Software SILDIS® (in Excel format) allows the computation of acoustic & aerodynamic performance of a rectangular splitter baffle silencer, as illustrated with a case study relating to an air-conditionning network (example 1.4.1 of User's Manual). It comes to a device limiting the propagation of sound through a duct or an opening by minimizing resistance to the flow of the transported fluid, using a sound-absorbing lining:
- being a multi-layered acoustic structure combining porous materials (e.g. wool, foam), cloths, and perforated protections
- consisting of parallelepiped-shaped elements, parallel to each other, with one of their transverse dimensions equal to a dimension of the cross-section of the silencer's casing
Wording of the problem (envisaged application relating to the computation of a rectangular baffle silencer with software SILDIS®)
It is wished to compute the acoustic and aerodynamic performance of a dissipative silencer with a rectangular cross section (width B=1200mm [1], height H=2000mm [2], length L=1500mm [3]), having rectangular edged [4] splitters with a core layer thickness 2d=200mm [5] with a open area ratio of 50% [6] made of an homogeneous in directions parallel to and perpendicular to its surface bulk absorber having the reference BYOb (airflow resistivity 12.5 kNsm-4) [7] in the database for porous media of SILDIS® modelled as a rock wool with model M76 [8] with a cloth [9] of thickness d’1=5/100 mm [10] having the reference BYO (airflow resistance 30 Nsm-3, mass density 90 g/m2)[11] in the series cloths database of SILDIS® without perforated protection [12]. It is foreseen to use the silencer with an air flow rate of 24.1 kg/s [13] at 20 °C [14] at a pressure of 101325 Pa [15]. It is decided to take into account a limitation of the propagation loss for L>1m with model FRO1 [16] and to take into account the reflection loss with model MUL [17]. The reference spectrum is supposed of the type “pink noise” with a sound power level of 130 dB/oct [18]. It is chosen to predict the total pressure loss with the model referred to as FRO [19]. It is chosen to predict the self noise of the silencer in the way described with the general model 2081e* [20], with the model of thermodynamic correction ZER [21], with the model of spectral correction 2081e [22]. Language to be used is English [23].
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Rectangular cross section of a splitter baffle silencer (h=extreme airway, 2h = central airway, 2d=thickness of sound-absorbing splitter baffles, B=duct cross-section width, H=duct cross-section height) |
Input data (to be entered in Excel worksheets of SILDIS® software Module 1)
The input data required for the computation are listed hereafter in reference with the above data (see figures in brackets in the previous §, used as placemarks for explaining the selection below). The input cells are referred to thanks to their Excel’s coordinates (column / line) in the following part extracted from user’s manual.
Worksheet [in COALA] COmputation of Acoustic LAyers
| Item | Cell for input | Foreseen action | Input | Placemark / comment |
| Language | AF2 | For English select E, for French select F | E | [23] |
| Temperature (°C) | D5 | Enter a real number | 20 | [14] |
| Pressure (Pa) | D6 | Enter a real positive number | 101325 | [15] |
| Reference (porous medium) | J21 | Select a reference (material in a list) | BYOb | [7] |
| Resistivity (Nsm-4) | M22 | Enter a real positive number | 12500 | [7] |
| General model (porous medium) | J27 | Select a model (in a list) | M76 | [8] |
| Thickness (porous medium) (m) | J56 | Enter a real positive number | 9.995E-2 | [5]&[9] |
| Incorporation of the series perforated protection (0/1) | J125 | For NO enter 0, for YES enter 1 | 0 | [12] |
| Reference (series cloth) | AB21 | Select a material (in a list) | BYO | [11] |
| Airflow resistance (Nsm-3) | AD22 | Entrer a positive real number | 30 | [11] |
| Mass density (kg/m2) | AD23 | Enter a positive real number | 90 | [11] |
| Incorporation of the series cloth (0/1) | AB26 | For NO input 0, for YES input 1 | 1 | [9] |
| Thickness (series cloth) (m) | AB27 | Enter a real positive number | 0.00005 | [10] |
| Lw0 (dB ref. 1 pW) | C163 to L163 | Enter a real positive number for 1/1 octave band sound power level | 130 | [18] |
Worksheet [in COSIL] COmputation of SILencers
| Item | Cell for input | Foreseen action | put | Placemark / comment |
| h/(d+h) | N33 | Input a positive real number <1 | 0.5 | [6] |
| Half airway (m) | D34 | Input a positive real number | =N34 i.e. 0.1 | [6] |
| Mass flow rate (kg/s) | D66 | Input a real number | 24.1 | [13] |
| Width B (m) | D72 | Input a positive real number | 1.2 | [1] |
| Height H (m) | D73 | Input a positive real number | 2 | [2] |
| Length L (m) | D74 | Input a positive real number | 1.5 | [3] |
| Model of by-pass correction for L>1m | F83 | Select a model (in a list) | FRO1 | [16] |
| Model of reflection loss | G86 | Select a model (in a list) | MUL | [17] |
| Aerodynamics upstream | D95 | Select a model (in a list) | R | [4] |
| Aerodynamics downstream | D96 | Select a model (in a list) | R | [4] |
| Friction (lineic pressure drop) model | D98 | Select a model (in a list) | FRO | [19] |
| Model of total pressure loss | T104 | Select a model (in a list) | FRO | [19] |
| General model for the flow acoustic power | T106 | Select a model (in a list) | 2081e* | [20] |
| Model of thermodynamic correction | T107 | Select a model (in a list) | ZER | [21] |
