2012 ces acoustics -...

AcousticsAcoustics

IntroductionIntroduction

What is sound?

How do we measure sound?

Sound power vs. sound pressure

Sound quality

AHRI 880/885

NC vs. RC

Installation effects

2

The Sound RoomThe Sound Room

Products are tested in a qualified reverberant chamber (per AHRI 220)

Reverberant chambers are used for quiet products

Anechoic chambers are used for noisy products

The reverberant field eliminates all directionality from a sound source

Sound levels within the reverberant field are equal at all points

3

The Comparison MethodThe 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 attenuationLw = Lp + room attenuationIf 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 attention is constantAll sound meters measure sound pressure (Lp)

4

The Decibel (dB)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’

5

Octave BandsOctave 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

Octave Band Designations Center Frequency 63 125 250 500 1000 2000 4000 8000 Band Designation 1 2 3 4 5 6 7 8

6

Octave BandsOctave 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

7

To add two decibel values:

80 dB+ 74 dB

Decibel Addition ExampleDecibel Addition Example

8

154 dB (Incorrect)



80 dB+ 74 dB

9


Difference in Values: 6 dBFrom Chart: Add 1.0 dB

to higher Value80 dB

+ 1 dB

81 dB81 dB (Correct)(Correct)

80 dB- 74 dB= 6 dB

Difference In Decibels Between Two Values Being Added (dB)

Cor

rect

ion

To B

e A

dded

To

Hig

her V

alue

(dB

)

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10


10

Good To KnowGood 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

11

Sound Power ChangesSound Power Changes

Equation for sound power changes = 10lognEquation for sound power changes = 10logn1 Fan on vs. 2 Fans on n=2 Add 3 dB

1 Fan on vs. 4 Fans on n=4 Add 6 dB

1 Fan on vs. 10 Fans on n=10 Add 10 dB

1 Fan on vs. 100 Fans onn=100

Add 20 dB

50 Fans on vs. 100 Fans on n=2 Add 3 dB

12

Proximity To Sound SourcesProximity 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

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1 dB not noticeable

3 dB just perceptible

5 dB noticeable

10 dB twice as loud

20 dB four times as loud

For High FrequenciesFor High Frequencies

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3 dB noticeable

5 dB twice as loud

10 dB four times as loud

For Low FrequenciesFor Low Frequencies

15

What We HearWhat We Hear

Our ears can be fooled by frequency– Both tones sound equally loud

65 dB65 dB

63 HZ63 HZ 1000 HZ1000 HZ

40 dB40 dB

A Difference of 25 dB 16

Acoustic QualityAcoustic Quality

Not too quiet Don’t destroy acoustic privacy

Not too loudAvoid hearing damage

Don’t interfere with speech

Not too annoying

No rumble, no hiss

No identifiable machinery sounds

No time modulation

Not to be felt No noticeable wall vibration

17

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63 125 250 500 1K 2K 4K 8KMID - FREQUENCY, HZ

OC

TAVE

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ND

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dB

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02 M

ICR

OB

AR

APPROXIMATETHRESHOLDOF HUMAN HEARING

NC-70

NC-20

NC-60

NC-50

NC-30

NC-40

NC CurvesNC Curves

18

Typical NC LevelsTypical 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 NC20‐2519

Sound Power Vs. Sound PressureSound 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)

20

Reference Sound SourceReference 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”

21

In a Reverb RoomIn a Reverb Room

Sound power (Lw) is calculated from measured sound pressure (Lp) and corrected for background

– Product sound should be 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

22

Catalog DataCatalog 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

23

Diffuser TestingDiffuser Testing

Current test standard for diffusers – ASHRAE 70‐2006

No significant changes in many years

24

Terminal Unit TestingTerminal 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

25

Sound TestsSound 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 26

Performance Rating Performance Rating

Current rating standard for terminals unit– 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

27

Sound Path Determination 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

28

Industry Standardization 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

29

AHRI 885AHRI 885‐‐2008 Catalog Assumptions2008 Catalog Assumptions

Radiated SoundOctave BandOctave Band

22 33 44 55 66 77Environmental Effect 2 1 0 0 0 0Ceiling / Space Effect 16 18 20 26 31 36Total dB Attenuation 18 19 20 26 31 36

mineral fiber tile5/8 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

Discharge SoundOctave BandOctave Band

22 33 44 55 66 77Environmental Effect 2 1 0 0 0 0Duct Lining 3 6 12 25 29 18End Reflection 9 5 2 0 0 0Flex Duct 6 10 18 20 21 12Space Effect 5 6 7 8 9 10Total dB Attenuation 25 28 30 53 59 40

Octave BandOctave Band22 33 44 55 66 77

300 ‐ 700 CFM 2 1 1 ‐2 ‐5 ‐1Over 700 CFM 4 3 2 ‐2 ‐7 ‐1

30

Certified Performance Data 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

31

AHRI Source AHRI Source –– Path Path –– Receiver ConceptReceiver Concept

DB (SND POWER) ‐ DB PATH LOSS = DB (SND PRESSURE)

Source Path Receiver

AHRI 880Air Terminals

AHRI 885Attn Factors

NC or RCSound Pressure Level

32

NC SpecifyingNC 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

33

Reverb Room InstallationsReverb Room Installations

Tests are under ideal conditions

Boxes have only the unit exposed to the room

Diffusers have ‘wonderful’ inlet conditions

Actual conditions will always be louder than reverb room tests indicate– Covered by AHRI 885‐2008

