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Noise reduction coefficient

The noise reduction coefficient (NRC) is a measure of how effective certain building materials are at controlling sound within a room.  This article describes briefly how the NRC is defined, what it does and does not mean, and how to use it.  Some examples of noise reduction coefficients for typical building materials are also provided.

When a sound wave in air hits one side of a planar material element, a portion of the incoming sound energy is reflected, a portion is absorbed, and a portion is transmitted through to the other side.  In room acoustics, the primary concern is usually with how sound behaves within the room.  From this point of view, when sound hits a wall or ceiling, it doesn’t really matter whether it is absorbed or transmitted; either way sound energy leaves the room.  So, the sound absorption coefficient of a material is usually defined to be the proportion of energy absorbed by or transmitted through the element.  In other words, the absorption coefficient is 1 minus the proportion of reflected energy.  It is therefore always a number between zero and one.  The absorption coefficient depends both on the frequency of the sound  and on the angle that the sound hits the surface.  Since one can not generally predict beforehand from which directions the sound in a room will strike the surface, the absorption coefficient can be averaged over random directions.  This is accomplished by measuring the absorption coefficient in a reverberation chamber, which is a room specially constructed to disperse sound in many different directions.

The noise reduction coefficient is defined to be the average of the random incidence absorption coefficient at four specific frequencies: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz, and then rounded off in increments of 0.05.  So for example, if the four-frequency average of the absorption coefficients is 0.27, the NRC would be 0.25.

Here are some things to keep in mind when considering NRC values.  First, it is important to understand that the NRC is merely an average at four different frequencies.  The 250, 500, 1000, 2000 Hz frequencies were chosen for the rating because these cover the range where interior noise generated by human speech is mostly concentrated.  The rating was designed for specification of materials for offices, waiting areas, restaurants, and so on.  Because NRC is an average, it gives absolutely no information about how effective the material is at controlling sound at specific frequencies.  For example, a material with absorption coefficients of  0.00 at 250 Hz, 0.00 at 500 Hz, 0.25 at 1000 Hz and 0.75 at 2000 Hz would have an NRC value of 0.25, even though it apparently provides no absorption at all at 500 Hz and below.  Furthermore, the frequency range of 250 Hz to 2000 Hz is a small fraction of the range of human hearing (usually taken to be about 20 Hz to 20,000 Hz).  The NRC provides no information at all about high frequencies or low frequencies.  When considering materials for a musical listening area or home theater, NRC ratings simply do not provide enough information to make informed decisions.

Second, remember that NRC is defined in terms of the sound absorption coefficient, which unfortunately in room acoustics does not measure sound absorption.  The classic example is an open window.  From a room acoustics point of view, the window has an absorption coefficient of 1.00 at all frequencies, because all of the sound incident upon the opening leaves the room.  The NRC rating for the open window would be 1.00.  But the window does nothing to reduce outside noise entering the room, and it also does not reduce the amount of noise escaping the room, perhaps to a room next door.  If a brick wall is built just outside the open window, suddenly the window’s NRC rating of 1.00 means nothing, since the brick will reflect most of the sound back into the room.  A common situation where this distinction is important is with dropped acoustical ceilings.  If the ceiling is suspended below a reflective surface, like a concrete slab, ceiling tiles with a large NRC rating would not be a good choice for sound control, unless some other form of sound insulation is introduced in the air space.

Finally, keep in mind that the NRC does not measure total absorption in a room, only absorption per unit area.  This is probably obvious, but a 1 square meter section of carpeted flooring will only be roughly half as effective at reducing sound as a 2 square meter section.

Here is a list of a few approximate NRC ratings for some common building materials.  These numbers can vary depending on the properties of the specific materials being used and who measures them.  Consult manufacturers or laboratory data before beginning an actual project.

Material NRC value
Gypsum drywall, 1/2 inch thick, nailed to 16 inch on center 2x4s 0.05
Plaster on lath 0.05
Plywood, 3/8 inch 0.15
Window glass 0.15
Medium weight drapery, pleated 0.55
Concrete floor 0.00
Marble or glazed tile floor 0.00
Linoleum over concrete floor 0.05
Wood floor 0.10
Heavy carpet on concrete 0.30
Heavy carpet on foam padding 0.55
Fiberboard suspended ceiling, 3/4 inch, medium texture 0.55
Thin, porous suspended ceiling, 3/4 inch 0.75

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