ACOUSTICAL INSULATION Stanley D. Gatland II Manager of Building Science Technology CertainTeed Corporation ABSTRACT Today’s residential and commercial property owners are concerned about the comfort and safety of the interior environment. Unwanted sound, or noise, is one factor that can influence these conditions. This paper will identify the benefits of sound control. The science of acoustics will be described to give a basic understanding of the physics behind different types of sound control techniques and products. Several commonly specified, standardized test methods and sound control practices developed by the American Society for Testing and Materials (ASTM) will be described in detail. Design concerns related to acoustically, insulated interior partitions will be presented, as well as, readily available references. INTRODUCTION The world is a noisy place. People are constantly exposed to sounds that are not wanted. Noise from many interior and exterior sources, within the workplace and the home, negatively impact people’s hearing and diminish productivity. People do not need to suffer the distracting and unhealthful consequences of noise. Practical and economical solutions to sound related problems exist for architects, engineers, contractors, building owners and homeowners. Acoustical products and systems are readily available today, however, a basic understanding of the science behind the different types of noise control solutions is necessary to realize the benefits. SCIENCE OF SOUND Environmental acoustics studies the characteristics and performance of materials, products, systems and services related to the science of sound and the effect on the surrounding environment. Sound is all around us. We are literally bathed in it all the time. We never know absolute quiet, although it is theoretically possible. Sound waves travel through air creating very small changes in atmospheric pressure. The sensation of hearing, produced by vibrating the eardrum, is due to these small pressure changes. The sound wave’s alternating fluctuation, above and below the static atmospheric pressure creates a sound pressure. Sound can sometimes be perceived through the vibration of a body or surface. However, sound is generally regarded as a disturbance in the air, like waves in the sea, but instead of just spreading out in circles on the surface, sound spreads out in spheres in three dimensions, like expanding soap bubbles, one inside the next. As a wave progresses, the sound pressure diminishes in proportion to the distance from the source, in the same manner that a water wave dies out as it spreads. Since sound pressure determines loudness, sound outdoors becomes fainter as one moves away from the source.
Transcript
ACOUSTICAL INSULATION
Stanley D. Gatland II Manager of Building Science Technology
CertainTeed Corporation ABSTRACT Today’s residential and commercial property owners are concerned about the comfort and safety of the interior environment. Unwanted sound, or noise, is one factor that can influence these conditions. This paper will identify the benefits of sound control. The science of acoustics will be described to give a basic understanding of the physics behind different types of sound control techniques and products. Several commonly specified, standardized test methods and sound control practices developed by the American Society for Testing and Materials (ASTM) will be described in detail. Design concerns related to acoustically, insulated interior partitions will be presented, as well as, readily available references. INTRODUCTION The world is a noisy place. People are constantly exposed to sounds that are not wanted. Noise from many interior and exterior sources, within the workplace and the home, negatively impact people’s hearing and diminish productivity. People do not need to suffer the distracting and unhealthful consequences of noise. Practical and economical solutions to sound related problems exist for architects, engineers, contractors, building owners and homeowners. Acoustical products and systems are readily available today, however, a basic understanding of the science behind the different types of noise control solutions is necessary to realize the benefits. SCIENCE OF SOUND Environmental acoustics studies the characteristics and performance of materials, products, systems and services related to the science of sound and the effect on the surrounding environment. Sound is all around us. We are literally bathed in it all the time. We never know absolute quiet, although it is theoretically possible. Sound waves travel through air creating very small changes in atmospheric pressure. The sensation of hearing, produced by vibrating the eardrum, is due to these small pressure changes. The sound wave’s alternating fluctuation, above and below the static atmospheric pressure creates a sound pressure. Sound can sometimes be perceived through the vibration of a body or surface. However, sound is generally regarded as a disturbance in the air, like waves in the sea, but instead of just spreading out in circles on the surface, sound spreads out in spheres in three dimensions, like expanding soap bubbles, one inside the next. As a wave progresses, the sound pressure diminishes in proportion to the distance from the source, in the same manner that a water wave dies out as it spreads. Since sound pressure determines loudness, sound outdoors becomes fainter as one moves away from the source.
