hvac noise control

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Commercial & Industrial Air Conditioning

Architectural Acoustics

HVAC Noise Control

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Resources

• Manufacturers Trane Industrial Acoustics Many others

• ASHRAE (American Society for Heating, Refrigerating, and Air-Conditioning Engineers)

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Basic HVAC Functionality

• Room air is blown over a heat exchanger through which heated liquid (hot water) or cooled liquid (cold water or other refrigerant) liquid is circulated.

• Unwanted thermal energy is released outdoors• This requires…

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aerMain HVAC Noise Sources

• Fans (to move the air) Axial Centrifugal Propeller

• Compressors (to convert gas to liquid) Piston Rotary Scroll Centrifugal Screw

• Pumps (to circulate liquids)

• Diffusers and Ductwork (to distribute air) Turbulent aerodynamic

noise “Break-out” noise

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

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Other MEP Noise Sources

• Waste and Rain Leader Piping• Transformers• Dimmer Racks• Lights & Ballasts• Elevator Equipment

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Noise Control Approaches

• Location of equipment• Sealing penetrations • Resilient mounting of equipment & connected

services• Flexible connections to equipment• Lower fluid velocities• Internal duct lining and duct attenuators• Routing of ductwork and piping• Enclosing ductwork and piping

From Kirkegaard Associates

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Fan Coil Units• Opportunity for

significant noise issues: Fan and coil in

close proximity: high turbulence

Applications: typically close to “listeners” (hotel rooms, etc.)

Water flow noiseFrom Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005

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Packaged Air Handler

• Includes fan or fans• Heating coil• Cooling coil• Air filters• Humidifier• Air dampers and

controls


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Packaged Air Handler


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aerTypical Air-Handler Design

MJR Figure 9.3, p. 192

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Equipment Location: Rooftop


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Equipment Location: Mechanical Equipment Room

• Noise inside the MER• Noise outside the

MER• Duct Breakout• Active Noise Control


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Isolator TypesElastomeric Pads

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Isolator TypesNeoprene-In-Shear Floor Mount

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Isolator Types

Open Spring Floor Mount

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Isolator Types

Restrained Open Spring Floor Mount

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Reciprocating and Centrifugal Chillers Noise

• Reciprocating chillers tend to be quieter than centrifugals for the same load


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Fan Noise Components

• 1 duct length• 3 duct length• 5 duct length


• Aerodynamic noise• Blade-passage noise

fB = (RPM/60) ·N N = number of blades

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Fan Noise

Fan noise depends on the fan operation point on the fan curve


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Estimating Fan Noise

• LW = fan sound power level

• KW = fan specific value

• Q = volume flow rate (cfm)• P = static pressure (in H20)

• BFI = blade frequency increment• C = efficiency correction

CBFIPQKL WW 1010 log20log10


1

log1010 10C

η = Hydraulic efficiency of the fan = Q·P/(6350 · HP)

HP = nominal horsepower of the fan drive motor

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Estimating Fan Noise

US Army TM 5-805-4 Technical Manual, “Noise and Vibration Control”, Table C-13

CBFIPQKL WW 1010 log20log10

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Diffuser Noise

• Flow sets the noise level at a given static pressure level forcing the flow

• Good aerodynamics are important to low noise from air terminals

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (Long Fig. 13.23, p. 474)

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Indoor Diffusers• Linear or Slot Diffusers • Round or Rectangular Diffusers • Grilles• Registers


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Specifications for Diffuser Noise

• Ideal: sound power data in octave bands versus static pressure & CFM

• Reality: most manufacturers only provide the NC “rating” at a fixed “room effect” (typically 10 dB)

• Sound power from NC:

• Sadly, this only provides a noise estimate based on a perfect NC curve (diffusers are typically high-frequency elements, therefore this tends to over-estimate low frequency power)

dB10)( NCLL PW


• 400 sabins• 12 feet

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aerEstimating Diffuser Noise

• LW = sound power level (dB re. 10-12 Watts)

• SD = cross-sectional face area of diffuser (ft2)

• UD = flow velocity prior to the diffuser (ft/s)

• ξ = normalized pressure-drop coefficient

3.31log60log30log10 101010 DDW USL

20

9.334DU

P

ΔP = pressure drop across the diffuser (in. H20)

ρ0 = density of air (0.075 lb/ft3)

Long, p. 475

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aerEstimating Diffuser Noise

Long, Fig. 13.24, p.476

DWW CLL Oct,

• Octave-band power levels can be calculated from the overall level LW

213.115.082.5 AACD 213.115.082.11 AACD

Generalized Diffuser Spectrum

for round diffusers

for rectangular diffusers

GP Uf 8.48

fNfNA BPB peak frequency

NB(x) = octave-band number of frequency x (32 Hz = 0, 63 Hz = 1, 125 Hz = 2, …)

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Recommended Velocity Limits

• Plant Rooms 5m/s• Aud. Shafts 4m/s• Within Aud. 2.5m/s

Branch RunoutsRC-35 2.75 m/sRC-25 2 m/sRC-15 1.25 m/s

Terminal velocities are critical because there is nothing after the diffuser to provide additional attenuation!


