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  Designers must understand acoustical ratings in order to write good specifications 2

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

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 4

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) 5

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

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

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 8

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 9

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

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

To add two decibel values: Difference in Values: 6 dB From Chart: Add 1.0 dB to higher Value 80 dB + 1 dB 81 dB (Correct) 80 dB - 74 dB = 6 dB Difference In Decibels Between Two Values Being Added (dB) Decibel Addition Example 12

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 13

Sound Power Changes Equation for sound power changes = 10logn 1 Fan onvs. 2 Fans onn=2Add 3 dB 1 Fan onvs. 4 Fans onn=4Add 6 dB 1 Fan onvs. 10 Fans onn=10Add 10 dB 1 Fan onvs. 100 Fans onn=100Add 20 dB 50 Fans onvs. 100 Fans onn=2Add 3 dB 14

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 15

 1 dBnot noticeable  3 dBjust perceptible  5 dBnoticeable  10 dBtwice as loud  20 dBfour times as loud For High Frequencies 16

 3 dBnoticeable  5 dBtwice as loud  10 dBfour times as loud For Low Frequencies 17

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

Acoustic Quality Not too quietDon’t destroy acoustic privacy Not too loud Avoid hearing damage Don’t interfere with speech Not too annoying No rumble, no hiss No identifiable machinery sounds No time modulation Not to be feltNo noticeable wall vibration 19

K2K4K8K MID - FREQUENCY, HZ OCTAVE BAND LEVEL _ dB RE MICROBAR APPROXIMATE THRESHOLD OF HUMAN HEARING NC-70 NC-20 NC-60 NC-50 NC-30 NC-40 NC Curves 20

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

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) 22

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” 23

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 24

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 25

Diffuser Testing  Current test standard for diffusers –ASHRAE  No significant changes in many years 26

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

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

Performance Rating  Current rating standard for terminal units –AHRI (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

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

Industry Standardization  AHRI contains Appendix E –Recommends standard attenuations to be used by all manufacturers for catalog data –First presented in ARI –Makes comparing catalog NC levels much less risky 31

AHRI Catalog Assumptions Radiated Sound Octave Band Environmental Effect Ceiling / Space Effect Total dB Attenuation mineral fiber tile 5/8 in thick 20 lb/ ft 3 density 5 ft, 1 in fiberglass lining 8 in flex duct to diffuser 2500 ft 3 room volume 5 ft from source The following dB adjustments are used for the calculation of NC above 300 CFM Discharge Sound Octave Band Environmental Effect Duct Lining End Reflection Flex Duct Space Effect Total dB Attenuation Octave Band CFM Over 700 CFM

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

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

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

Example K2K4K8K MID - Frequency, HZ Octave Band Level_ dB RE Microbar Approximate threshold of human hearing NC-70 NC-20 NC-60 NC-50 NC-30 NC-40 Sound PowerSound Power less 10 db in each band NC rating given is NC-30 since this is highest point tangent to an NC curve 36

Example K2K4K8K MID - FREQUENCY, HZ Octave Band Level dB RE Microbar NC-70 NC-20 NC-60 NC-50 NC-30 NC-40 Approximate threshold of human hearing NC rating given is NC-45 since this is highest point tangent to an NC curve 37

Both noise spectrums would be rated NC-35, However, they would subjectively be very different! K2K4K8K Mid - Frequency, HZ Octave Band Level_ dB RE Microbar NC-30 NC-70 NC-20 NC-60 NC-50 NC-40 Typical grille noise at a distance of 10FT (high-frequency) Typical fan noise from adjacent mechanical room (low-frequency) Approximate threshold of human hearing Example 38

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 39

Room Criteria (RC) Curves RC C ADAPTED FROM 2009 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA Region A High 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 K2K4K Octave Band Center Frequency, HZ A B Threshold of audibility Octave Band Sound Press. Level, dB 40

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 41

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 = ( ) / 3 = 40 dB 42

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 43

Measured data is outside the reference region by >5 dB, below the 500 Hz octave band, therefore the noise is likely to be interpreted as “rumbly” PSIL=( ) / 3 = 34 RC-34(R) Rumbly Spectrum (R) K2K4K Octave Band Center Frequency, HZ Octave Band Sound Press. Level, dB 44

