PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Comparison between prediction and measurement of soundattenuation associated to ventilation network elements
Catherine GUIGOU-CARTER1; François BESSAC2; Simon BAILHACHE3
1,3
2

CSTB, France

CETIAT, France

ABSTRACTThe AcouReVe Project (2015-2018) aimed to improve the knowledge and the quality of acoustic calculationin ventilation ductworks. Such calculations are based on simplified models and the main issue is the input
data. For each component of the ductwork, acoustic insertion loss and/or sound generation due to air velocity
has to be known. The present work concentrates on sound attenuation across different ventilation network
elements, such as bends, branches and manifold. A 2D prediction method was developed; it involves a multimodal model for elements associated to guided waves propagation and a finite elements model for complex
elements. Measurements have been performed at CETIAT laboratory on different elements; the experimental
setup is described along with the different evaluated configurations. The comparison between the measured
results, the predicted results as well as the results from simple standard formulas applicable for the consideredelements is presented and discussed in detail.
Keywords: Ductwork, Noise, Ventilation

1. INTRODUCTION
Indoor noise remains a key issue for most building occupants. Ventilation is one of the noise
sources in buildings and efforts are made by most manufacturers to design silent solutions, b oth forcomponents and ductwork. Acoustical consultants implement calculations to predict the sound levels
in rooms taking into account the ventilation system and ductwork but they often face several issues:- Lack of information about the acoustical characteristics of ductwork components, both for noise
attenuation and regeneration- Lack of confidence in the calculation process: it usually uses a simplified approach in which each
ductwork component is considered independently from the others without inter action,- Lack of confidence in the literature results: simplified tables or empirical relations are used but
their scope and limits of application are not well known.
The goal of the AcouReVe research project (2015-2018) was to make the ductwork noise
calculations more reliable by providing answers to many of these issues. Such calculation needs tosplit ductwork in several parts (elements) and associates to each of these elements the two following
acoustic characteristics: sound generation by the air velocity and sound attenuation by the element.
The calculation is then rather simple to implement if it uses an energetic approach with uncoupled
elements but the key factor is the acoustic input data for each element, which are often missing or
uncertain (low reliability).
Part of the work then focused on characterizing ductwork components such as junctions, straightducts, bends, dampers and manifolds for balanced ventilation systems. Other ductwork components
such as air inlet or air outlet transfer devices are expected to be accurately characterized bymanufacturers as they are located at the end of the ductwork, visible, and with specific design which
can greatly influence the noise behavior. For this reason, they were not targeted in the AcouReVe
project. Acoustic characteristics of silencers are usually known since their role i s to reduce the noiselevel (even if their noise generation is sometimes forgotten), so they were not considered in the project.
Laboratory tests allowed to check how the assumption of independent components used in the1catherine.guigou@cstb.fr
francois.bessac@cetiat.fr
3simon.bailhache@cstb.fr
2

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classical noise calculation methods leads to differences between calculation and measurements.
The project also investigated the way to implement such calculation in aerodynamics software,checking which data are necessary, and if their approach is compatible with that of acoustic calculation.
In addition, the implementation of a database was started and made available to users, giving a place
to manufacturers to share these specific data with users or software developers.
This paper focuses on sound attenuation in junctions, along straight ducts, in 90° bends, in dampers
and rectangular manifolds.

2. JUNCTIONSThe junctions are used to split the air flow from one duct to two (or more) other ducts, i.e., todistribute the air flow to several sub-branches or terminal devices. Several tests have been performedon 3 main configurations, described in Figure 1. The measurements are supplemented by a noise
calculation with a 2D approach, as well as with the common simple expression prediction model.
The results are presented as in terms of sound level variation, i.e. the difference between thedownstream and upstream sound levels, so that -3 dB means that the level in the considereddownstream branch is 3 dB lower than in the incoming upstream branch.
(a)

(b)

(c)

Figure 1 – Evaluated junctions; (a) T90-junction, (b) T45-junction and (c) Y-junction.
2.1 Simple prediction modelThis simple geometrical configuration has been widely described in the literature (e.g. Ashrae (1))
and is easy to implement in calculations. The calculation of the sound variation ΔL w Bi uses two
formulas, depending on cut-off frequency f0 (i.e. the frequency below which only plane waves travelin the duct) of the upstream branchσ ೄಳ൏Ͳ ȟ‫ܮ‬ௐ஻೔ ൌ ͳͲ݈‫ ݃݋‬቎ͳ െ ൭

