Flooring Design with Acoustics in Mind

Controlling sound in today’s smooth surface designs
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Sound vs. Noise

“Noise” is a subjective term. One person’s brilliant guitar solo might be perceived by another as a hideous racket. But in general, sounds that register as “noise” have some common characteristics: they are loud enough or of such high intensity that they are uncomfortable; they are intermittent and unpredictable rather than continuous; and they compete or interfere with the sounds you really want to hear.

The basic goal of design for good acoustics is to reduce unwanted sound and make desired sound clearer and easier to hear. This is possible because of the way soundwaves interact with different materials and barriers between sound sources and receivers (ears).

There are three basic physical possibilities for a soundwave. Sound can be:

Reflected: It bounces off the surface back into the room. This may create echoes and reverberation. Hard surfaces are highly reflective. The more reflective surfaces there are in the room, the more the echoes will bounce back and forth.

Absorbed: It hits a soft, fibrous material, where sound energy is converted into heat via friction, or a dense, resilient material like rubber, which acts more like a damper and “absorbs” sound by dissipating the acoustic energy within the material itself.

Transmitted: This can happen when a wave goes right through a thin material or an opening, like a crack or gap. It also happens when the wave causes part of the structure itself—for example, the floor or wall between two spaces—to vibrate and transmit the sound.

In general, higher frequencies are more likely to be reflected by materials in a room. The relatively long, slower wavelengths of lower sounds can more easily pass through and around materials. The simplest example is how well you can hear the bass line from music upstairs or a car going by, even if you can’t make out anything else.

In most spaces, absorbing sound within the room and blocking transmission of sound between adjacent spaces are both important. For example, in a hospital, it is critical that staff and patients can clearly understand speech in the room, so finishes should absorb excess noise. But for privacy and to control noise in the whole area, sound transmission into the corridor or between adjacent rooms needs to be controlled too.

How much sound is reflected, and how much is absorbed or transmitted, will be greatly influenced by the room’s materials. Some materials, like insulation, foam, cork or rubber, can muffle or mitigate noise. Others can dramatically increase noise—for example, from the reverberation when soundwaves impact hard surfaces like tile, stone, and glass.

This is why the testing and rating of materials is so important. They give designers verified and quantifiable ways to predict how products and materials will work in the space and enhance the way people will experience sound.

Key Numbers (And Letters)

Small changes in the ratings can make a big difference in acoustics. For flooring specifically, there are four ratings designers should understand and use when selecting surfaces.

The first, noise reduction coefficient (NRC), measures how the material absorbs sound. The second and third ratings are closely related: impact insulation class (IIC) and delta IIC (written as ∆IIC). These are measures of how structure-borne sound is transmitted from one room to the room below, through the floor-ceiling assembly. These are probably the most important factors for evaluating flooring, and also probably the most often confused. The fourth rating, sound transmission class (STC,) is a measure of airborne sound transmitted through a structure (e.g., a partition or floor-ceiling assembly), often used but with some limitations when applied to flooring.

An additional new measure relevant to flooring acoustics, surface generated sound (discussed later in the course) has been developed and is nearing release by ASTM.

But right now, the following four are the indispensable basics.

(Also, see Quick Guide to the Ratings sidebar.)

Noise Reduction Coefficient (NRC)

Based on ASTM C423: Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method

NRC measures how much sound a finish material absorbs when hit by a soundwave. Any sound the material does not absorb will either be reflected or transmitted. NRC is a single number from zero (absorbing no sound) to one (absorbing all the sound that hits it). So, for example, if a material has an NRC of 0, all the sound that hits it will be reflected back into the space. A theoretical material that absorbed all sound would have an NRC of 1. A basic smooth surface flooring might have an NRC rating of 0.20.

Rooms with very hard surfaces and low NRC ratings absorb very little sound. Most of the sound from voices, laughter, footsteps, music, chairs sliding in and out, and many other sources is reflected right back into the room and bounces around with all the other soundwaves, causing echoes, reverberations, and the perception of confused loudness.

Carpet has always been the most absorptive flooring material, particularly useful at absorbing footsteps, but as discussed below, carpet has distinct drawbacks in many applications. Some resilient flooring and underlayment products are now being engineered to offer acoustic performance approaching that of carpet.