A B RC-33(RV) K2K4K Octave Band Center Frequency, HZ Octave Band Sound Press. Level, dB 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 Rumbly & Induced Vibration (RV) PSIL= ( ) / 3 = 33 45

Octave Band Center Frequency, HZ Octave Band Sound Press. Level, dB Neutral Spectrum (N) C Measured data must not lie outside the reference region by >5 dB, below the 500 Hz octave band Measured data must not lie outside the reference region by >3 dB, above the 1000 Hz octave band RC-34(N) K2K4K PSIL=( ) / 3 = 34 46

C 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” RC-35(H) K2K4K PSIL = ( ) / 3 = 35 Hissy Spectrum (H) Octave Band Center Frequency, HZ Octave Band Sound Press. Level, dB 47

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

Terminal Unit Installations  Sound characteristics  Optimal installation  Attenuators  Liners 49

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 50

Ideal Terminal Unit Installation VAV UNIT Lined Sheet Metal Plenum (Max velocity 1,000 FPM) Lined Flexible Ducts To Diffusers Flexible Connectors For Fan-powered Units D > 3 D Ceiling Maximize Height Above Ceiling 4' Min. 51 Max velocity 2,000 FPM

Attenuators  Single duct –Equivalent to lined ductwork  Dual duct –Provides a mixing area for unit, but not much sound attenuation  Fan powered –Lined elbow or “boot” may provide 2dB attenuation by removing line of sight to motor –Carefully engineered attenuators can provide additional sound reductions 52

Liners  Different liners in single ducts do not affect discharge sound much –Unit is too short for the air to interact with liner  1" liner does not significantly decrease sound compared to ½"  Foil faced liners add 6-8 dB  Fiber free adds 4-6 dB  Double wall is variable –Kettle drum effect increases sound, but it is directional 53

Flex Duct  Don’t forget about flex duct  5' of flex can reduce mid frequencies by 20 dB or more  Flex is better than lined duct or attenuators in reducing low frequencies  You can have too much of a good thing 54

Diffuser Tests - ASHRAE Conditions 10 equivalent Diameters, min Pressure Measured Air Flow Discharge Velocity Sound 55

Inlets: 3 Equivalent Diameters - Ideal  ~1 NC add to catalog data Pressure Measured Air Flow 3 equivalent Diameters Discharge Velocity Sound Flex Duct, 1 radius bend 56

Inlets: Long 90 at Diffuser  ~3 NC add to catalog data Pressure Measured Air Flow Discharge Velocity Sound Flex Duct 57

Inlets: Hard 90 at Diffuser  ~5 NC add to catalog data Pressure Measured Air Flow Discharge Velocity Sound Flex Duct 58

Inlet ‘Kinked’  ~7-9 NC add to catalog data Pressure Measured Air Flow Discharge Velocity Sound 2 equivalent Diameters Flex Duct 59

Summary of Results  Minimum add for flex duct = 1 NC  Worst case add, ‘Kinked’ = 7-9 NC  Air distribution pattern can be greatly effected – Plaque / Perforated shows most effect – Multi-Cone / Louvered shows least effect  Results were not the same for all diffuser types  Don’t forget that catalog NC’s are based on typical offices (-10 dB across all bands) 60

Some Diffuser Solutions  Locate balancing dampers at branch takeoff  Keep flexible duct bends as gentle as possible –Flex duct is a great attenuator of upstream noise sources  Keep duct velocities as low as possible –But over-sizing can result in higher thermal loss 61

Additional Resources  Noise and Vibration Control for HVAC Systems –Mark Schaffer, 2005  ASHRAE Fundamentals –Chapter 8, 2009 Edition  ASHRAE HVAC Applications –Chapter 48, 2011 Edition 62

Summary  NC remains the preferred sound specification  RC is often used after-the-fact  Specified max sound power levels are safest  Lining materials affect sound levels  Careful selection, design and installation are required to avoid problems 63