David HerrinUniversity of Kentucky
Using Simulation to Determine Attenuation of HVAC Ductwork
University of Kentucky
March 18, 2021
Vibro-Acoustics Consortium
March 18, 2021
2
Overview
• Unlined and Lined Duct Insertion Loss
• Elbow Insertion Loss
• Plenum Insertion Loss
• Duct Breakout Noise
Vibro-Acoustics Consortium
March 18, 2021
3
Prior ASHRAE Sponsored Work• Vér, I.L. 1978. A review of the attenuation of sound in straight lined and
unlined ductwork of rectangular cross section. ASHRAE Transactions84(1):122-149.
• Kuntz, H.L. and R.M Hoover. 1987. The interrelationships between the physical properties of fibrous duct lining materials and lined duct sound attenuation. ASHRAE Transactions 93(2):449-470.
• Reynolds, D.D. Reynolds, D.D. and J.M. Bledsoe. 1989a. Sound attenuation of unlined and acoustically lined rectangular ducts. ASHRAE Transactions 95(1):90-95.
• J.M. Bledsoe. 1989b. Sound attenuation of acoustically lined circular ducts and radiused elbows. ASHRAE Transactions 95(1):96-99.
Limitation Measurement-based studies based on a limited number of duct cross-sectional areas and lining lengths.
Vibro-Acoustics Consortium
March 18, 2021
4
RP-1408 Campaign
Receiving Room (Reverberant Room)
Test Duct
Source Room with Loudspeaker Array
ASTM E477
Source Room
ReceivingRoom
Vibro-Acoustics Consortium
March 18, 2021
5
Modeling Approach
20 Monopoles
Automatically Matched Layer
Structural FEMRigid Ductwork
Acoustic FEM
Automatically Matched Layer
Automatically Matched Layer
Automatically Matched Layer
Poroelastic FEM
Acoustic FEM
Acoustic FEM
Vibro-Acoustics Consortium
March 18, 2021
6
Modeling Approach
Poroelastic Mesh
Structural Mesh
Acoustic Mesh
Vibro-Acoustics Consortium
March 18, 2021
7
Sound Absorptive Material Properties
0.0
0.2
0.4
0.6
0.8
1.0
0 1000 2000 3000 4000 5000
Soun
d Ab
sorp
tion
Frequency (Hz)
Measurement Curve Fit
• Johnson-Champoux-Allard model (Allard and Atalla 2009)• Curve fit (using ESI Foam-X software) used to identify the flow
resistivity, characteristic viscous length, characteristic thermal length, and mass density for the fiber.
• Alternative sound absorbing models can be used with little impact.
Vibro-Acoustics Consortium
March 18, 2021
8
Metrics for Duct Attenuation
Insertion Loss – Difference in sound pressure level in receiving room without and with absorbing lining.Noise Reduction – Difference in sound pressure level between source and termination (independent of source).Transmission Loss – Difference between incident and transmitted sound power (independent of source and termination).
Source Room
Noise Control Treatments
Vibro-Acoustics Consortium
March 18, 2021
9
Noise Reduction – Unlined Duct
24 in x 24 in (0.61 m x 0.61 m), 10 ft (3.05 m) Length Square Duct
-5
0
5
10
15
62.5 125 250 500 1000 2000 4000
Noi
se R
educ
tion
(dB)
Frequency (Hz)FEM (Rectangular) ASHRAE Handbook (Rectangular)
Vibro-Acoustics Consortium
March 18, 2021
10
Noise Reduction – Unlined Duct24 in (0.61 m) Diameter, 20 ft (6.10 m) Length Circular Duct
-5
0
5
10
15
62.5 125 250 500 1000 2000 4000
Noi
se R
educ
tion
(dB)
Frequency (Hz)FEM (Circular) ASHRAE Handbook (Circular)
Vibro-Acoustics Consortium
March 18, 2021
11
Insertion Loss – Lined Duct24 in x 24 in (0.61 m x 0.61 m), 10 ft (3.05 m) Length Square Duct2 in (5 cm) fiber lining)
0
10
20
30
40
50
60
50 100 200 400 800 1600 3200 6400
Inse
rtion
Los
s (d