| Model of spectral correction | T108 | Select a model (in a list) | 2081e | [22] |
Outcome: main results of simulation with Module 1 of software SILDIS®
Sound power level with silencer 125.1 dB(A), after consideration of regenerated noise which limits sound attenuation
Silencer insertion loss 12.0 dB(A)
Silencer total pressure loss 120 Pa
Outcome: screenshots of worksheet [in-out CODIS1] (COmputation of DISsipative silencers) of Module 1 of software SILDIS®
Acoustics
In what follows, performance indicators are linked by the formulas Di’ = Da.L + Dc + Dr ; Lw1 = 10 * log [10^ (0.1 * (Lw0 – Di’)) + 10^ (0.1 * Lw)] ; Di = Lw1 - Lw0
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Result of the simulation of the acoustic performance of a rectangular baffle silencer with software SILDIS® |
Aerodynamics/aeraulics
In what follows, performance indicators are linked by the formulas Δpt = ζf * 0.5 * ϱ * (Vf) 2 = ζp * 0.5 * ϱ * (Vp) 2 with ϱ = density (kg/m3), Vf = front speed (m/s), Vp = passage speed (m/s)
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Result of the simulation of the aeraulic/aerodynamic performance of a rectangular baffle silencer with software SILDIS® |
Remarks regarding performance of dissipative baffle splitter silencer simulated with Module 1 of software SILDIS®
The input and output data (simulation results) above are those of the version of Module 1 of the SILDIS® software normally sold; some cells (e.g. model selections for different calculation steps) are pre-filled by default with robust selections (which can be modified by the user) for the considered silencer mounting. In order to limit questions from possibly inexperienced users in relation to the input data to be considered (explanations being provided in user's manual), it is possible to restrict the functionalities of the software package by fixing (instead of allowing them to vary) some input data/some models for different steps of the calculations (depending on applications), so as to further simplify the use.
The Module 1 of software SILDIS® allows dissipative baffle splitter silencer simuled performance results:
- not necessarily using english language: french is possible too (also for entering input data)
- with possibly lower total pressure loss when round (semi circular) geometry/rectangular geometry without sharp angles are considered for upstream and/or downstream extremity of splitter baffles or when a profiled geometry is considered for downstream extremity of splitter baffles
- possibly in line with some engineers associations computation uses (when appropriate models are selected in software drop-down menus)
- possibly comparable with some laboratory measurement standards e.g. NF EN ISO 7235 Acoustics - Laboratory measurement procedures for ducted silencers and air terminal units- Insertion loss, flow noise and total pressure loss (when appropriate models are selected in software drop-down menus)
- which can be obtained in 9 minutes only, including time necessary for data entry (cf. tutorial of which link is provided in present post bottom)
The Module 1 of software SILDIS® also allows silencer performance prediction for more complex case studies, as encountered for some industrial applications:
- with more severe service conditions e.g. increased temperature, increased pressure, increase fluid speed in airways, fluid being not dry air
- with sound absorber being not only rockwool (e.g. glass wool, polyester wool, foam of various kinds), possibly with more sophisticated modeling of acoustical behaviour beyond the consideration of sole flow resistivity e.g. with the consideration of porosity, tortuosity, thermal & viscous charactersitic lengths (with some libraries available in some versions of the software)
- with more complex acoustic structure for the sound absorbing filling e.g. with perforated sheets (eventually accounting effect of grazing flow in case of high value of Mach number, and effect of high sound levels which are very important in the case of Micro-Perforated Panels - MPP - i.e. with sub-millimeter perforations), with Helmholtz resonators (i.e. with cavities), with superficial or internal flexible membranes (for resonance effect), with laminated filling (combining several porous media layers in each spliter baffle),with unsymetrical sound absorbing lining (splitter baffles constitution being not unique)
- with different assumptions for sound propagation in porous media e.g. absorber locally reacting or anisotropic (besides homogen absorber considered in general)
- with other geometries e.g. with round cross section (with or without central pod/concentric annular sound absorbing linings), with pine-tree shaped splitter baffles
With Module 1 of software SILDIS®, solving the equation of sound propagation both in silencers airways and in sound absorbing lining/splitter baffles by means of an analytic method is the basis for the simulation of the propagation loss - fundamental component of acoustical performance -, and also of reflection loss with some of the models implemented ; a consequence is that none of the limitations often associated to other approaches are detrimental here. Regressions based on a wide range of measurements for reflection loss with some of the models implemented, and for by-pass correction allows a very good agreement for comparisons with laboratory testing results (the later also basing - via parametric regressions - the prediction of total pressure loss).
