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Warnock, A. C. C.
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Controlling the transmission of Impact sound
through floors
Warnock, A. C. C.
A version of this paper is published in / Une version de ce document se trouve dans : Construction Canada, vol. 42, no. 5, September 2000
www.nrc.ca/irc/ircpubs
NRCC 44483
12/01/002:55 PM
Controlling the Transmission of Impact Sound through Floors
by A.C.C. Warnock
Impact sounds, such as those created by footsteps, the dropping of an object or the moving of furniture, can be a source of great annoyance in residential buildings. While there are no requirements in the National Building Code for floor impact sound
attenuation, some degree of sound control is necessary for occupant comfort.
A standard tapping machine1 incorporating five steel-faced hammers has been used for decades to rate floors for impact sound attenuation. The noise levels generated by the machine are used to calculate a rating called the impact insulation class (IIC).2 The higher the IIC, the better the attenuation of impact sound. Experience has shown that the minimum IIC value needed to achieve occupant satisfaction is 50.
A tapping machine used in tests to evaluate the ability of floor coverings to reduce the transmission of impact sound from one floor to another in multi-family dwellings
The character and level of impact noise generated depends on the object striking the floor, on the basic structure of the floor, and on the floor covering. This article, based on recent research at NRC’s Institute for Research in Construction, discusses some of the difficulties encountered in controlling the transmission of impact noise and explains how to achieve good sound control with three different types of base floors. One of the
difficulties relates to limitations in the use of the tapping machine in the rating of floor systems; the machine, for instance, is unable to simulate the low-frequency thumping sounds characteristic of a person walking. This limitation can mean that even though a floor has a good IIC rating, thumping sounds can still be annoying.
Floor Coverings or Toppings
The choice of floor covering has a major effect on the attenuation of impact sound. There are two main ways of providing an acceptable finished floor surface, or “topping,” for the reduction of impact noise: 1) a resilient (flexible) layer, such as vinyl or carpet, and 2) a “floating floor,” which comprises a slab of rigid material supported on a resilient mat or pads. The degree of noise reduction achieved with a particular floor topping can be quite different with different base floors such as concrete and wood.
Concrete slab floors
When a hard object, such as a marble or a chair leg, strikes a concrete floor slab containing a hard finish such as ceramic tiles, the sound is clearly audible below as a sharp click. This “efficient” transmission of sharp tapping sounds through such a floor results in low IIC values for and creates annoyance for occupants below.
Hardwood flooring adhered directly to a concrete slab gives only slightly better impact sound attenuation than bare concrete or concrete covered with ceramic tiles. However, placing a resilient layer below the hardwood flooring improves matters significantly, resulting in acceptable impact attenuation. The resilient materials most commonly used and the most effective are shredded or foamed rubber, foamed plastic or cork mats. Increasing the thickness of the resilient material usually, but not always, increases the IIC.
Soft finishing layers can achieve very effective impact sound attenuation: the softer and thicker the floor covering, the higher the IIC. Thus, thick carpet with good underlay usually provides very high IIC ratings because impacts are well cushioned. Most people living below carpeted concrete floors are not disturbed by footstep noise. Vinyl
coverings, however, are typically thin, not very resilient, and not nearly as effective as carpet.
Gypsum board ceilings suspended resiliently (independently) from a concrete slab offer another way to increase impact sound attenuation. There is little information available for such floor systems, but it is known that increasing the mass of the gypsum board, the depth of the cavity, or the amount of sound-absorbing material all increase the IIC.
Joist floors with wood subfloors
Attaching gypsum board directly to joists, or to wood or stiff metal furring, gives poor impact sound attenuation. Resilient support of the gypsum board is necessary for best results; common resilient metal channels are adequate for most cases. (For this discussion, differences between joist types may be ignored).
A basic joist floor that follows good acoustical principles is one that has resilient metal channels supporting the gypsum board and sound-absorbing batts in the cavity. In such floors, the most important factor influencing the impact sound attenuation is the total mass of the subfloor and the ceiling layers. Each doubling of the total mass increases the IIC by about seven points. Increasing the thickness of the sound-absorbing material increases the IIC but to a lesser degree.