ೄೌ
σ ೄಳ
ೄೌ

೔ ିଵ
೔ ାଵ

ଶ
ௌಳ

൱ ቏ ൅ ͳͲ݈‫ ݃݋‬൬σ ೔ ൰ௌಳ೔ௌಳ൐Ͳ ȟ‫ܮ‬ௐ஻೔ ൌ ͳͲ݈‫ ݃݋‬൬σ ೔ ൰
ௌಳ೔

(1)

(2)

where Sa is the section of upstream branch and S Bi, the ith section of downstream branch. The cut-off
frequency f 0 for a circular cross-section duct is given by f 0=(0.59 c0 )/D, with D the diameter in m andc 0 the sound speed in air expressed in m/s ; and for a rectangular section duct f0=(0.5 c 0)/H, with H
the largest dimension in m.
For frequencies below the cut-off frequency, the first term of Equation (1) is associated to areflection effect of the plane waves in the low frequency range. For frequencies above the cut -offfrequency, Equation (2) only depends on the section of the considered downstream branch and thesections sum of all the downstream branches. It is obvious that since Equations (1) and (2) do notinclude geometrical parameters, they are most relevant for symmetrical Y-junctions or T-junctions.
But for the T-junction with a main duct and a lateral branch, the sound transmission could be
distributed unsymmetrically, and Equations (1) and (2) might not be appropriate.
2.2 2D prediction model
A 2D acoustic model was also developed. Three types of geometries were distinguishe d: straightduct elements, bends and elements with complex arbitrary geometry (such as junctions). A distinct
modelling strategy was implemented for each of these three types of elements . For straight ductelements as well as bends, a multi-modal approach of guided waves propagation is implemented; forthe complex element a finite elements model approach is preferred. The model is described in detailin (2) and is therefore not discussed further here.
Two types of attenuation are evaluated based on the developed model: either based on the acoustic

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power evaluated over a duct section (denoted Prediction -Lw ), or based on pressure level obtained at
2 positions on the duct section at half the radius (denoted Prediction -Lp ). The difference between
these two evaluations is that for the method denoted Prediction -Lp the pressure level L p in the
upstream duct branch (sound emission) combines the incident and reflected waves due to thediscontinuity inducing some interference effects; while for the method denoted Prediction -Lw thepower level Lw in the upstream duct branch is obtained from the power associated to the incident
waves only.
2.3 Results
All ducts are 250 mm in diameter, of the round spiral steel type, as widely used in ventilation
ductworks. Sound power level is measured inside ducts (ISO 5136 (3)) at a distance of 1 m upstreamof the junction and 2.5 m after the junction in each downstream branch. The sound level is generated
by a upstream loudspeaker emitting a broadband noise, in an axial or lateral position. Anechoic
terminations are placed at the end of each branch.
Figure 2 shows for the T90-jounction the theoretical sound level variation according to Equations(1-2), the measured sound power level variation Lw , and the 2D model predicted sound level variation(considering sound pressure levels L p and sound power levels L w ). The accident observed in themeasurements and the prediction based on pressure level in the one-third octave band 125 Hz and 250
Hz is associated to the fact that the microphones in the upstream branch (incident sound) are locatedin a section corresponding to a sound pressure node (interference between the incident wave andreflected wave due to the junction discontinuity). Figure 2(a) shows that for frequencies below 500
Hz, the plane wave theory (see Equation (1)) is quite well respected, with a 3.5 dB attenuation. Above
this frequency (approx. two one-third octave bands below the cut-off frequency (800 Hz), the theory
is far away from the measurement and 2D model prediction results, showing a large attenuation around
8-10 dB. By contrast, Figure 2(b) shows a smaller attenuation, around 1 dB, instead the 3 dB givenby theory. It seems obvious that it is easier for noise to continue straight ahead in the duct than to takea way with right angle.
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(b)

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'Lw (dB)(a)Frequency (Hz)Figure 2 – Sound level variation for T90-junction; (a) side branch and (b) direct branch – axial loudspeaker.
Similar results have been obtained for a T45-junction, as presented in Figure 3. The sameconclusions as for T90-junction apply for low frequencies. For higher frequencies, the same
dissymmetric results occur, with only a little lower amplitude with the T45 junction compared to T90
junction. It has to be noted that the dissymmetrical behavior is reduced when the exc iting loudspeakeris positioned on the lateral side of the upstream duct branch, instead of at the axial end. This position
leads to higher order modes excitation and may be more representative of sound fields in realductworks.
The Y-junction shows that the symmetrical effect is achieved. The difference with the theory
decreases especially around the cut-off frequency. On the higher range (1600-3150 Hz), higher
attenuation is seen for experimental results, probably due to the losses in the ducts. The pred iction
obtained by the 2D model fits with the literature theory.
The literature theory does not include a parameter to adjust the attenuation in the downstream
branches according to the geometry of the junction. A proposal is to add to Equation (2) a new term
taking into account the angle between the upstream and the downstream branches. The following terms
are proposed for a junction with a downstream branch (branch 1) in line with upstream branch and asecond branch (branch 2) forming an angle T B2 with the upstream branch ( S/2dTB2 )