B)
Frequency (Hz)
FEM Measurement (RP-1408)
Vibro-Acoustics Consortium
March 18, 2021
12
+
-
+
+
-
Insertion Loss – Lined Duct
𝑓𝑐
2𝑑
𝑓 𝑐𝑑
Vibro-Acoustics Consortium
March 18, 2021
13
Insertion Loss – Lined Duct
0102030405060708090
50 100 200 400 800 1600 3200 6400
Inse
rtion
Los
s (d
B)
Frequency (Hz)
FEM Measurement (RP-1408)
24 in x 24 in (0.61 m x 0.61 m), 30 ft (9.15 m) Length Square Duct2 in (5 cm) fiber lining)
Vibro-Acoustics Consortium
March 18, 2021
14
Likely Flanking Path
Receiving Room (Reverberant Room)
Source Room with Loudspeaker Array
Flanking Path
Vibro-Acoustics Consortium
March 18, 2021
15
Insertion Loss – Lined Duct
0
10
20
30
40
50
60
70
80
50 100 200 400 800 1600 3200 6400
Inse
rtion
Los
s (d
B)
Frequency (Hz)FEM Measurement (RP-1408)
24 in (0.61 m) Diameter, 20 ft (6.10 m) Length Circular Duct2 in (5 cm) fiber lining)
Vibro-Acoustics Consortium
March 18, 2021
16
Attenuation vs. Length
010
20
3040
50
6070
80
90
0 5 10 15 20
Atte
nuat
ion
(dB)
Distance (Acoustic Wavelengths)315 Hz (10 ft) 630 Hz (10 ft) 800 Hz (10 ft) 1250 Hz (10 ft)315 Hz (30 ft) 630 Hz (30 ft) 800 Hz (30 ft) 1250 Hz (30 ft)
Vibro-Acoustics Consortium
March 18, 2021
17
Overview
• Unlined and Lined Duct Insertion Loss
• Elbow Insertion Loss
• Plenum Insertion Loss
• Duct Breakout Noise
Vibro-Acoustics Consortium
March 18, 2021
18
Introduction
Airborne direct path
Airborne indirect path or duct breakout
Structure‐borne path
HVAC Building Equipment
Vibro-Acoustics Consortium
March 18, 2021
19
Prior Elbow Work• Vér, I.L. 1983. Noise generation and noise attenuation of duct fittings
– a review: Part II (RP-265). ASHRAE Transactions 90(2A):383-390.
• Mechel, F. 1975. Mufflers, Chapter 19, Pocket Book of Technical Acoustics. Ed. H. Heckl and M.A. Mueller, Springer (in German).
• VDI.2001. VDI Technical Report 2081. Noise Generation and Noise Reduction in Air-Conditioning Systems.
Limitation Studies limited to certain duct sizes, bend angles, and specific sound absorptive linings.
Vibro-Acoustics Consortium
March 18, 2021
20
Modeling Approach
20 Monopoles
Rigid Ductwork
Acoustic FEM Poroelastic FEM
Structural FEM
Automatically Matched Layer
Acoustic FEM
Acoustic FEM
Automatically Matched Layer
Automatically Matched Layer
Automatically Matched Layer
Vibro-Acoustics Consortium
March 18, 2021
21
Modeling Approach
Acoustic Mesh with Automatically Matched Layers
Structural Mesh
Poroelastic Mesh
Vibro-Acoustics Consortium
March 18, 2021
22
Elbow Insertion Loss
Source
Source
𝐿 ,
𝐿 ,
𝐼𝐿 𝐿 , 𝐿 ,
Vibro-Acoustics Consortium
March 18, 2021
Insertion Loss – Unlined Elbow
23
24 in x 24 in (0.61 m x 0.61 m)
-5
0
5
10
15
20
50 100 200 400 800 1600 3200
Inse
rtion
Los
s (d
B)
Frequency (Hz)
FEM ASHRAE Handbook
Vibro-Acoustics Consortium
March 18, 2021
Insertion Loss – Lined Elbow
24
24 in x 24 in (0.61 m x 0.61 m)1 in (2.5 cm) fiber lining
-5
0
5
10
15
20
50 100 200 400 800 1600 3200
Inse
rtion
Los
s (d
B)
Frequency (Hz)
FEM ASHRAE Handbook
Vibro-Acoustics Consortium
March 18, 2021
25
Lined Elbow Insertion Loss
Source
Source
𝐿 ,
𝐿 ,
𝐼𝐿 𝐿 , 𝐿 ,
Vibro-Acoustics Consortium
March 18, 2021
26
Insertion Loss – Lined Elbow24 in x 24 in (0.61 m x 0.61 m)1 in (2.5 cm) fiber lining
-10
0
10
20
30
40
50 100 200 400 800 1600 3200
Inse
rtion
Los
s (d
B)
Frequency (Hz)
FEM ASHRAE Handbook
Vibro-Acoustics Consortium
March 18, 2021
27
Overview
• Lined Duct Insertion Loss
