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Sound Advice Presented by Randy Zimmerman

Sound Advice Presented by Randy Zimmerman. Introduction. Good acoustical design Comfortable and productive environments Systems Comfort vs. energy efficiency Proximity to occupants Air terminal units Air outlets

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Sound Advice Presented by Randy Zimmerman

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  1. Sound AdvicePresented by Randy Zimmerman

  2. Introduction • Good acoustical design • Comfortable and productive environments • Systems • Comfort vs. energy efficiency • Proximity to occupants • Air terminal units • Air outlets • Designers must understand acoustical ratings in order to write good specifications 2

  3. What You Will Learn Sound power vs. sound pressure Sound power determination End reflection correction Sound criteria Determining NC ratings for catalog data Specifying in terms of NC Specifying maximum allowable sound power levels Sound paths Mock-up room testing 3

  4. The Sound Room • Products are tested in a qualified reverberant chamber (per AHRI 220) • Reverberant chambers are used for quiet products • Low absorption • Low background • The reverberant field eliminates all directionality from a sound source • Sound levels within the reverberant field are equal at all points

  5. The Comparison Method • Determine the sound power (Lw) by comparison to a known reference sound source (RSS) • Measure the sound pressure (Lp) of the RSS in order to determine the room attenuation • Lp = Lw – room attenuation • Lw = Lp + room attenuation • If we know that the RSS creates Lw = 80 dB in the first octave band (63 Hz), but we read only Lp = 70 dB, we know that we have 10 dB of room attenuation in that octave band • Room attenuation is constant • All sound meters measure sound pressure (Lp)

  6. The Test Procedure Set-up any ductwork and equipment to be tested Remove any unnecessary material from the test chamber Turn off equipment and close test chamber doors Take background sound pressure level Switch RSS on Take RSS sound pressure level Switch RSS off and set test conditions Record sound pressure levels at various conditions by changing flow rates, pressures, etc 6

  7. The Decibel (dB) • Because of the great differences in energy (or pressure) available, the log of the actual value is used • Reference power is 10-12 watts • Reference pressure is 0.0002 microbars • dB is measured vs. frequency • An infinite number of frequencies, so they are averaged into bands, typically called ‘Octave Bands’

  8. Octave Bands • Octave bands are centered about increasingly wider frequency ranges, starting with 63 cycles/second (Hz) • Each band doubles in frequency • Bands are traditionally numbered, in our industry, as shown

  9. Octave Bands • Fan-powered products usually create their highest sound levels in octave band 2 (125 Hz), but sometimes octave band 3 (250 Hz) • Grilles, registers and diffusers create their highest sound levels in octave bands 4 (500 Hz), 5 (1000 Hz) or 6 (2000 Hz) • Octave bands 4-6 are known as the speech interference bands • It’s industry convention to report sound data for octave bands 2-7 only • Sound room size and design can cause problems with readings in octave bands 1 and 8

  10. Decibel Addition Example To add two decibel values: 80 dB + 74 dB

  11. Decibel Addition Example To add two decibel values: 80 dB + 74 dB 154 dB (Incorrect)

  12. From Chart: Add 1.0 dB to higher Value 80 dB + 1 dB 81 dB (Correct) Decibel Addition Example 3 To add two decibel values: 80 dB - 74 dB = 6 dB 2.5 2 1.5 Difference in Values: 6 dB 1 0.5 0 0 2 4 6 8 10 Difference In Decibels Between Two Values Being Added (dB)

  13. Good To Know • Any sound source 10 dB lower than background level will not be heard • Add 3 dB (or 3 NC) to double a sound source • Two NC40 terminal units over an office would probably create an NC43 sound level • Two NC20 diffusers in a room would create a worst case sound level of NC23 (if they are close together) • Don’t try to add-up dissimilar products in this manner

  14. Sound Power Changes

  15. Proximity To Sound Sources • Would you really expect to hear 100 fans running at the same time? • Properly selected diffusers shouldn’t be heard from more than 10 feet away • Although there may be multiple diffusers in a space, it’s unlikely that more than one or two are within 10 feet of an occupant • We would only expect to be able to hear a 10 foot section of continuous linear diffuser from any single location

  16. For High Frequencies • 1 dB not noticeable • 3 dB just perceptible • 5 dB noticeable • 10 dB twice as loud • 20 dB four times as loud

  17. For Low Frequencies • 3 dB noticeable • 5 dB twice as loud • 10 dB four times as loud

  18. A Difference of 25 dB What We Hear • Our ears can be fooled by frequency • Both tones sound equally loud 65 dB 40 dB 63 HZ 1000 HZ

