BUILDING SYSTEMS NOISE CONTROL

Transcription

1 BUILDING SYSTEMS NOISE CONTROL Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 1 First An Acoustics Refresher from ARCH 273 Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 2 1

2 Sound conceptually An audibly evaluated pressure variation Sound is energy that is heard Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 3 Vibration conceptually A tactilely evaluated pressure variation Vibration is energy that is felt Vibration generally becomes an architectural design issue at frequencies below the audible range (< 20 Hz) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 4 2

3 Noise conceptually Unwanted sound Noise is unwanted sound. Noise is a qualitative concept. Noise is analogous to glare; it is a negative phenomenon perceived by an individual. Saying unwanted noise is totally redundant. Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 5 Basic Architectural Acoustics Terms Sound is heard Vibration is felt Noise is unwanted sound (or possibly vibration although almost any vibration in a building is going to be unwanted) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 6 3

4 Properties of Sound SPEED used as a scientific reference value the speed of sound (Mach) 340 meters per second in air (vs. 299,792,458 m/s for light) in most building situations this means sound transfer is NOT instantaneous transit time for sound in large spaces is often discernable plane breaking the sound barrier: Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 7 Properties of Sound FREQUENCY 20 to 20,000 Hz (statistically) Hz = Hertz Hertz = cycles per second these frequencies correspond to 17 to meters wavelength vs nm for light Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 8 4

5 Frequency the frequency distribution of a sound defines its tonal character; similar to the way wavelength distribution defines the color of light Frequency Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 9 Frequency There are 19,980 discrete integer frequencies within the range of normal human hearing (20,000 Hz minus 20 Hz); it is difficult to consider/discuss all these frequencies individually unless it is done graphically Averaging across frequencies is a bad idea (valuable perceptual information will be lost through the averaging process) The octave band concept comes to the rescue Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 10 5

6 Octave Bands An octave is a doubling (in this case, a doubling of frequency) An octave band is a range (or span) of frequencies identified and named by a center frequency (these include 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, 16,000 Hz) Information about sound characteristics (such as intensity) at several (perhaps 4, or 6, or 8) octave bands is generally used to describe architectural sounds Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 11 Octave Bands as a Good Idea octave band these three sounds are perceived very differently at most octave bands, although they all have the same average Intensity average octave band center frequencies Frequency Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 12 6

7 Properties of Sound ENERGY sound is a lower quality form of energy than light it is mechanical versus electromagnetic raw energy and acoustical effect are not equivalent you wouldn t want to heat a building with sound, but the ear is very sensitive the ear is not equally responsive to all frequencies (some sound energy has a greater impact on people than other sound energy) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 13 Sound and Materials Sound that encounters a building material may be: -- reflected (reflectance, rho) -- absorbed (absorbtance, alpha) -- transmitted (transmittance, tau) Due to the law of conservation of energy: rho + alpha + tau = 100% these properties/concepts parallel those that apply to light Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 14 7

8 Sound Measurement and Units Sound power (W, watts) source property Sound pressure (Pa, pascals) space property Sound intensity (W / sq cm) space property Sound power level (db, decibels) source property Sound pressure level (db) space property Sound intensity level (db) space property Loudness receiver perception Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 15 The Magnitude of Sound Sound power A property of a sound source; independent of surroundings; not affected by the environment (i.e., by building characteristics); used in architectural acoustics Sound pressure The effect of a sound source (or sources) at a particular location is affected by the environment; used in architectural acoustics Sound intensity An alternative to sound pressure (same concept, different units); typically not used in architectural acoustics Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 16 8

9 Measuring Sound Pressure A or C weighting scale setting integrating sound level meter (we have several) gives a single-number (frequency-weighted-average) value for sound pressure level (in dba or dbc) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 17 The Weighting Scales Most commonly used for architectural acoustics Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 18 9

10 Noise Reduction (NR) NR is related to sound transmission NR NRC NRC is related to sound absorption Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 19 Noise Reduction (NR) barrier blocks some sound; receiving room absorbs some sound source Δ SPL = NR NR is a function of barrier and receiving space characteristics; SPL = sound pressure level (db) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 20 10

11 Noise Reduction in Action ambient SPL (a result of PWL) = 55 desired SPL (from NC) = 35 Δ SPL = NR sound barrier required? = 20 db of NR PWL = sound power level (db); NC = noise criterion (design target) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 21 Transmission Loss (TL) The ability of a barrier to prevent sound transmission (actually, its efficiency in transmitting sound) TL = -10 log (acoustic energy transmitted) (acoustic energy incident) (in db) reminder: log 1 = 0 log 0.5 = -0.3 log 0.1 = -1 Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 22 11

12 Transmission Loss (TL) TL = x db at x Hz TL is a function only of barrier construction; it is an inherent characteristic of a wall/floor/ceiling assembly Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 23 Relating TL and NR NR is affected by the TL of a barrier and by the characteristics of the receiving space NR = TL -10 log (S) (A) (in db) where, S = surface area of the barrier A = total absorption in receiving room Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 24 12

13 Approximate TL and NR Relationships (useful for early design decision making) receiving room hinders NR NR = TL 1 if a live receiving space receiving room helps NR receiving room helps more NR = TL + 4 if an average receiving space NR = TL + 7 if a dead receiving space live average dead Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 25 Equipment Noise Control Three elements are of primary concern in the control of building mechanical/ electrical equipment noise: sound sources sound paths noise receivers Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 26 13

14 Equipment Noise Sources Chillers Pumps Cooling towers Exhaust fans Air-handling units (the fan) Fan-coil units (the fan) Air distribution components (ductwork, VAV boxes, etc.) Air delivery devices Transformers Emergency generators DWV piping Owner equipment (photocopiers, printers ) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 27 Equipment Noise Receivers Building visitors (short-term occupants) Building users (long-term occupants) Building residents (overnight occupants) Each group has different schedules, expectations, adaptations remember that noise is a perception Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 28 14