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A basic floor joist
The softer surface of a wood subfloor changes the sound made by the hammers of the tapping machine, producing a more muffled sound in comparison to the sharp clicks heard beneath a concrete floor. However, joist floors must still be provided with a finish layer, and an unwise choice can actually increase sound transmission or achieve nothing of significance.
Hard ceramic tiles adhered directly to the subfloor, for instance, reduces the IIC because of the increased generation and transmission of sharp, high-frequency sounds.
Thin resilient coverings such as vinyl, while reducing the sharpness of noise, do not significantly increase the IIC of joist floors. They are more effective on concrete floors. Wood or laminate flooring on a thin resilient layer usually does not increase the IIC significantly. Any benefit comes mainly from the added weight of the wood.
High IIC values can be obtained with lightweight joist construction by using a carpet and pad. However, while the IIC rating will be high, such floors may still engender complaints about excessive low-frequency “booming” or “thumping” sounds produced by people walking. The IIC rating does not reflect the degree of annoyance caused by these sounds on this type of floor. This problem has been studied extensively but no easy solutions have been found.
Joist floors with heavy subfloors
A heavier floor vibrates less when walked on, and so it generates less low-frequency sound. Thus, a common approach to reducing the thumping noise associated with joist floors is to increase the mass of the floor by adding a layer of concrete, gypsum concrete or the like on top. But, while this reduces low-frequency thumping, it creates another problem: the sharp clicking heard on any hard surface. In fact, the effect of adding a layer of concrete on top of a basic joist floor is to reduce the IIC by several points. The
solution to this problem is to “treat” the hard surface of the added concrete layer in the same way as described earlier for a solid concrete slab—with soft floor coverings or floating layers.
Alternatively, a resilient material can be placed between the wood subfloor and the concrete layer. Using this approach, a ceramic or hardwood finish may then be safely applied directly to the concrete without fear of reducing the IIC. The effectiveness of this approach depends on the resilient material used. Manufacturers’ test data should be consulted.
Installation and edge details for floating slabs. The slabs are installed on wood furring and resilient pads (left), and on a resilient layer (right)
Summary
The IIC rating for a complete floor system tells nothing about the floor topping itself — to compare toppings, they must be tested on the same basic floor. For example, vinyl attached to concrete improves the IIC by about 9 points, but does little when added to a wood joist floor.
A thick carpet and underpad may result in a very high IIC even though the floor transmits excessive thumping sounds when walked on and has low airborne sound attenuation. For further information, see references 3, 4 and 5.
Table 1: Some approximate IIC ratings. These are given only as examples.
Joist Floor ♪ Topping 150-mm Concrete Slab ♫ Wood subfloor Concrete topping Ceramic tiles 28 40 40 Vinyl flooring 37 47 50 Hardwood flooring 33 47 47 9-mm-thick wood layer on 6- 47 47 55
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mm-thick resilient layer
Carpet and underlay 75-85 75-85 75-85
♪
Single-layer subfloor and ceiling.
♫
A 200-mm-thick concrete slab would give IIC ratings about three points higher.
Dr. A.C.C. Warnock is a senior researcher in the Indoor Environment Program of the National Research Council’s Institute for Research in Construction.
1
ASTM E492, Standard Test Method for Laboratory Measurement of Impact Sound Transmission through Floor-Ceiling Assemblies using the Tapping Machine, Annual Book of ASTM Standards, Vol. 04.06
2
ASTM E989, Standard Classification for Determination of Impact Insulation Class (IIC), 1997 Annual Book of ASTM Standards, Vol. 04.06.
3
Warnock, A.C.C. and Birta, J.A. Summary Report for Consortium on Fire Resistance and Sound Insulation of Floors: Sound Transmission Class and Impact Insulation Class Results, Institute for Research in Construction, National Research Council of Canada, Internal Report 766, 121 p.
4
Warnock, A.C.C. Controlling the transmission of airborne sound through floors, Institute for Research in Construction, National Research Council of Canada, Construction Technology Update 25, 4 p.
5. Warnock, A.C.C. and Quirt, J.D. Controlling the Transmission of Impact Sound through Floors, Institute for Research in Construction, National Research Council of Canada, Construction Technology Update 35, 6 p.