  19. Acoustic Quality 19

  20. 80 70 NC-70 60 NC-60 50 NC-50 40 OCTAVE BAND LEVEL _ dB RE 0.0002 MICROBAR NC-40 30 NC-30 APPROXIMATE 20 THRESHOLD NC-20 OF HUMAN HEARING 10 63 125 250 500 1K 2K 4K 8K MID - FREQUENCY, HZ NC Curves

  21. Typical NC Levels Conference Rooms < NC30 Private offices < NC35 Open offices = NC40 Hallways, utility rooms, rest rooms < NC45 NC should match purpose of room Difficult to achieve less than NC30 Select diffusers for NC20-25 (or less) 21

  22. Sound Power Vs. Sound Pressure • Sound power (Lw) cannot be measured directly • Sound pressure (Lp) is measured with a very fast pressure transducer (i.e. a microphone) • Calculate sound power (Lw) by correcting sound pressure (Lp) readings in a reverberant chamber to a known power source • Reference Sound Source (RSS)

  23. Reference Sound Source • Correction device for a reverb room is the RSS (per AHRI 250) • Calibrated in an anechoic chamber to simulate a free field condition • Used in a reverberant field, so there is a known error called the “Environmental Effect”

  24. In a Reverb Room • Sound power (Lw) is calculated from measured sound pressure (Lp) and corrected for background • Unless product sound is 10 dB above background • RSS is used to “calibrate” the room • Data is recorded per octave band (or 1/3 octave band if pure tones are anticipated), for each operating condition

  25. Catalog Data • Sound pressure data is collected by a frequency analyzer that samples microphones via a multiplexer • Data is collected and sound power recorded • Spreadsheets are used to check the linearity of data sets • Catalog data is prepared from actual sound power data sheets using accepted regression techniques

  26. Diffuser Testing • Current test standard for diffusers • ASHRAE 70-2006 • No significant changes in many years 26

  27. Terminal Unit Testing • Current test standard for terminal units • ASHRAE 130-2008 • ASHRAE 130 is currently under review • SPC 130 • It will be updated to include more products including exhaust boxes

  28. Sound Tests • Discharge sound, VAV terminals • Unit mounted outside room • Discharging into reverb room • Radiated sound, VAV terminals • Unit mounted inside room • Discharging outside reverb room • All ductwork lagged to prevent ‘breakout’ • Diffuser supply/return sound • Unit mounted flush to inside the reverb room wall 28

  29. Performance Rating • Current rating standard for terminal units • AHRI 880-2011 (effective Jan 1, 2012) • Increases discharge sound levels due to end reflection • This affects all published data and selection software • The boxes will still sound the same, but now the acoustical consultants will be happier 29

  30. Sound Path Determination • Current standard for estimating sound levels in rooms • AHRI 885-2008 • Provides sound path data from ASHRAE research • Attenuation factors for duct lining, ceiling tiles, room volume, elbows, flex duct, etc 30

  31. Industry Standardization • AHRI 885-2008 contains Appendix E • Recommends standard attenuations to be used by all manufacturers for catalog data • First presented in ARI 885-98 • Makes comparing catalog NC levels much less risky 31

  32. AHRI 885-2008 Catalog Assumptions mineral fiber tile 5/8 in thick 20 lb/ ft3 density 5 ft, 1 in fiberglass lining 8 in flex duct to diffuser2500 ft3 room volume 5 ft from source The following dB adjustments are used for the calculation of NC above 300 CFM 32

  33. Certified Performance Data • AHRI Program • Directory of Certified Product Performance • www.ahrinet.org • Random samples subjected to annual third party lab testing • Verifies that performance is within established test tolerances • Failures result in penalties • Voluntary program 33

  34. The dBA Scale Used for outdoor noise evaluation Also used for hearing conservation measurements Basis of most non-terminal sound ratings 34

  35. NC Specifying Specifying and unqualified NC value is an ‘open’ specification Specifying an NC with specific path attenuation elements could result in acceptable sound quality It is far preferable to set maximum allowable sound power levels than to specify NC 35

  36. NC rating given is NC-30 since this is highest point tangent to an NC curve NC-70 Sound Power NC-60 Sound Power less 10 db in each band NC-50 NC-40 NC-30 NC-20 Example 80 70 60 50 40 Octave Band Level_ dB RE 0.0002 Microbar 30 Approximate 20 threshold of human hearing 10 63 125 250 500 1K 2K 4K 8K MID - Frequency, HZ

  37. NC rating given is NC-45 since this is highest point tangent to an NC curve NC-70 NC-60 NC-50 NC-40 NC-30 NC-20 Example 90 80 70 60 Octave Band Level dB RE 0.0002 Microbar 50 40 30 20 Approximate threshold of human hearing 10 63 125 250 500 1K 2K 4K 8K MID - FREQUENCY, HZ

