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Acoustical Presentation to the Rocky Mountain ASHRAE Chapter

Acoustical Presentation to the Rocky Mountain ASHRAE Chapter. April 16, 2010. Discussion Topics . Acoustics Overview Frequency Ranges of Mechanical Noise Design Criteria for Typical Spaces Mechanical Noise Control : Areas of Interest Sound Transmission Paths

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Acoustical Presentation to the Rocky Mountain ASHRAE Chapter

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  1. Acoustical Presentation to theRocky Mountain ASHRAE Chapter April 16, 2010

  2. Discussion Topics • Acoustics Overview • Frequency Ranges of Mechanical Noise • Design Criteria for Typical Spaces • Mechanical Noise Control : Areas of Interest • Sound Transmission Paths • Typical Ductborne Mitigation Methods • Typical Duct Breakout Mitigation Methods • Typical Structure-borne Noise and Vibration Mitigation Methods • Mechanical Design affecting Sound Isolation • LEED for Schools

  3. Acoustics - A Brief Overview 1. Sound Isolation 2. Noise Control 3. Vibration Control 4. Interior Acoustics 1 2 3 4

  4. Acoustics 101 • Frequency is the rate of repetition of a periodic event. • Most sound sources, except pure tones, contain energy ever a wide range of frequencies. • For measurement and analysis of sound, the frequency range is divided into sections labeled as octave bands

  5. Acoustics 101 • Decibel (dB): Measure on logarithmic scale of the magnitude of sound pressure, sound power, or sound intensity level with respect to a standard reference value. • L = 20 log (Prms/Pref) Pref = 20µPa • Human Hearing • Threshold of Audibility: 0 dB • Threshold of Pain: 120 dB • Ear cannot differentiate less than 1 dB of change • Due to log scale, dB does not add algebraically 2 Vacuums ≠ 180 dB 2 Vacuums = 93 dB 1 Vacuum = 90 dB

  6. Acoustics 101

  7. Definitions of Terms: Sound Power vs Sound Pressure

  8. Definitions of Terms: dBA • A-Weighted Sound Levels (dBA) • dBA does not completely represent human perception of noise. • dBA is used primarily in environmental noise studies and LEED for Schools Requirements.

  9. Definitions of Terms: NC • Noise Criteria Level (NC) • Industry Standard • Does not address frequencies below 63 Hz • Does not provide sound quality assessment.

  10. Definitions of Terms: RC • Room Criteria (RC) • Probable industry standard for future • Addressed frequencies below 16 and 31.5 Hz • Provides sound quality assessment. • N, R, H, RV Excerpted from Chapter 7, “Sound and Vibration,” of the 1993 ASHRAE Fundamentals Handbook

  11. Comparison of dBA, NC, RC NC 36 RC 18 (R) 39 dBA 39 dBA

  12. Perception of Sound • Decrease of 3 dB represents a halving of sound energy but is a just noticeable difference. • Decrease of 10 dB represents a halving of perceived sound levels • Decrease of 20 dB represents ¼ of the perceived sound levels Picture from Bell Telephone Laboratories

  13. Frequency Ranges of Mechanical Noise

  14. Design Criteria for Typical Spaces

  15. Mechanical Noise Control : Areas of Interest • Equipment Selections • Type of Fans, Variable vs Constant, Diffusers/Grilles • Noise Data for Equipment Selections • AHU’s, RTU’s, VAV Boxes, Cooling Towers, Fan Coil Units, etc… • Ductwork layouts • Overhead Ducted, Displacement, Under Floor Distribution • Ducted vs. Plenum Return • Airflow Velocities • Plumbing noise • Vibration Isolation

  16. Sound Transmission Paths Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  17. Sound Transmission Paths Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  18. Sound Transmission Paths Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  19. Sound Transmission Paths Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  20. Sound Transmission Paths Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  21. Sound Transmission Paths Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  22. Typical Ductborne Mitigation Methods • Internal Ductliner • Attenuates Mid to High Frequencies • Distance of ductwork from mechanical equipment • Sound Attenuators • Most effective at attenuating Mid to High Frequencies • Increases Static Pressure Drop • Lined Plenum • Most effective method for attenuating low frequencies • Can be incorporated into AHU and RTU Casing