• Elbow Insertion Loss
• Plenum Insertion Loss
• Duct Breakout Noise
Vibro-Acoustics Consortium
March 18, 2021
28
𝐼𝐿 10 log 𝑆𝑐𝑜𝑠𝜃2𝜋𝑑
1𝑅
Above the cutoff frequency
𝑑 𝐿 𝑙 𝐻 )
cos𝜃 𝐻/𝑑
𝑆 𝑙𝑊
𝑅 𝑆𝛼 1 𝛼⁄ )
Wells, 1958
𝑙
𝐿
𝐻 𝑊
Unlined and Lined Plenums
Vibro-Acoustics Consortium
March 18, 2021
29
Mouratides and Becker, 2003
Unlined and Lined Plenums
𝐼𝐿 𝐴 𝑆 𝑊 𝐸
Below the cutoff frequency
𝐴 ~ empirically defined surface area coefficient𝑆 ~ total inside surface area of plenum less the inlet and outlet areas𝑊 ~ wall effect (dB)𝐸 ~ offset angle effect (dB)
Vibro-Acoustics Consortium
March 18, 2021
30
Mouratides and Becker, 2003
Unlined and Lined Plenums
Above the cutoff frequency
𝐼𝐿 𝑏𝑆 𝑄4𝜋𝑟
𝑆 1 𝛼𝑆𝛼 𝐸
𝑆 ~ outlet cross-sectional area𝑆 ~ surface area of the plenum𝑟 ~ distance between the centers of the inlet and outlet sections𝑄 ~ directivity factor, and 𝛼 is the average absorption coefficient in the plenum𝑏, 𝑛 ~ empirically determined constants (3.505 and -0.359)𝐸 ~ offset angle effect (dB)
Vibro-Acoustics Consortium
March 18, 2021
31
Finite Element Model
Poroelastic Mesh
Structural Mesh
Acoustic Mesh
Automatically Matched Layers
Vibro-Acoustics Consortium
March 18, 2021
32
Insertion Loss – Unlined Plenum
4 ft x 6 ft x 5 ft
unlined plenum
0
5
10
15
20
100 200 400 800 1600 3200
Inse
rtion
Los
s (d
B)
Freqeuncy (Hz)
Measurement (RP-1026)FEMMouratidis and Becker (2004)Wells (1958)
Vibro-Acoustics Consortium
March 18, 2021
33
Insertion Loss – Lined Plenum
0
10
20
30
40
100 200 400 800 1600 3200
Inse
rtion
Los
s (d
B)
Freqeuncy (Hz)
Measurement (RP-1026)FEMMouratidis and Becker (2004)Wells (1958) 4 ft x 6 ft x 5 ft lined
(2 in fiber) plenum
Vibro-Acoustics Consortium
March 18, 2021
34
0
10
20
30
40
100 200 400 800 1600 3200
Inse
rtion
Los
s (d
B)
Freqeuncy (Hz)
Measurement (RP-1026)FEMMouratidis and Becker (2004)Wells (1958)
Insertion Loss – Lined Plenum with Right Angle
4 ft x 6 ft x 5 ft lined
(2 in fiber) plenum
Inlet and outlet ducts at right angles
Vibro-Acoustics Consortium
March 18, 2021
35
Overview
• Lined Duct Insertion Loss
• Elbow Insertion Loss
• Plenum Insertion Loss
• Duct Breakout Noise
Vibro-Acoustics Consortium
March 18, 2021
Prior Breakout TL Work
36
• Vér, I.L. 1983. Prediction of Sound Transmission through Duct Walls; Breakout and Pickup. ASHRAE TRP-319.
• Cummings, A. 1983. Acoustic noise transmission through the walls of air conditioning ducts. Final Report, Department of Mechanical and Aerospace Engineering, University of Missouri, Rolla.
• Cummings, A. 1985. Acoustic noise transmission through duct walls. ASHRAE Transactions 91(2A):48-61.
• Cummings, A. 2001. Sound transmission through duct walls. Journal of Sound and Vibration 239(4):731-765.
• Lilly, J. 1987. Breakout in HVAC duct systems. Sound & Vibration 21(10).
Limitation Studies limited to certain duct sizes and specific sound absorptive linings.
Vibro-Acoustics Consortium
March 18, 2021
Measurement (Cummings, 1985)
Receiving Room (Reverberation Room)
Source Room with Loudspeaker Array
Sound Absorptive Termination
𝑇𝐿 𝐿 𝐿 10 log𝑆𝑆
Method 1 (low frequencies)
Vibro-Acoustics Consortium
March 18, 2021Measurement of Breakout Transmission Loss (Cummings,
1985)
Receiving Room (Reverberant Room)
Source Room with Loudspeaker Array
Method 2 (high frequencies)
Measure sound pressure in reverberation room with cap as on prior slide then measure again with horn.
