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Technical Library

Lab & Field Testing

Currently, the most widely accepted standard for ranking the acoustical performance of accordion and operable partitions is STC or Sound Transmission Class. Virtually every manufacturer conducts laboratory tests.

FINDING STC. Sound Transmission Class is a two-digit number describing the laboratory performance of a single building element in stopping the transmission of sound through it. It is found from individual STL figures of 16 frequencies between 125 and 4000 Hertz. The STLs are found through the formula described earlier, completely defined by ASTM Standard E90. The procedure for finding the STC is defined in ASTM E413. Refer to the graph while following the steps below:

Accoustics In Decibels

  1. Plot the STLs on a graph.
  2. Plot the standard STC curve.
  3. Move this standard curve as high as possible so that:
    1. no frequency's STL falls below the standard curve by more than 8 dB. (Each dB below the curve is called a deficiency.)
    2. total deficiencies do not exceed 32.
  4. Locate the level on the STANDARD curve corresponding to the 500 Hz frequency. This is the STC.

How Good is an STC Rating?

Use the following chart to get a rough idea what various STC levels mean in terms of privacy afforded. Note that this is only a very rough guide.

25 Normal speech easily understood
30 Normal speech audible, but unintelligible
35 Loud speech understood
40 Loud speech audible, but unintelligible
45 Loud speech barely audible
50 Shouting barely audible
55 Shouting not audible

Source: U.S. Dept. of Commerce / National Bureau of Standards. Handbook 119, July, 1976: Quieting: A Practical Guide to Noise Control; Page 61.

STCs of Familiar Constructions

Listed below are some typical building items and materials, to give you an idea how the typical operable or accordion partition compares.

18 Hollow-metal door without seals
22 Solid wood door without seals
26 1/4" plate glass
32 1/2" plate glass
38 1/2" drywall on wood studs, both sides
41 4" painted concrete block wall
42 5/8" drywall on steel studs, both sides
46 8" hollow concrete block
48 12" painted concrete block wall
50 Double 5/8" drywall on steel studs, insulated
53 12" poured solid concrete wall

Field Testing and NIC

Perhaps the most practical way to state the acoustical performance of movable partitions already installed is Noise Isolation Class (NIC). This is a number describing the performance of ALL building elements in isolating one room from another. It is found from the Noise Reduction (NR) figures of the same 16 frequencies used in the laboratory STL. The same steps are used in the measurement and calculation, except the test is done in the field, and no effort is made to measure or use the absorption in the calculation.

The specific test procedure is defined by ASTM E336 while the NIC value is found exactly the same way as STC, using ASTM E413, except the Noise Reduction figures are plotted, rather than STL. Note: In this type of testing the emphasis is on the overall isolation of one room from another, so no attempt is made to measure flanking paths or the effect of room absorption on the results. In effect, it tells what the users of the room will experience.

Caution: It is very difficult to transfer the NIC obtained at an existing installation to a new facility. Very seldom are two buildings identically designed, and almost never do the same workmen build them. There are often major differences in ductwork, return air plenums, floor levelness, wall plumbness, etc. The only sure way to guarantee the results of a new installation is to require a field test for NIC upon completion. Also, one can expect NIC values to be 5 to 10 dB lower than the corresponding laboratory STC values for the same product.

Why the Big Difference Between Lab and Field Ratings?

STC is sort of like the EPA gas-mileage ratings for cars: you can use it for comparison but your actual acoustics will probably be worse. That is because real-Life buildings are not as well built as laboratories. The floors are not as level, the permanent walls are not as plumb, the joints are not as well sealed, the structure is not as heavy, etc. In a laboratory, test specimens tend to fit perfectly because the lab is built just for that purpose.

Generally, it is not a good idea to design a building to be as good acoustically as a laboratory. Purely and simply, it costs too much. And money must be available for decor, effective HVAC, and fire safety.

What to Look for in Tests

When acoustical tests are performed in laboratories, they are conducted under well-defined ASTM standards. Only a handful of labs across the U.S. have received accreditation from the National Bureau of Standards under the National Voluntary Laboratory Accreditation Program (NVLAP). When requesting a lab test for STL and STC, you should look for a NVLAP-accredited lab, or at least one with an excellent national reputation.