Impact Insulation Class (IIC)

Based on ASTM E492: Standard Test Method for Laboratory Measurement of Sound Transmission Through Subfloor-Ceiling Assemblies Using the Tapping Machine

IIC is a measure of sound (in dB) transmitted through the structure from upstairs to downstairs. Impact noise is not airborne sound like a radio playing or people talking. IIC tests measure sound from impacts like high heels hitting the floor above your ceiling or heavy weights being dropped in the fitness center above. The higher the IIC rating, the more effective the floor-ceiling assembly is at isolating the vibrations and impact sound.

IIC ratings can be greatly affected by the flooring. Materials improve the IIC rating according to their physical properties. More rigid materials are less effective than materials that offer internal damping of sound. For example, the IIC of a very basic wood floor assembly with no underlayment might be 20–25. Adding an effective underlayment might give the entire floor-ceiling assembly an IIC in the 50s, dramatically reducing the transferred vibrations and thus the noise.

This measure—if used properly—is probably the most important for the accurate evaluation of acoustic flooring products, particularly when they are to be used anywhere in buildings of more than one story. According to the International Building Code (IBC), the minimum IIC rating is 50 (if using laboratory tests; 45 if testing in the field, written as FIIC). Most local building codes are based on the IBC, and most have minimum acoustic requirements for multistory buildings.

However, these are minimums and not likely to come anywhere near meeting the needs of most carefully designed spaces today, from health care and educational settings to high-end apartments.

One key thing to remember: no flooring product has an IIC rating all its own. All IIC ratings are for an entire floor-ceiling assembly. (See Quick Guide to the Ratings sidebar.) The type of materials, the thickness of the flooring and of the underlayment, the construction of the assembly, and the installation methods all make big differences that can cause wide swings in IIC ratings.

Delta IIC (∆IIC)

Based on ASTM E2179: Standard Test Method for Laboratory Measurement of the Effectiveness of Floor Coverings in Reducing Impact Sound Transmission through Concrete Floors

It’s easy to miss the small Greek letter ∆ (delta) that makes a very important difference in evaluating how much a flooring product will help reduce impact noise. “Delta” IIC, written as ∆IIC, is a way of determining the rating of the flooring product itself, subtracting it out from the total measurement for the rest of the building assembly (the concrete slab). The ∆IIC rating will give you a good idea of what the product might add to a floor-ceiling assembly.

Testing under this ASTM standard first measures the bare 6-inch concrete slab assembly without any flooring products to get a baseline IIC rating (28 is usually considered the baseline reference for a 6-inch concrete slab). Then the entire assembly is tested again after flooring products are installed for an overall IIC rating (say, 58). The difference between those two tests is the ∆IIC for the flooring products only (in this example, 30).

Understanding the distinction helps avoid unhelpful product comparisons. For example, under the standard, the base reference is 28 for a concrete floor without any flooring material. So a flooring product with a stated IIC of 35 probably only has a ∆IIC of 7—not a significant contribution. But a flooring product with a ∆IIC of 35 would indicate that the product alone contributes significant impact protection, beyond the protection provided by the slab alone.

Another important detail to understand about a ∆IIC rating is what products are included: Is it a finished flooring product alone, or did the test also include an acoustical underlayment? The same caution is true when reviewing ∆IIC ratings for underlayments. It’s important to determine which floor covering was tested in conjunction with the underlayment. Underlayments are never installed without floor coverings, and therefore their ∆IIC ratings should always include a floor covering. The same underlayment tested with different floor coverings will have different ∆IIC ratings (for example, tile vs. engineered wood over the same underlayment).

∆IIC tests at this time are only conducted over 6-inch concrete slabs with no ceiling below.

Manufacturers of effective, properly tested flooring products should be able to provide ∆IIC numbers, as well as data on alternative assemblies so that designers can make realistic comparisons based on accurate predictions of performance.

STC: Sound Transmission Class

Based on ASTM E90: Standard Test Method for Laboratory Measurement of Airborne Sound

STC is a measure of how partitions and/or floor-ceiling assemblies reduce the airborne sound being transmitted through them. A partition could be a floor/ceiling assembly, but more often STC is used to describe barriers like walls, doors, room dividers, etc. Higher values are better as they indicate less transmission of airborne sound. For example, you could probably hear loud speech through a wall with an STC of 30, but not through a wall with an STC of 60. The International Building Code specifies the same minimums for STC as for IIC: 50 if tested in the lab, and 45 in the field.

STC is primarily controlled by mass and therefore is not significantly affected by flooring systems. For example, whether a floor assembly is made of concrete slab or wood joist is going to be more of a determining factor in the STC rating of a floor-ceiling assembly than the type of floor covering used.


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