USG Presents Understanding Acoustics in …

1 Architectural Record Version:02 Issue Date:06/02 Page No:167 Filename:USG_06 / Designer:Kenneth KaiserLast Revision:xxBlack CyanMagentaYellowRHP525 50 75 95 AIA/ARCHITECTURAL RECORDCONTINUING EDUCATIONS eriesIn today s architectural environment, good acoustical design isn t a luxury it s a necessity. Acoustics impacts everything from employee productivity inoffice settings to performance quality in auditoriums to the market value ofapartments, condominiumsand single-family the sciencebehind sound is wellunderstood, using thatscience to create desiredacoustical performancewithin a specific building orroom is complex. There s nosingle acoustical solution that can be universallyapplied to building built environment offersits own unique set ofacoustical parameters. Theacoustical design for abusiness conference room, forinstance, differs greatly fromthe design needed for akindergarten thesedifferences and knowing howto utilize building materials, system design and technologies are key factorsbehind successful acoustical design.

2 This article will provide basic background onthe science and measurement of sound, as well as insights into some of theprinciples of wall partition and ceiling system acoustical Science of Sound Technically speaking, sound is defined as a vibration in an elastic medium. Anelastic medium is any material (air, water, physical object, etc.) that has theability to return to its normal state after being deflected by an outside force suchas a sound vibration. The more elastic a substance, the better it is able toconduct sound waves. Lead, for instance, is very inelastic and therefore a poorsound conductor. Steel, on the other hand, is highly elastic and an excellentsound vibrations travel through elastic mediums in the form of smallpressure changes alternating above and below the static (at rest) nature of theconducting material. Picture a vibrating tuning fork. As it moves in onedirection, it compresses the air particles next to it.

3 They, in turn, pass on thereaction to adjacent particles of air. As the tuning fork vibrates in the otherdirection, it leaves a void or rarefaction. This rarefaction follows along behind thecompression. It, in turn, is followed by another compression, and then anotherrarefaction and so of these compression/rarefaction cycles is called a wave. The numberof waves that occur per second is termed frequency. Frequency is measured interms of hertz (Hz). One Hz is equal to one cycle per second. The human ear candiscern sounds ranging from approximately 20 to 20,000 Hz. Human speechranges between 125 and 4,000 supplement provided by USG CorporationUse the learning objectives below to focusyour studyas you readUnderstandingAcoustics in Architectural one AIA/CES Learning Unit includingone hour of health safety welfare credit,answer the questions on page 297, thenfollow the reporting instructions on page368 or use the Continuing Education selfreport form located at Objectives: Know how sound waves form and how theytravel through elastic mediums Understand how sound can be isolated and absorbed in building design Realize the benefits that sound masking provides for closed and open-office spacesAIA/ARCHITECTURAL RECORDCONTINUING EDUCATIONS eriesUnderstanding Acoustics in Architectural Designby:James D.

4 Janning, AIA, CSIA rchitectural Systems Manager, USG CorporationUSG Presents167 USG_06 10/29/02 4:04 PM Page 167 Understanding Acoustics in Architectural DesignAIA/ARCHITECTURAL RECORDCONTINUING EDUCATIONS eriesArchitectural Record Version:01 Issue Date:06/02 Page No:168 Filename:USG_06 / Designer:Kenneth KaiserLast Revision:xxBlack CyanMagentaYellowLHP525 50 75 95168 The amplitude of sound waves how far they travel above and below the staticpressure of the elastic medium they are traveling through is measured in decibels(dB). The higher the decibel level, the higher the volume, or loudness of a sound. Ajet airplane has an amplitude of 140dB, while a human whisper is approximately20dB. A typical office environment usually falls in the 40 to 60dB MovementArchitectural Acoustics is the process of managing how both airborne andimpact sound is transmitted and controlled within a building design.

5 Whilevirtually every material within a room from furniture to floor coverings tocomputer screens affects sound levels to one degree or another, wall partitions,ceiling systems and floor/ceiling assemblies are the primary elements thatdesigners use to control moves through building spaces in a variety of ways. Mostcommonly, it is transmitted through air. But wall partitions, ceilings andfloor/ceiling assemblies can also transmit both airborne sound, such as humanvoices and ringing telephones, and impact sound, such as footsteps on a waves actually travel through many physical objects faster and with lessloss of energy than they travel through air. Sound waves travel at a rate of 1,128feet per second through air (at 70 degrees F); 11,700 feet per second throughwood; and 18,000 feet per second through reflection occurs when sound waves bounce off smooth, hard wall,ceiling and floor surfaces.