Measurement (Cummings, 1985)
Vibro-Acoustics Consortium
March 18, 2021
Modeling Approach
39
20 Monopoles
Automatically Matched Layer
Structural FEM
Rigid Ductwork
Acoustic FEM
Automatically Matched Layer
Automatically Matched Layer
Automatically Matched Layer
Poroelastic FEM
Acoustic FEM
Acoustic FEM
Vibro-Acoustics Consortium
March 18, 2021
Structural FEM Models
40
30 ft (9.15 m) Rectangular Duct
20 ft (6.10 m) Circular Duct
Vibro-Acoustics Consortium
March 18, 2021
Damping Properties
41
Accelerometers
Data Acquisition
Rectangular Duct
Vibro-Acoustics Consortium
March 18, 2021
Measured Damping
42
Measured Damping Loss FactorsCircular Rectangular
Frequency (Hz)10 in Diameter
(25.4 cm)10 in × 10 in
(25.4 cm × 25.4 cm)16 in × 16 in
(40.6 cm × 40.6 cm)32 in × 32 in
(81 cm × 81 cm)Unlined Unlined Unlined Lined Unlined Lined
63 0.016 0.011 0.012 0.014 0.012 0.021125 0.010 0.006 0.009 0.038 0.007 0.011250 0.007 0.004 0.006 0.036 0.004 0.030500 0.016 0.003 0.010 0.026 0.003 0.026
1000 0.008 0.003 0.007 0.022 0.003 0.0222000 0.004 0.003 0.004 0.012 0.002 0.0144000 0.002 0.004 0.004 0.007 0.002 0.0098000 0.001 0.002 0.002 0.005 0.002 0.006
Vibro-Acoustics Consortium
March 18, 2021
43
Rectangular Duct Breakout TL Curve
From Vér, I.L. 1983
Vibro-Acoustics Consortium
March 18, 2021
Breakout TL – Rectangular Duct
44
16 in x 48 in (0.4 m x 1.2 m) Rectangular Duct; 10 ft (3.1 m) Length2 in (5 cm) Fiber Lining
10
20
30
40
50
60
70
50 100 200 400 800 1600 3200 6400
Brea
kout
Tra
nsm
issi
on L
oss
(dB)
Frequency (Hz)
FEM (Unlined) FEM (Lined)ASHRAE Handbook (Unlined) Cummings (1983)
Vibro-Acoustics Consortium
March 18, 2021
45
Circular Duct Breakout TL Curve
From Vér, I.L. 1983
Vibro-Acoustics Consortium
March 18, 2021
46
Breakout TL – Circular Duct24 in (0.61 m) Diameter Duct; 20 ft (6.1 m) Length2 in (5 cm) Fiber Lining
20
30
40
50
60
70
80
90
100
50 100 200 400 800 1600 3200 6400
Brea
kout
Tra
nsm
issi
on L
oss
(dB)
Frequency (Hz)
FEM (Unlined) FEM (Lined)ASHRAE Handbook (Unlined) ASHRAE Handbook (Lined)Cummings (1983)
Vibro-Acoustics Consortium
March 18, 2021
47
Importance of Breakout TL
0
10
20
30
40
50
60
50 100 200 400 800 1600 3200 6400
Atte
nuat
ion
(dB)
Frequency (Hz)
Insertion Loss Breakout Transmission Loss (Unlined)Breakout Transmission Loss (Lined)
Vibro-Acoustics Consortium
March 18, 2021
48
Summary
• FEA approach qualified for predicting insertion loss of unlined and lined ducts, elbows, and plenums.
• FEA approach qualified for predicting breakout transmission loss.
• Approach can be used for design purposes and to fill in the gaps in the ASHRAE Handbook.
Vibro-Acoustics Consortium
March 18, 2021
49
References• K. Ruan and D. W. Herrin, “A Simulation Approach to Determine the
Insertion and Transmission Losses of Unlined and Lined Ducts (RP-1529),” ASHRAE Transactions, Vol. 122, Part 1 (2016).
• K. Ruan and D. W. Herrin, “Using Simulation to Determine Elbow and Side Branch Attenuation and Duct Breakout Transmission Loss (RP-1529),” ASHRAE Transactions, Vol. 122, Part 1 (2016).
• D. W. Herrin and K. Ruan, “A Review of Prior ASHRAE Research Efforts to Characterize Noise Propagation in Ducts,” ASHRAE Transactions, Vol. 125, Part 1 (2019).