The three fundamental properties of sound are frequency (pitch), wavelength and amplitude (loudness). Frequency, perceived as pitch in air, is defined as the number of times per second that the sound pressure alternates above and below the ambient atmospheric pressure. Each complete alternation is called a cycle, and frequency is expressed in cycles per second, or hertz. The higher the frequency, the higher is the pitch. The extreme range of frequencies, which the ear can perceive, is approximately 20 to 20,000 hertz, although the upper limit decreases considerably with advancing age. Typical human speech ranges from 200 Hz to 3000 Hz. Most adults can hear sounds from about 50 Hz to about 15,000 Hz. The wavelength of a sound wave is the distance between the start and end of a sound wave cycle or the distance between two successive sound wave pressure peaks. The sound wave’s speed in a medium divided by the frequency of the sound wave will determine the wavelength. Sound travels at the speed of 750 mile per hour in air, this speed is known as the sonic barrier for aircraft. The speed varies with temperature and pressure. A sound wave’s frequency is inversely proportional to the wavelength, meaning the lower the frequency the larger the wavelength. The amplitude or loudness of a sound wave is expressed by sound pressure level. Since the sound pressure of a sound wave varies over a wide range, a change in magnitude of ten million to one, sound pressure is expressed using a logarithmic scale. This is the basis of the decibel scale, which compresses the range of sound pressure into a scale from 0 to 150. The decibel (dB) expresses the ratio between a given sound pressure and a reference sound pressure. The scale runs like a Fahrenheit thermometer, 30 being quiet, 70 being noisy and 120 being the threshold of pain. The sound levels normally in our homes range from a quiet 10 dB to a loud stereo of 80 dB. Some typical sound pressure levels in decibels are given below in Table 1. People’s response to sound levels depends on how many decibels the level increases. Remember that sound pressure level combinations are not linear. Table 2 provides the subjective human response to changes in sound pressure level. Table 1. Typical sound pressure levels
Pain Threshold 120 dB Thunder 115 dB
Subway Train 100 dB Noisy Office 80 dB
Average Conversation 50 dB Whisper 20 dB
Threshold of Hearing 0 dB Table 2. Subjective human response to sound pressure level changes
0 – 2 dB Not Noticeable 2 – 4 dB Slightly Noticeable 5 – 8 dB Clearly Noticeable 9 – 10 dB Twice or Half as Loud
TYPES OF SOUND PATHS Sound waves can travel through any media, which includes air, water, wood, masonry or metal. The type of media that sound travels through determines whether the sound is either airborne or structureborne. Airborne sound is directly transmitted from a source into the air. All sound that reaches your ear is airborne. Some examples of airborne sound are passing traffic, music or voices from an adjacent room, or the noise from machinery and aircraft. Structureborne sound travels through solid materials, either from direct contact with the sound source or from an impact on the material. All structureborne sound must eventually become airborne sound in order for people hear it, otherwise, the disturbance is felt as a vibration. Examples of structureborne noise are footsteps, door slams, plumbing vibrations, mechanical vibrations and rain impact. Most noise control situations require that both airborne and structureborne sound be considered. Effective sound control addresses both sound paths by controlling, or reducing, noise at the source, reducing paths or blocking noise along its path, or shielding the receiver from the noise. SOUND ABSORPTION - ASTM C 423 Sound absorption is the ability of a material to transform acoustical energy into another form of energy, usually heat. To be an effective sound absorber, a material must have interconnecting air pockets or cells, like, fiber glass insulation. A resonator is another effective sound absorber that employs small perforations or slots, which allow sound to enter but not escape easily. Concrete masonry units and wood slat panels operate using this principal. A sound absorption coefficient is the decimal fraction of the sound energy absorbed and not reflected by a material. Most building products absorb and reflect a portion of the incident sound energy, as illustrated in Figure 1. Sound absorption coefficients are measured in North America using the American Standards for Testing and Materials (ASTM) standard test method ASTM C 423, “Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method.” Measurements are performed at one-third octave band center frequencies form 125 to 4000 Hz, since materials absorb different amounts of sound energy at different frequencies. A reverberation room is designed to have a sound field that closely approximates a diffuse sound field through the use of highly reflective, massive surfaces. The decay rate of an initial sound pressure level is measured and sound absorption is calculated in sabins, see Figure 2. The increase in reverberant room absorption divided by the test specimen area is the sound absorption coefficient for each third-octave band frequency.
Figure 1. Sound absorption Decibel (dB)
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Figure 2. Measurement of decay rate Building materials are generally rated by their noise reduction coefficient (NRC) value. This single number rating is the average of the sound absorption coefficients at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05. The Sound Absorption Average (SAA) is the average of the sound absorption coefficients of a material from 200 through 2500 Hz, inclusive, rounded to 0.01. A material is considered to be a sound absorber if the SAA value is greater than 0.35. Products with hard reflective surfaces, like gypsum board, have low SAA values of 0.05. Acoustically absorptive materials, like 6 ¼ inch thick unfaced, fiber glass building insulation, have high SAA values of 0.90 or better.