  38. Typical grille noise at a distance of 10FT (high-frequency) Typical fan noise from adjacent mechanical room (low-frequency) Example 90 80 NC-70 Both noise spectrums would be rated NC-35, However, they would subjectively be very different! 70 NC-60 60 NC-50 50 Octave Band Level_ dB RE 0.0002 Microbar 40 NC-40 30 NC-30 20 NC-20 10 63 1K 2K 4K 8K 125 250 500 Approximate threshold of human hearing Mid - Frequency, HZ

  39. NC vs. RC • NC rates speech interference and puts limits on loudness • NC gives no protection for low frequency fan noise problems • NC stops at 63 Hz octave band • RC includes the 31.5 Hz and 16 Hz octave band • RC rates speech interference and defines key elements of acoustical quality

  40. 60 50 RC 40 50 45 30 40 35 20 30 25 10 Room Criteria (RC) Curves 90 A 80 Region AHigh probability that noise induced vibration levels in light wall and ceiling structures will be noticeable. Rattling of lightweight light fixtures, doors and windows should be anticipated. Region B Moderate probability that noise-induced vibration will be noticeable In lightweight light fixtures, doors and windows. B 70 Octave Band Sound Press. Level, dB C Threshold of audibility 16 63 1K 2K 4K 125 250 500 31.5 Octave Band Center Frequency, HZ ADAPTED FROM 2009 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA

  41. Two Parts of RC • Example – RC 40 N • The number is the speech interference level • The letter tells you speech quality • (N) = neutral spectrum • (R) = too much rumble • (H) = too much hiss • (V) = too much wall vibration

  42. RC Number Calculation • Average of level of the noise in the octave bands most important to speech • 500Hz Octave band = 46 dB • 1000Hz Octave band = 40 dB • 2000 Hz Octave band = 34 dB • RC = (46+40+34) / 3 = 40 dB

  43. RC Letter Determination • Plot room sound pressure on RC chart • Determine rumble roof • 5 dB greater then low frequency • Determine hiss roof • 3 dB greater then high frequency • R - room sound pressure crosses rumble roof • H - room sound pressure crosses hiss roof • V - room sound pressure goes into vibration zone • N - room sound pressure does not cross

  44. Rumbly Spectrum (R) 90 80 70 60 Octave Band Sound Press. Level, dB 50 Measured data is outside thereference region by >5 dB, below the 500 Hz octave band, therefore the noise is likely to be interpreted as “rumbly” 40 30 20 PSIL=(38+35+29) / 3 = 34 RC-34(R) 10 16 63 1K 2K 4K 125 250 500 31.5 Octave Band Center Frequency, HZ

  45. Rumbly & Induced Vibration (RV) 90 A 80 B 70 Even though the PSIL Is only 33 dB, the noise spectrum falls within regions A & B indicating a high probability of noise-induced vibration in lights, ceilings, air diffusers and return air grilles 60 50 40 Octave Band Sound Press. Level, dB 30 20 PSIL= (38+32+29) / 3 = 33 RC-33(RV) 10 16 63 1K 2K 4K 125 250 500 31.5 Octave Band Center Frequency, HZ

  46. NeutralSpectrum (N) 90 80 Measured data must not lie outside the reference region by >5 dB, below the 500 Hz octave band 70 60 50 40 Octave Band Sound Press. Level, dB C Measured data must not lie outside the reference region by >3 dB, above the 1000 Hz octave band 30 PSIL=(38+35+29) / 3 = 34 20 RC-34(N) 10 16 63 1K 2K 4K 125 250 500 31.5 Octave Band Center Frequency, HZ

  47. Hissy Spectrum (H) 90 80 70 Measured data is outside the reference region by >3 dB, above the 1000 Hz octave band, therefore the noise is likely to be interpreted as “hissy” 60 50 40 Octave Band Sound Press. Level, dB C 30 PSIL = (35+36+34) / 3 = 35 20 RC-35(H) 10 16 63 125 250 500 1K 2K 4K 31.5 Octave Band Center Frequency, HZ

  48. Who Uses RC? NC remains the best way to make product selections RC is preferred as an analysis tool Acoustical consultants will typically report whether or not equipment meets NC spec but will describe the resulting sound spectrum in terms of RC You should continue to see catalog application data in terms of NC 48

  49. Terminal Unit Installations • Sound characteristics • Optimal installation • Attenuators • Liners

  50. Sound Characteristics • Radiated sound is primary issue with fan-powered terminals • Discharge sound is primary issue with non-fan terminals • Fan-powered sound is typically set in 2nd (125 Hz) and 3rd (250 Hz) octave bands • Long sound waves • Harder to attenuate • Discharge sound is easily attenuated with lined ductwork and flex duct

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