Generally, the same sensitive equipment used in the lab can be carried to the job site for a field test. Some on-site preparation is necessary to determine the noise source location and proper microphone paths. To assure a completely unbiased test, the procedure should be witnessed by an independent acoustical consultant. If one was hired in the planning stages, he or she would be the ideal choice for the final test.

On jobs so small that the cost of a test by an independent acoustician cannot be justified, consider requiring the manufacturer to conduct the test, with his own equipment, in the presence of the owner's representative.

Flanking Paths

Accoustics Flanking Paths

Sound, like water, follows the path of least resistance. If there are leaks in the surrounding construction, even the best movable partition will not provide a good sound barrier. Shoddy construction, customary construction practices, or poor installation of the partition can all contribute to the leaks, known as 'flanking paths'.

The chart here shows how flanking paths limit even a 55 STC partition to a 30 STC performance when a path one tenth of one percent of the total area exists.

Keeping an Eye on Details

Flanking paths can be present even when the surrounding construction is of good quality. Direct HVAC ductwork between rooms, common lobbies and corridors, and open plenums above suspended ceilings are all perfect escape routes for sound. The ceiling tiles themselves, whose porous properties help prevent reverberation, allow sound to pass through easily. Uneven floors and out-of-plumb walls also contribute to leaks as do recessed lighting, access panels, projection and lighting booths, and other design details.

Beating Flanking Paths

In dealing with sound transmission, there's no substitute for quality construction materials: simple mass is what stops sound waves. Naturally stud-and-drywall construction is going to have a higher STC than 1/4" hardboard panelling. Extra attention to quality techniques will pay off in sound control and ensure a good fit for the partitions.

Above all, the best defense against flanking paths is careful planning in the early stages of the project. ASTM Recommended Practice E557 is an excellent guide to installation conditions. It includes such issues as:

  1. flat, level floors
  2. level track suspended from a non-sagging structure
  3. sealed, insulated plenum barrier
  4. plumb end walls or adjustable jamb
  5. indirect HVAC ducts.
Hint: Using a screed directly under the track line when pouring a ballroom floor keeps the floor even and level for the seals.

Building a barrier in the plenum above the partition track with a construction as good as the partition itself, will prevent sound from leaking over the top.

The permanent walls with which the movable partitions intersect must be vertical and braced so that the horizontal pressure does not force the partition to become uneven. (ASTM Recommended Practice E497 is helpful for designing and building stud-and-drywall walls for acoustical performance.)

Although wall insulation (glass or mineral wool) is not an effective barrier, adding it in the cavity of a wall greatly improves its performance. It is also useful above the suspended ceiling to reduce reverberation.

Both the supply and return air ducts should branch from main lines outside the rooms containing movable partitions, rather than running directly from one side to the other. If this is impossible, at least using lined ducts with several zig-zags will help some.

Putting your Knowledge to Practical Use

The theories and terminology of acoustics come into practical application when you begin to plan the division of space with movable walls. What and how you specify relies on how you answer some very critical questions:

  1. What will the rooms be used for?
    The intended use of the building or room versus the use of the surrounding areas will, in part, determine the amount of sound control you need. For example, a folding wall dividing a high school science classroom from a student lab doesn't need to have as good a barrier as an operable partition between meeting rooms in a hotel. New offices near a heavily travelled corridor will need nearly as much sound control as a children's day care centre adjacent to the worship area of a church.

  2. What will the level of ambient noise be?
    It's important to realize that there is no such thing as 'sound proof'. There will always be some sort of ambient or background noise in a room. However, we can use that fact to our advantage. Let's take the case of the offices on the noisy corridor. The constant hum of a computer, or the woosh of the HVAC system can actually make the offices seem quieter, since these everyday sounds will partially mask the outside noise.

  3. What types of materials will be used in the room?
    The materials used in building and furnishing the room can either absorb or reflect the sound. Ideally there will be some of both. The walls, floors, ceiling decks, and doors should be able to block the sound from passing through. Soft surface treatments like carpet, drapes, wall hangings, etc., will reduce unwanted reverberation. In rooms where it is difficult to include adequate soft decor, such as a gym or hospital room, a better barrier may be needed to offset the extra loudness due to echoes.