AIRBORNE SOUND TRANSMISSION – ASTM E 90 & E 413 Building partitions and elements are evaluated for the ability to reduce airborne sound transmission through the assembly. The sound insulation property of a material indicates the ability of the system to reduce the loudness of a noise created in one room, or enclosure, and measured in another room, or enclosure, separated from the first room by a partition of the material, see Figure 3. Sound transmission loss of building systems, like walls, floor-ceiling assemblies, roofs, doors, windows operable partitions, and other space-dividing elements, is measured in a laboratory using the standard test method ASTM E 90, “Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.” Measurements are performed at one-third octave band center frequencies from 125 to 4000 Hz. Figure 3. Airborne sound transmission Building assemblies are rated using a Sound Transmission Classification, or STC. Values are determined using normalized airborne sound transmission loss data from ASTM E 90 and calculated with ASTM E 413, “Classification for Rating Sound Insulation.” Results from ASTM E 90 are compared with a reference contour curve to calculate the STC value. The reference data in general correlates with subjective impressions of sound transmission for speech. Once the calculation criteria are met the STC value is the sound transmission loss value in decibels at 500 Hz on the reference contour curve. Figures 4 and 5 provide an STC comparison of two wood stud wall systems. Figure 4 gives the graphical result of a nominal 2 by 4 wood stud wall with an empty cavity and a single layer of ½ inch thick gypsum board on either side. The perimeter edge was not air sealed. The STC value for the system was 29.
Sound Transmission Class (STC) using Classification E 413
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No cavity insulation, no air seal, ½ inch gypsum board both sides STC = 29 @ 500 Hz on the Reference Contour Curve Test data below the reference curve determines deficiencies
Sound Transmission Class (STC) using Classification E 413
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Figure 5. Nominal 2 by 4 inch wood stud, spaced 16 inches on center Cavity filled with 3 ½ inch fiber glass insulation and air sealed ½ inch gypsum board both sides, one side mounted to resilient channel STC = 46 @ 500 Hz on the Reference Contour Curve Test data below the reference curve determines deficiencies
Figures 5 and 6 illustrate an acoustically treated wood stud wall system and provide the graphical results. The cavity has been filled with 3 ½ inch thick fiber glass insulation, one layer of gypsum board has been mounted to resilient channel spaced 24 inches on center, and the perimeter edge has been sealed. The STC value has increased to 46. The combined effect of absorptive material in the cavity, using a resilient channel to reduce the structural tie between the gypsum board layer and the wood studs and air sealing the perimeter edge resulted in the increase in system acoustical performance. All sound transmission loss testing evaluates the entire system, unlike sound absorption, which is material specific. Figure 6. Acoustically Treated Wood Stud Wall IMPACT SOUND TRANSMISSION – ASTM E 492 & E 989 Floor-ceiling assemblies are evaluated for the ability to reduce impact sound transmission, like footsteps or dropped objects on the floor surface, through the system to the space below. See Figure 7. The test specimen is the primary sound transmission path. Impact sound transmission loss of floor-ceiling assemblies is measured in a laboratory using the standard test method ASTM E 492, “Laboratory Measurement of Impact Sound Transmission Through Assemblies Using a Tapping Machine.” Measurements are performed at one-third octave band center frequencies from 100 to 3150 Hz. Impact noise is generated using a tapping machine, which drops five hammers in rapid succession at equal intervals on the test specimen surface. Floor ceiling assemblies are rated using an Impact Insulation Class, or IIC. Values are determined using normalized impact sound transmission loss data from ASTM E 492 and calculated with ASTM E 989, “Classification for Determination of Impact Insulation Class (IIC).” Impact sound pressure levels measured in the receiving room below the test specimen during ASTM E 492 are compared with the IIC Reference Contour curve to calculate the IIC
Figure 7. Impact sound transmission value. The reference data in general correlates with subjective impressions of sound transmission for speech. Once the calculation criteria are met the IIC value is the sound transmission loss value in decibels at 500 Hz on the reference contour curve subtracted from 110 dB. Figure 8 gives the graphical result of a 6 inch thick reinforced concrete slab floor. The IIC value for the system was 27. The same system has an STC value of 53. STC and IIC values do not always correspond with one another, due to the strong structural influence of the impact vibration through the assembly. Resilient floor coverings, like a carpet and pad, or isolated, suspended ceilings combined with sound absorptive material, like fiber glass insulation, will increase the systems performance with respect to impact insulation. However, the