter heights. You might have difficulty getting fire approvals for a design that has never been tested.
Much of the information available from suppliers of materials can be misleading. Some test results may be the best ever achieved with a design and not representative. Laboratory results can differ greatly from expected results in the field because laboratory results do not include flanking of impact sound into walls. Wood-frame structures behave very differently from concrete structures. Some products that can work with concrete structures do not work on wood frame. Most test data is for either wood-frame or heavy concrete construction. Risk is greater for any other structure type due to lack of information.
The features that provide acoustical quality are not visible or immediately noticeable by a potential buyer. However, those buyers expect the acoustical quality to match the visual quality they see. Reducing the money spent on acoustical isolation is false economy and very risky. We strongly encourage developers and designers to consider their floor-ceiling designs carefully and get independent advice before construction.