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Unlined Ducts

• Not much attenuation in unlined ducts Little absorption from surfaces (although some

energy is lost to break-out noise) Plane-wave propagation → no spreading loss

• Plane-wave propagation when duct dimensions (not length) are less than half a wavelength

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Attenuation in Unlined Ducts


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Duct Liner

MJR Figure 9.5 and 9.7, pp. 193 and 194

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Duct Liner• Attenuation in lined rectangular ducts can be

approximated with this equation

P = duct perimeter (ft) S = duct cross-sectional area (ft2) t = thickness of lining (in)

DC

duct tS

PBL

63 125 250 500 1000 2000 4000 8000

B 0.0133 0.0574 0.2710 1.0147 1.7700 1.3920 1.5180 1.5810

C 1.959 1.410 0.824 0.500 0.695 0.802 0.451 0.219

D 0.917 0.941 10.79 10.87 0.000 0.000 0.000 0.000

Octave-Band Center Frequency (Hz)

Long, Eq. 14.12, p. 487

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Duct Liner

MJR Figure 9.5 and 9.7, pp. 193 and 194

x xx

x

xx

x

x

x

x

x x

x

Data from Long’s equation

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Duct Liner Data

http://www.owenscorning.com/comminsul/documents/FiberglasDuctBoardLiner.pdf

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Internal Fiberglass Duct Lining


Duct Liner

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aerAirflow: Turbulent Noise in

Ductwork

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Fig. 9.12, p. 198)

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aerAirflow: Turbulent Noise in

Ductwork

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005, (MJR Table 9.1, p. 197)

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aerHow Ductwork Radiates Noise

(Break Out)


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Duct Shape and Noise Control


• Stiffness of round ductwork reduces break-out noise since motion of the duct walls is restricted

• However, this means that more noise energy stays within the duct and may produce higher noise levels at the outlet

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Long, p. 486

• The ratio of perimeter to cross-sectional area is also important, and can be used to approximate duct attenuation.

P = perimeter (ft) S = cross-sectional area (ft) l = duct length (ft) f = octave-band center frequency between 63 and 250 Hz

3 ,0.17 85.025.0

S

Plf

S

PLduct

3 ,64.1 58.073.0

S

Plf

S

P


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• For octave bands above 250 Hz

P = perimeter (ft) S = cross-sectional area (ft) l = duct length (ft)

lS

PLduct

8.0

2.0

Long, p. 486


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Long, p. 486

Frequency (Hz)

63 125 250 500 1000 2000 4000

Loss (dB/ft)Circular

0.03 0.03 0.03 0.05 0.07 0.07 0.07

Loss (dB/ft)Square

0.36 0.20 0.11 0.06 0.06 0.06 0.06

• Data for circular duct from Long, Table 14.1• Data for square duct from previous equations with P/S = 4


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Discharge Noise


High noise levels near the discharge of the AHU

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Discharge Noise Control

• Stiffen the initial 25-50 ft of the discharge duct

• Often done by wrapping the duct with gypsum board or loaded vinyl


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Duct Lagging

Make the ducts stiff using lagging, typically fire-rated drywall.


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Duct Lagging


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Duct Lagging

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Duct Penetrations

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Ductwork Crossing an Isolation Joint

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Resilient Duct Hangers

Elastomeric Hanger


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Spring-and-Neoprene-in Series Isolator (Hanger)


Precompressed Spring-and-Neoprene-in-Series

Isolator (Hanger)

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Spring-and-Neoprene-in-Series Isolator (Hanger)


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Resilient Hangers


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aerFlexible Duct Connections


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Improvements in Design for Noise Performance


Poor Design

Better Design

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End Effects

• Change in cross-sectional area when a duct terminates in a room

88.1

010 1log10

fd

cLend

88.1

010

8.01log10

fd

cLend

Termination in free space:

Termination flush with wall:

c0 = speed of sound f = frequency d = duct diameter ( for a rectangular duct)

S

d4

Long, p. 490

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Air Plenums, Passive and Active Silencers

• Plenum used near equipment outlet; promotes laminar airflow and provides acoustical insertion loss (< 12 dB)

• Passive silencers used when large insertion loss is required; must account for pressure drop

• Active silencer has no pressure drop, but is typically impractical


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Application of Duct Liner in Underfloor Plenum


Lined Plenum(For under-floor air supply)

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Silencer Location


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Duct Sound Attenuators


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Active Noise Control in Ducts


Using data from the input microphone, the controller generates a signal to be played by the loudspeaker which is out of phase (180º) with the duct-borne noise at the loudspeaker position. Feedback from the error microphone (which ideally senses no noise) helps fine tune the process.

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A Sample Interior Noise Prediction

From Paul Henderson, Acoustics for Mechanical Engineers, ASHRAE Expo 2005 (MJR Table 9.2, p. 204)

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1/3 vs. 1/1 Octave Band Data


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aerFan and Compressor Noise


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Thanks!

hvac noise control

Engineering

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