only way to effectively compare systems is to have the corresponding system specific IIC test results. CEILING AIRBORNE SOUND ATTENUATION – ASTM E 1414 & E 413 Suspended ceiling systems are evaluated for the ability to reduce airborne sound transmission between rooms connected by a common air plenum, see Figure 9. Airborne sound attenuation, or reduction, is measured in a laboratory using the standard test method ASTM E 1414, “Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum.” Two acoustical chambers are arranged to simulate a pair of small adjacent rooms that are separated by a partition and share a common plenum space above. Measurements are performed at one-third octave band center frequencies from 125 to 4000 Hz. Suspended ceiling systems are rated using a Ceiling Attenuation Class, or CAC. Values are determined using normalized ceiling attenuation (airborne sound transmission loss) data from ASTM E 1414 and calculated with ASTM E 413, “Classification for Rating Sound Insulation.” Historically, CAC was referred to as a ceiling STC. Results from ASTM E 1414 are compared with a reference contour curve to calculate the CAC value. The reference data in general correlates with subjective impressions of sound transmission for speech. Once the calculation
Impact Insulation Class (IIC) using ASTM Classification E 989
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IIC = 27; 110-dB @ 500 Hz on Reference Contour Test data above the reference curve determines differences criteria are met the CAC value is the normalized ceiling attenuation value in decibels at 500 Hz on the reference contour curve. Figure 10 gives the graphical result of a 24 by 48 inch fiber glass ceiling tile system suspended in a metal grid. The CAC value for the system was 26. The same fiber glass tile has an SAA value greater than 0.90. Usually, ceiling tile systems with good sound absorption properties have low ceiling attenuation. Acoustical ceiling tile manufacturers will use combinations of absorptive and reflective materials and surfaces to achieve the desired space conditions. Historical industry
Figure 9. Airborne sound transmission through suspended ceilings data has shown that some acoustical ceiling tile system’s CAC number will increase 8 to 9 points with the addition of 3 ½ inches of fiber glass batt insulation above the tile. INTERIOR LIGHTWEIGHT PARTITION DESIGN CONCERNS – ASTM E 497 Most of the benefits of using acoustical insulation are realized in the construction of lightweight partition walls. However, the benefits of systems with high STC ratings can be lost because of improper installation or poor construction details. Sound flanking paths, sound leaks and structural short circuits due to fasteners are a few conditions that decrease the effectiveness of sound insulating systems, as illustrated in Figures 11 to 13. The ASTM standard practice, E 497 “Installing Sound-Isolating Lightweight Partitions” provides recommendations for preventing situations or conditions that will detract from the acoustical performance of various types of partitions, such as, wood and steel stud walls, floor-ceiling assemblies and roof-ceiling systems. Combinations of dense sound barrier materials, like gypsum board, sheet metal or wood framing, and acoustical caulk can block many above ceiling, between floor-ceiling, and below floor sound flanking paths, as illustrated Figures 14 to 21. Systems must be airtight, since sound will always take the path of least resistance, no matter how small the opening. All penetrations and perimeter joints should be sealed with combinations of gaskets and acoustical caulk. Framing members, fastening systems, plumbing and electrical conduits should be vibration isolated when possible to minimize the structureborne transfer of sound energy through the assembly.
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Figure 10. 24 by 48 inch fiber glass ceiling tile suspended in a metal grid
CAC = 26 @ 500 Hz on the Reference Contour Curve Test data below the reference curve determines deficiencies
Figure 11. Sound flanking paths.
Figure 12. Air leakage paths Figure 13. Fastener short circuits
Figure 17. Between floor blocking and caulking Figure 18. Under floor blocking
Figure 19. Wall corner and intersection structural breaks Figure 20. Air sealing and blocking electrical penetrations Figure 21. Overlap gypsum board seams
CONCLUSION Understanding the basics of sound control is the first step to achieving the benefits of acoustically comfortable buildings. Integrating sound control techniques and products into the design process are the best ways to ensure success. Noise problems that exist after a building is occupied are sometimes difficult to solve, and usually are much more expensive than if addressed during the design phase. REFERENCES 1. American Society for Testing and Materials, Annual Book of ASTM Standards, Section
Four, Construction – Thermal Insulation; Environmental Acoustics, Volume 04.06, 2002. 2. CertainTeed Corporation, Noise Control in Buildings, Guidelines for Acoustical
Problem-Solving, Literature Code No. 30-25-047, February 2002. 3. North American Insulation Manufacturers Association (NAIMA), Sound Control for
Commercial and Residential Buildings, PUB # BI405, December 1997.