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ௌಳ
݂‫ͳ݄ܿ݊ܽݎܾݐ݄݃݅ܽݎݐݏݎ݋‬ͳȟ‫ܮ‬௪஻೔ ൌ ͳͲ݈‫ ݃݋‬൬σ ೔ ൰ ൅ ݂ሺߠ஻௜ ሻ݂ሺߠ஻௜ ሻ ൜ௌ ಳ೔െʹǤͷ  ߠ஻ଶ ݂‫ʹ݄ܿ݊ܽݎܾ݁݀݅ݏݎ݋‬
4

(b) 4

2

2
0

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Measurement-4''Lw (dB)-2

Prediction - Lw

-6

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Fréquence (Hz)Figure 3 – Sound level variation for T45 junction; (a) side branch and (b) direct branch – axial loudspeaker.

3. STRAIGHT CIRCULAR DUCTThe total length of a ductwork can be more than several tens or hundred meters. The sound travels
on long distances and can be damped by air relaxation effects and the acoustic losses on th e duct walls(mainly noise transmitted to the duct and to its surrounding). This part only deals with circular ducts,
which are known to produce few losses.
A typical data from literature for ducts around Ø 200 mm is a loss of 0.1 dB/m for low frequencies
and 0.3 dB/m for high frequencies. These values are small but can lead to significant attenuations forlong ducts. Practice shows that design contractors often neglect this phenomenon in calculation,
probably for conservative reasons.
Several tests were performed to assess these losses in the well-known galvanized steel spiral ductused in ventilation. The test set-up consists in a loudspeaker, an upstream duct with in-duct soundpower measurement (used as reference), the ducts under test (length 6 m, resul ting from the assemblyof 2 ducts of 3 m), and downstream an anechoic termination. A microphone is moved into the duct tomeasure the sound pressure levels every 20 cm, located at the ¼ of the diameter.
Tests were carried out on 4 different diameters: Ø 80/100/125/160 mm. For the 160 mm diameter,
one additional case consisted in a 5 m length duct made of 5 sections of 1 m.
Figure 4 shows the attenuations in dB/m for the 5 tested configurations. All of them present a low
attenuation for frequencies below 1600 Hz. The common literature value of ~ 0.1 dB/m seems to berealistic. For frequencies higher than 1600 Hz, the 4 diameters ducts made of 2 x 3 m give comparable
results, between 0.5 and 1.5 dB/m, that can be rounded to 1 dB/m. The case of the Ø 160 mm duct
made of 5 x 1 m leads to a different result, with higher attenuation, locally around 3 dB/m at 2000 and2500 Hz and between 1 and 2 dB/m above; the regeneration of 1.3 dB/m at 1600 Hz is rather unrealistic
and unexplained. This difference is obtained although the numerous duct connections were tightenedwith care, using adhesive tape around the male connectors.
Littérature5

Ø80 mm (2 x 3 m)

Attenuation (dB/m)

4Ø100 mm (2 x 3 m)

3

Ø125 mm (2 x 3 m)

2

Ø160 mm (2 x 3 m)
Ø160 mm (5 x 1m)

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Figure 4 – Measured attenuation in dB/m in a straight duct.

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The frequency from which the acoustic behavior changes should be related to the cut -off frequency
when the travelling waves are no longer plane waves (for Ø80 mm at 2530 Hz, Ø100 mm at 2030 Hz,
Ø125 mm at 1624 Hz and Ø160 mm at 1270 Hz), but Figure 5 does not show such differences, as thefrequency with an accident appears to be always around 1600 Hz. This remains unexplained ; the useof a loudspeaker in axial position does not favor the generation of higher order mode in the duct.