15 Equipment Noise Paths form structure partitions volume blogs.utexas.edu/housing/2010/03/25/dr-steven-a-moore-school-of-architecture/ Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 29 Equipment Noise Path Groups Air-borne paths (requiring consideration of noise reduction and transmission loss) Structure-borne paths (requiring specialized isolation/attenuation) Duct-borne paths (requiring specialized attenuation) Vibration (requiring isolation) Typically structure-borne we feel we hear vibration control can be very specialized and will often involve a consultant Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 30 15

16 Equipment (Noise Source) Locations Source may be external to building For example: cooling tower, transformer easiest condition to mitigate (can use distance, location, barrier) Source may be in a central mechanical room For example: chillers, pumps, fans, emergency generators fair condition to mitigate (can use location, barrier) Sources may be distributed throughout building and may be adjacent to occupied spaces For example: air-handlers, VAV boxes, step-down transformers iffy condition to mitigate (options are often constrained) Sources may be distributed throughout building and within occupied spaces For example: diffusers, fan-coil units; unitary AC units terrible condition to mitigate (few, if any, options) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 31 Air-borne Noise Control Lowest-cost design solution is: smart location of equipment Away; away from critical areas (use distance) Low-cost solution: use spatial buffers Interpose less-acoustically-sensitive spaces between source and receiver (use hierarchy of spaces) Expensive* solution: wise selection of equipment Chose quiet equipment (to reduce sound power) Expensive solution: sound barriers Involves mass and money (to deliver TL) * but, the investment may improve energy efficiency and extend equipment life Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 32 16

17 Structure-borne Noise Control Lowest cost solution is: smart location of equipment Away from critical areas For example: slab-on-grade versus mid-span on bar joists Expensive* solution: wise selection of equipment Chose quiet and/or less dynamic equipment To reduce sound power; reduce vibration Expensive solution: structural isolation Floating constructions, springs, etc. Especially if impact noise is a particular problem Barriers Not an option in the traditional sense (a barrier is structure ) * but, may improve energy efficiency and extend replacement life Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 33 Floating Floor concrete structure-borne noise control solution Architectural Acoustics: Mehta, Johnson, Rocafort Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 34 17

18 Duct-borne Noise Control Good solution is: smart selection of equipment Chose quiet equipment To reduce sound power and thus sound pressure level OK solution: smart location of equipment Away from critical areas But, remember that the pathway is ductwork acting as a transmission channel OK solution: add attenuation Distance, duct splits, elbows, acoustical liner, sound absorbers, active noise cancellation, etc. By inserting acceptable things into the transmission channel Not an option: using barriers You cannot block air flow with a barrier duct-borne noise control is conceptually and practically different from air-borne Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 35 Duct-borne Noise Control Context noise sources ductwork transitions can both cause noise (via turbulence) and attenuate noise (via reflection) space of concern AHU noise noise caution! supply return attenuation opportunities the return air path is often acoustically ignored not a good idea Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 36 18

19 Duct-borne Noise Control bad layout (little opportunity for attenuation) -- sound enters one diffuser and exits another diffuser better layout (more attenuation opportunities) Architectural Acoustics: Mehta, Johnson, Rocafort Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 37 Duct-borne Noise Control low-cost duct silencer glass-fiber pipe insulation segments in a return air duct (probably a desperate retrofit) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 38 19

20 Active Duct-borne Noise Control replaces a section of ductwork electronically produces a valley where there is naturally a crest Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 39 Vibration Control Very effective solution: smart location of equipment Away from resonance-prone areas Place at the least-flexible structural location (on grade, not mid-span) Effective solution: smart selection of equipment Chose equipment with less propensity for shaking and rattling To reduce source-vibration potential Effective solution: add inertia Attach equipment to a heavy mass To dissipate vibration through mechanical frustration Effective solution: install equipment on vibration isolators Pads, springs, flexible connectors, industrial balloons To interrupt transmission path Not a solution: barriers Because building elements can/will transmit vibration Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 40 20

21 Vibration Control Options step-down transformer (in electrical closet) suspended using springs Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 41 Vibration Control Options pads: effective for high frequencies springs: effective for low frequencies Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 42 21

22 Vibration Control Options air springs (industrial balloons) springs and flexible connectors Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 43 Vibration Control Options housekeeping pad inertia block housekeeping pad keeps water away from metal springs inertia block adds mass to an object that wants to move Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 44 22

23 Bad Noise Control Specifications yet commonly seen The chiller shall not be noisy noisy is a perception, it cannot be measured by a contractor The fan noise shall not disturb people disturb is an opinion ( I say, you say) The pump will not cause vibration as above (plus if it does, it is too late to do anything) The diffuser will not exceed NC 35 NC is a sound pressure level limit, sound power is produced by a diffuser and the manufacturer should not do the design analysis required to connect cause (power) and effect (pressure) Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 45 there are serious legal and ethical implications to punting on specs drawings and specifications are contract documents and must be enforceable Good Noise Control Specifications The chiller sound power level shall not exceed xxx Sound power will be known by the manufacturer (and thus the contractor) the designer will need to do the math to convert desired sound power to sound pressure The fan sound power level shall not exceed xxx as above; it is not appropriate to pass design work off to the contractor (it is in fact unethical) The pump sound power level and vibration spectrum shall not exceed xxx as above The diffuser sound power level shall not exceed xxx as above lack of understanding of acoustics is the biggest impediment to good specs Ball State Architecture ENVIRONMENTAL SYSTEMS 2 Grondzik 46 23