Returning to rectangular silencers with splitter baffles, as modeled in this calculation example, the applications are numerous:
- in the building sector, e.g. air conditioning systems, particularly for sections installed close to Air Handling Units (AHUs), for ventilating technical rooms (engine rooms, generator sets, etc.), and for noisy equipment such as air-cooled condensers
- in industry (both to comply with regulations relating to workplaces and to those relating to the prevention of environmental noise pollution), e.g. ventilation systems (for buildings or machine sound enclosures), dust collection and fan intake/exhaust systems, turbomachinery air intake/exhaust systems, and test benches
Thus, Module 1 of the SILDIS® software allows, with a single Excel workbook and a single data entry, the calculation - using various models - of the aerodynamic and acoustic performance of silencers. The latter is not limited to insertion loss (transmission loss with infinite input impedance and anechoic termination), but includes (which is not common for sizing silencers with rectangular baffle sections) the calculation of reflection loss - related to the change in cross-section at inlet and outlet - and a bypass correction (accounting the fact that the performance of a 3-meter-long silencer is not three times that of a 1-meter-long silencer), in addition to calculating self-noise (due to airflow).
In fact, the combination of such features makes the SILDIS® software package - at the very least - a rarity, especially when the simulation capabilities it offers are accompanied by numerous other strengths:
- Module 1 of the SILDIS® software eliminates the often tedious (complex, delicate, time-consuming, and costly) tasks associated with other calculation methods, thus offering several advantages (related to the fact that its use consists exclusively of filling in Excel spreadsheet cells with numerical values and/or selecting models from drop-down menus):
- no specific prerequisites for the user
- no need for extensive training, as getting started is easy after a very quick initial support session
- no need to import or create the silencer geometry, as it is simply configured by selecting dimensions
- no need for meshing
- the calculation time is not counted in hours, or even minutes
- for some versions of the software, the choice of materials can be made (via drop down menu in software) among library embedded in software:
- for porous media (rock wool, glass wool, basalt wool, polyester wool and ceramic fiber of different densities for wich flow resistivity, porosity, tortuosity, viscous & thermal characteristic lengths are recorded based on laboratory measurements, also with a reference BYO - Bring Your Own - allowing alternatively free enter by user of relevant parameters)
- for series cloths (glass cloth, fabric for wich airflow resistance and mass density are recorded based on laboratory measurements also with a reference BYO - Bring Your Own - allowing alternatively free enter by user of relevant parameters)
- for perforated protections (round perforation with various diameters & open area ratios are recorded, also with a reference BYO - Bring Your Own - allowing alternatively free enter by user of relevant parameters)
- the basic investment is limited to a single (multi-user) license with initial onboarding support for getting started (advanced support is available as an option), at no additional cost for subsequent years
- the license cost is such that even occasional use is sufficient to recoup, all the more so when it comes to a software package providing - notably for parametric studies - silencer performance evaluations that would be more expensive to obtain otherwise (assuming they are then as complete and reliable ?)
Furthermore, the software package has nothing to do with a tool that returns, as a response to a selection query, the performance stored in a database - as some silencer manufacturers have - (or an interpolation of such data), based on measurements (sometimes carried out in time immemorial with poorly documented packing materials or materials that have undergone modifications in terms of the properties induced by their manufacturing methods changes, not to mention the fact that no difference is made for the presence or absence of fabric or perforated sheet) for a limited number of geometric configurations and which are only valid for air under laboratory conditions (with a flow with such a low speed that its direction - with respect to that of the propagation of sound waves - is considered negligible).
The Module 1 of software SILDIS® (based on Excel) is the outcome of decades of development and validation (in conditions meeting requirement of ISO 9001 relating to quality management systems) in the domain of acoustics (with a specialization in sound propagation and transmission in ducts and multilayer structures) and aeraulics by a human ressource able to combine theoretical considerations with field technical feedback (laboratories, measurements sites). It is a polyvalent, user friendly and reliable tool for sizing dissipative/resonant silencer in all contexts: from easiests cases (ventilation/air handling systems with ambiant thermodynamic conditions) up to most demanding applications in industry e.g. in energy production sector or for test benches (customizations are possible for a tailor-made tool, even for non specialized users).
Programming, marketing, training & hotline are available from one company: Isolation Technologie Services aka ITS. Being available with a near-perpetual licence (100 years), Module 1 of software SILDIS® is a must-have for anyone involved in acoustical insulation/noise control, for engineering studies as well as for Research & Development (e.g. silencer manufacturers or integrators, engineering companies, acoustic consultants & architects offices).