4. BENDS
Bends, as any obstacle in a duct with air velocity, can produce noise. The leve l of this noise greatly
depends on the air velocity itself and as a first approximation on the pressure loss coefficient (i.e. theshape of the obstacle). Most of bends used in ventilation have a soft radius of curvature, approx imatelyaround 1 diameter, even if they can be built differently, such as pressed steel (round bend) or assemblyof several straight parts. Several types of bends (see Figure 5) have been tested to determine theirsound power level and sound attenuation.
The generated noise level for such rounded bends was found very similar to a straight ductreference configuration, meaning that 1D curvature radius bends can be considered not generating
noise in ventilation ductworks. This is not the case for the sharp bend which is rather unrealist ic andnot used in practice, the generated noise was about 10 dB(A) higher than the standard rounded bend(see (4) for results).
(b)

(a)

(c)

Figure 5 – Tested bended elements: (a) rounded bend, (b) bend by straight parts, (c) sharp bend.
Since the bend represents an impedance change in the duct for the traveling waves , it generates
sound reflections to upstream which reduces the sound energy transmitted to downstream, resulting
in some attenuation. Measured insertion loss is presented for 2 bends of Ø 160 mm, round bend andsharp bend (without airflow). Test set-up consists from upstream to downstream in a loudspeaker(axial or lateral with reference microphone), a straight duct, the tested bend, a straight duct with asection to measure the average sound pressure level and an anechoic termination. The insertion loss
is then obtained as the difference of downstream sound level measured without bend (replaced bystraight element) and then with the bend. Figure 6 shows the obtained insertion loss for the 2 bends,
compared to results based on (5). Round bends have a lower pressure loss; their noise attenuation islow, equal to zero at low frequencies, and around 2 dB at higher frequencies. The measured attenuation
is between the predicted results (close to zero) and the results based on (5). On the contrary, the sharp
bend presents a greater pressure loss, thus a higher sound attenuation. Predicted results are in relative
accordance to those based on (5), but the measured maximum attenuation appears at a d ifferent onethird octave band. Note that the bend by straight parts provides similar results to the rounded bend.
10
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VDI - Rounded Bend
VDI - Abrupt Bend
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Frequency (Hz)Figure 6 – Sound attenuation of sharp and rounded bends.

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It must be noted that when the loudspeaker is located on a lateral position of the upstream duct,
the attenuation is a little bit lower for the sharp bend.

5. DAMPERS
Dampers are used to adjust air flow rates in the various branches of a ductwork. Dampers createpressure loss, which in turns can create noise. Five types of dampers of Ø 160 mm have been
investigated, as displayed in Figure 7: iris, blade (full or perforated), blast gate, flexible opening foam
(made with agglomerated PU foam) with removable pieces to adapt pressure drop. Sound generation
associated to these dampers was measured but is not presented in this paper. It was obtained that theoverall sound power level dB(A) could be fairly well evaluated using expression given in VDI 2081
(5).
(e)
(d)
(a)
(b)
(c)

Figure 7 – Tested dampers: (a) iris damper, (b) damper with full blade or (c) perforated blade,(d) blast gate, (e) flexible opening foam.
As for bends, the damper is an obstacle in the duct that changes the impedance, creating reflection
and reducing transmission. For the downstream point of view, the damper creates a sound attenuation(insertion loss). The tests use an upstream loudspeaker to generate broadband noise. The attenuation
is calculated by comparing sound pressure levels in the reverberant room with and without the damper.
Figure 8 compares several types of damper in a configuration close to a pressure loss coefficient[ = 10. Metal dampers have no effect between one-third octave bands 125 and 630 Hz, before to reach
2 dB (full blade damper) or about 4 dB (iris damper). The perforated blade damper has no effect,
although it is in closed position (90°!). On the contrary, the foam damper, made of absorbing
agglomerated PU foam, is associated to sound attenuation, above 10 dB above 6.3 kHz.
20

Iris damper - P6 (ksi = 10)18

Full blade damper - 45° (ksi = 10)

16

Perforated blade damper - 90° (ksi = 6)

Insertion loss (dB)

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Foam damper - 4 holes (ksi=11)

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Figure 8 – Insertion loss of different dampers for [ # 10.

6. ACOUSTIC BEHAVIOR OF MANIFOLD
Manifolds are used in ventilation systems to distribute the air coming from the ventilation unit to
the downstream branches bringing air to the different rooms. Their size and shape vary, fromrectangular to circular, with spigots on lateral sides or on all sides as well. As proposed in the Ashrae
handbook (1), one could be tempted to consider them as a plenum, but their small volume makes thisnot suitable. One manifold has been tested and results compared with calculations (2D prediction
model). For practical reasons, the tested manifold as seen in Figure 9 is in wood with an inlet of Ø
250 mm and 6 outlets of Ø 160 mm. Internal size is L = 88 cm, w = 50 cm, d = 25 cm.