  23. Typical Ductborne Mitigation Methods • Double Wall Ductwork • Utilized when internally lined ductwork is not allowed. • Hospitals, Laboratories • Diffuser/Grille Selection • Diffusers/grilles should be selected 5 NC points below room criteria. • Flex duct connection • Airflow velocity • Ductwork • Airflow velocity • Number of elbows and junctions

  24. CONSTANT OR VARIABLE AIR VOLUME FAN-POWERED, SERIES FLOW, VAV Terminal Units • Integral Sound Attenuators • Manufacturer NC Ratings

  25. Good Design Practices • Fan Discharge Configurations • Inlet Configuration Excerpted from Chapter 7, “Sound and Vibration,” of the 2003 ASHRAE Fundamentals Handbook

  26. Typical Duct Breakout Mitigation Methods • Lagging or Wrapping • Attenuates Mid to High Frequencies • Utilized primarily for plumbing noise • Ductwork Enclosures • Most effective at attenuating low frequencies • Primarily used for RTU’s • Utilized as an extension of Mechanical Room

  27. Typical Structure-borne Noise and Vibration Mitigation Methods • Concrete Inertia Bases • Pumps/Large Fans • Spring Isolators • Pumps • Rotating Equipment • Above Grade Chillers/Cooling Towers • Neoprene Pads • On Grade Chillers/Cooling Towers

  28. Typical Structure-borne Noise and Vibration Mitigation Methods • Rooftop Isolation Curb • RTU’s • Spring/Neoprene Hangers • Ductwork/Piping • 30 foot critical distance • Flex Connections • Double Bellows

  29. Mechanical Design affecting Sound Isolation • Crosstalk between Spaces • Length of ductwork • Junctions and Elbows • Internal Ductliner • Plenum Return • Z or U Shaped Internally Line Transfer Ducts Excerpted from No Noise Classroom Acoustics Publication

  30. Mechanical Design affecting Sound Isolation • Penetrations Full Height Partitions

  31. Mechanical Design affecting Environmental Noise Control • Most states, counties, cities, and towns have property line noise ordinances. • Typical Day/Night level of 55/50 dBA • Typical Equipment Culprits • Emergency Generators • Radiator, Exhaust, Intake • Cooling Towers • Fans • Rooftop Units • Alignment of Compressor/Condenser Section

  32. Mechanical Design affecting Environmental Noise Control Mitigation Measures • Equipment Locations • Adjacent Properties • Barrier Walls/Screens • Materials • Height • Louvers • Type

  33. LEED for Schools • Acoustics is now a mandatory LEED credit for Schools • Prerequisite 3 Background Noise Requirements: • Max BNL of 45 dBA OR • Achieve an RC (N) Mark II level of 37 • EQ Credit 9: Enhanced Acoustical Performance Background Noise Requirements • Max BNL of 40 dBA (1 point) or 35 dBA (2 points) • 2: Achieve an RC (N) Mark II level of 32 (1 point) or 27 (2 points)

  34. Thanks for Attending • Any Questions???? • Additional Resources • ASHRAE Application Handbook Chapter 47 • Architectural Acoustics: David Egan

  35. Case Studies • Ritz Carlton Denver • Boardroom and Conference Areas • NC 45+ due to breakout noise (125 Hz) • NC 35 Criteria • Remedial Measures: Incorporated ductwork enclosure around high pressure running over spaces. • NC 34 after implementation of remedial measures

  36. Case Studies • UCDHSC Research 1 Facility • Vibration Issues in NMR and Crystallography Growth Chambers • Acoustical testing: Issues at 32 Hz • Short circuited spring isolators in AHU fans: Still bolted down for shipping

  37. Case Studies

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