6 Concave surfaces tend to concentrate or focus reflectedsound in one area. Convex surfaces do just the opposite; they tend to dispersesound in multiple reverberation is the persistence of sound reflection after the source ofthe sound has ceased. Reverberation can have both a positive and negative effect inarchitectural design. For example, specifying highly reflective ceiling panels directlyabove the stage area in an auditorium will help direct sound toward specific seatingareas, thus enhancing the room s acoustical performance. However, that samereflective performance will become a negative factor if highly reflective wall andceiling materials are installed in the rear of the auditorium. That s because thesound reflections from the rear of the room take too long to reach the audience,resulting in a distractingecho can alsodiffract, or bend andflow around an object orthrough a small space oropening.

7 This givessound waves the abilityto squeeze throughvery small openings withlittle loss of energy. Thesmall openings underand around doors, floortracks, electrical boxesand conduit and HVAC ducting are typicalsources of sounddiffraction. These arecommonly referred to as flanking or leaking paths. They can becontrolled by the properapplication of Sound A primary goal of a wall partition,ceiling system and floor/ceilingassembly design is to minimize theflow of airborne and impact soundthrough the use of specialmaterials, methods of constructionand effectiveness of anassembly s ability to isolate airbornesound is quantified by SoundTransmission Class (STC) is expressed as a single numberand usually ranges fromapproximately 35 to 70. It quantifiesthe transmission loss (TL) of anassembly. A wall partition orfloor/ceiling assembly that reducesthe overall incoming sound levelsfrom 80dBA to 20dBA would have an STC rating of approximately related measurement is Ceiling Attenuation Class (CAC).

8 This ratingquantifies how much sound is lost when it is transmitted through the ceiling ofone room into an adjacent room through a common plenum. Like STC, a higherCAC rating indicates that the ceiling system allows less sound transmission. Forclosed-office environments, a CAC of 40 to 44 is usually sound transmission in a floor/ceiling assembly is quantified byImpact Isolation Class (IIC). This is a single number rating that quantifies anassembly s ability to isolate impact sounds generated from footsteps and otherimpact sources. It is tested in laboratory conditions by a tapping machine thatimpacts the floor of a source room. The sound of the tapping is measured in a receiving room, located directlybeneath the source Partitions and STCR educing sound transmission throughwall partitions can be accomplished in avariety of ways, including isolation (theseparation of adjoining wall partitionsurfaces), mass, absorption, decoupling(inelasticity) and the elimination offlanking paths (sound leakage).

9 Increasing the mass of a partitionforces sound waves to work harderand expend more energy to passthrough the medium. Specifically,doubling the mass of a partition canreduce sound transmission by up to5dB. However, using mass alone toincrease sound control has definitelimitations. To achieve a 60dB reduction, a total mass of 320 pounds per squarefoot is required. This is equivalent to approximately 3 feet of solid concrete,which is obviously impractical for virtually any building air space within a partition is an effective means for raising STCperformance, but like mass, it has its limitations. Doubling the partition air spacecan reduce sound transmission by up to 5dB, but to achieve a reduction of 60dBrequires an isolated air space 4 feet wide. Again, this is hardly practical forbuilding design. The effectiveness of air isolation is limited by the fact that theWater-felted mineral fiber panels have face perforations toimprove acoustical performance.

10 They are the most economicalacoustical ceiling ceiling panels feature smooth, naturaltextures, excellent sag resistance and excellent NRCand CAC supplement provided by USG CorporationDesigning High-STC Wall Partitions: This gypsumboard wall assembly uses mass (2 gypsum boardlayers are on one side, and 3 layers are on the otherside), decoupling (resilient channels are screw-attached to the steel studs) and absorption (a layerofmineral fiber insulation is friction-fitted in thepartition cavity) to enhance STC 10/29/02 4:04 PM Page 168 Architectural Record Version:01 Issue Date:06/02 Page No:169 Filename:USG_06 / Designer:Kenneth KaiserLast Revision:xxBlack CyanMagentaYellowRHP525 50 75 95 AIA/ARCHITECTURAL RECORDCONTINUING EDUCATIONS eriesUnderstanding Acoustics in Architectural Designwood or steel studsattaching both sides ofthe partition assemblytransmit sound no matterhow wide the isolatedspace.