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The experimental set-up consists in a lateral loudspeaker on the inlet side, with a measurement ofsound power level at the inlet, and the measurement of sound power level at 4 of the 6 outlets. All
branches with sound level measurement are equipped with an anechoic termination. The test result isthe attenuation of the sound power level observed in one branch compared to the incomin g soundpower level.
2m
0,5 m
1m
1m
0.5 m
2m
(a)
(b)
G3 D3
G2 D2
G1 D1

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0.74 m3m

Figure 9 – (a) manifold used for tests; (b) set-up for the acoustic characterization of manifold.
The numerical simulation is based on a 2D-approach. Many configurations have been investigated,for 1 to 4 connected outlets in the experiments (for 1 to 6 in the predictions)Figure 10(a) shows the experimental and calculated transmission variations from inlet to outlet.
The prediction and measurement results are rather consistent, at least for the frequencies where
maxima and minima appear. These maxima and minima are related to the modal situation in the
manifold volume and the branches position. For example, as shown in Figure 10(b), it is observed thatat 125 Hz, the first outlet G1 (close to inlet) is positioned on a node of the manifold pressure field
distribution leading to a low transmission. On the contrary, the 3rd outlet G3 is positioned on an antinode of the manifold pressure field distribution, and the sound energy is then easily transferred frominlet to outlet. At high frequencies, prediction results are higher probably because no damping wasconsidered. The general level of attenuation appears to be between 2 and 20 dB for the measurements,
with high differences depending on frequency regions. The first outlet (G1) exhibits a high attenuation
at 125 Hz whereas the second G2 has a maximum attenuation at 200 Hz.
The analysis of all predictions (Figure 16) results for one outlet shows that all the possiblecombinations of all other outlets is less important than the position of the considered outlet. Increasing
the number of connected outlets reduces the amplitude, acting like a damping.
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50'Lw (dB)(a)Frequency (Hz)Figure 10 – (a) Comparison of measured and predicted sound level variation for only one connected outlet
and (b) Pressure distribution in the system with one connected outlet at 125 Hz.

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(b)

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Frequency (Hz)Figure 11 – Sound level variation for G1 outlet, for 1 to 4 outlets connected;(a) Measurement and (b) Prediction.

7. CONCLUSIONS
The acoustic calculation for ventilation ductworks is a challenge as it requires numerous input data,
e.g. attenuation and sound generation for all the components of the ductwork. The method of
calculation is based on a simplified energetic approach for which the crucial point is the quality of
input data. Some are usually well known such as for terminal diffusers, but some others are rarely
available, such as the noise generated and/or attenuated by junctions, straight ducts, bends, dampers
and manifolds. The AcouReVe project has experimentally investigated several components and thispaper has focused on sound attenuation associated to some of these elements . The main conclusions
are as follow:• The literature about junctions is correct for symmetrical configurations. For T90 or T45°
junction branches, the calculation requires a different treatment for each branch, the lateral one
receiving less sound energy than the straight one.
• For low frequencies, losses in straights duct are consistent with literature and are very low, but
the circular ventilation ducts provide higher losses than expected at high frequencies, with more than
1 dB/m for the considered galvanized spiral steel duct.
• Bends provide limited sound attenuation, as given in the literature. Furthermore, if the curvatureradius is at least 1D, there is generally no noise generation compared to a similar straight duct.
• Dampers have to be considered as an important source of noise rather than an attenuating
element. However, the tested foam damper was associated to a large insertion loss in the highfrequency range.
• Manifolds provide an attenuation at frequencies that are related to their geometry (pressure field
distribution in manifold). The number of connected outlets slightly influences the attenuation levels.

ACKNOWLEDGEMENTS
The authors acknowledge the French Environment & Energy Management Agency ADEME, theFrench ventilation systems manufacturers and CSTB for their financial support to the AcouReVe
project.

REFERENCES
1. ASHRAE Handbook – HVAC Applications – Chapter 48: Noise and Vibration Control. 2015.
2. Coguenanff C., Guigou-Carter C., Bailhache S., Bessac F. Modélisation du comportement acoustiqued’éléments de réseau de ventilation. Proc. CFA 2018; 23-27 April 2018; Le Havre, France 2018. p. 989993.
3. ISO 5136 – Acoustics – Determination of sound power radiated into a duct by fans and other air-moving
devices – In-duct method. 2003.
4. Bessac F., Guigou-Carter C., Bailhache S., Lefebvre C. Ductwork noise calculations: main outputs of
AcouReVe project. Proc. 39th AIVC 2018; 18-19 September 2018; Antibes Juan-les-Pins, France 2018.
5. VDI 2081 Part 1 – Air-conditioning - Noise generation and noise reduction. 2016.

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