34

Single Number RatingsSingle Number Ratings

A number of single number rating schemes have been developed to deal with a spectrum of sound

These include:– NC, NR and RC

– dBA (A scale, B scale and C scale)

– Sones, Bels

– STC, NRC

35

The dBA ScaleThe dBA Scale

Used for outdoor noise evaluation

Also used for hearing conservation measurements

Basis of most non‐terminal sound ratings

36

ExampleExample

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63 125 250 500 1K 2K 4K 8KMID - Frequency, HZ

Oct

ave

Ban

d Le

vel_

dB

RE

0.00

02 M

icro

bar

Approximatethresholdof human hearing

NC-70

NC-20

NC-60

NC-50

NC-30

NC-40

Sound Power

Sound Power less 10 db in each band

NC rating given is NC-30 since this is highest point tangent to an NC curve

37

ExampleExample

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63 125 250 500 1K 2K 4K 8K

MID - FREQUENCY, HZ

Octave Band Level dB RE 0.0002 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

38

Both noise spectrums would be rated NC-35, However, they would subjectively be very different!

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63 125

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Mid - Frequency, HZ

Oct

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vel_

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0.00

02 M

icro

bar

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

ExampleExample

39

NC vs. RCNC 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

Room Criteria Room Criteria (RC) Curves(RC) Curves

RC50454035

3025

C

ADAPTED FROM 2009 ASHRAE FUNDAMENTALS HANDBOOK - ATLANTA, GA

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 BModerate probability that noise-induced vibration will be noticeable In lightweight light fixtures, doors and windows.

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16 31.5 63 125

250

500 1K 2K 4K

Octave Band Center Frequency, HZ

A

B

Threshold of audibility

Oct

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, dB

41

Two Parts of RCTwo 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 CalculationRC 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 DeterminationRC 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

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”

PSIL=(38+35+29) / 3 = 34

RC-34(R)

Rumbly Rumbly Spectrum (R)Spectrum (R)

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16 31.5 63 125

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45

A

B

RC-33(RV)10

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Oct

ave

Ban

d So

und

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evel

, dBEven though the PSIL

Is only 33 dB, the noise spectrum falls within regionsA & B indicating ahigh probability ofnoise-induced vibration in lights,ceilings, air diffusersand return air grilles

Rumbly & Induced Rumbly & Induced Vibration (RV)Vibration (RV)

PSIL= (38+32+29) / 3 = 33

46


Oct

ave

Ban

d So

und

Pres

s. L

evel

, dB

NeutralNeutralSpectrum (N)Spectrum (N)

C

Measured data mustnot lie outside the reference region by>5 dB, below the 500 Hz octave band

Measured data mustnot lie outside the reference region by >3 dB, above the 1000 Hz octave band

RC-34(N)10

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16 31.5 63 125

250

500 1K 2K 4K

PSIL=(38+35+29) / 3 = 34

47

C

Measured data is outside the referenceregion by >3 dB, above the 1000 Hz octave band,therefore the noise is likely to beinterpreted as “hissy”

RC-35(H)10

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16 31.5 63 125

250

500 1K 2K 4K

PSIL = (35+36+34) / 3 = 35

Hissy Hissy Spectrum (H)Spectrum (H)


Oct

ave

Ban

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s. L

evel

, dB

48

Who Uses RC?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

49

Terminal Unit InstallationsTerminal Unit Installations

Sound characteristics

Optimal installation

Attenuators

Liners

50

Sound CharacteristicsSound 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

51

Ideal Terminal Unit InstallationIdeal Terminal Unit Installation

VAVUNIT

Lined Sheet Metal Plenum(Max velocity 1,000 FPM)

Lined Flexible DuctsTo Diffusers

Flexible ConnectorsFor Fan-powered Units

D

> 3 D

Ceiling

Maximize HeightAbove Ceiling

4' Min.

52

Max velocity 2,000 FPM

AttenuatorsAttenuators

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 53

LinersLiners

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

54

Flex DuctFlex 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

55

Diffuser Tests Diffuser Tests ‐‐ ASHRAE ConditionsASHRAE Conditions

10 equivalent Diameters, min

Pressure

Measured Air Flow

Discharge VelocitySound 56

Inlets: 3 Equivalent Diameters Inlets: 3 Equivalent Diameters ‐‐ IdealIdeal

~1 NC add to catalog data

Pressure

Measured Air Flow

3 equivalent Diameters

Discharge VelocitySound

Flex Duct, 1 radius bend

57

Inlets: Long 90 at DiffuserInlets: Long 90 at Diffuser


Pressure

Measured Air Flow


Flex Duct

58

Inlets: Hard 90 at DiffuserInlets: Hard 90 at Diffuser


Pressure

Measured Air Flow

Discharge Velocity

Sound

Flex Duct

59

Inlet Inlet ‘‘KinkedKinked’’

~7‐9 NC add to catalog data

Pressure

Measured Air Flow


2 equivalent Diameters

Flex Duct

60

Summary of ResultsSummary 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)

61

Some Diffuser SolutionsSome 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

62

Additional ResourcesAdditional Resources

ASHRAE Fundamentals– Chapter 8, 2009 Edition

ASHRAE HVAC Applications– Chapter 48, 2011 Edition

Cognizant Technical Committee– TC 2.6

63

SummarySummary

NC remains the preferred sound specification

RC is often used after‐the‐fact

Max sound power levels are safest

Lining materials affect sound levels

Careful selection, design and installation are required to avoid problems

64

2012 ces acoustics -...

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