Software SILDIS® (in Excel format) allows the calculation of acoustic & aeraulic performance of a circular/annular silencer silencer, whta can be illustrated by an example (referred to as 1.4.4 of User's Manual). Such a sound attenuator is an aeraulic circuit component which should not oppose too much to flow circulation, being based on the recourse to a sound-absorbing lining with the following characteristics (when modeled with SILDIS® software):
- acoustic structure is multi-layered with a porous core material (e.g. mineral or polyester wool), and possibly a surfacing (cloth or fabric) or perforated protective layer
- it consists in general of a peripheral sound absorbing lining (inner diameter being equal to upstream & downstream round duct diameter) and a central pod
Wording of the problem (envisaged application relating to the calculation of a circular/annular silencer with a central sound absorbing pod with software SILDIS®)
It is wished to calculate the acoustic and aerodynamic performance of a dissipative silencer with a circular cross section with a central pod (annular silencer), the area of the duct upstream and downtream (before and after the silencer) being not equal to the area of the overall section of the silencer (overall diameter D1A=1000mm [1’’’]) but being equal to the inner width, length L=1200mm [3]), having a (peripheral) lining core layer of thickness d=d1=200mm [5] made of an homogeneous in directions parallel to and perpendicular to its surface bulk absorber having the reference BYOb (airflow resistivity 16 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 4 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 FRO [16] and to take into account the reflection loss with model ZER [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 2081 [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].
With a central pod having a lining core layer of (radius) thickness d=d2=150mm [105] made of an homogeneous in directions parallel to and perpendicular to its surface bulk absorber having the reference BYOb (airflow resistivity 16 kNsm-4) [107] in the database for porous media of SILDIS® modelled as a rock wool with model M76 [108] with a cloth [109] of thickness d’1=5/100 mm [110] having the reference BYO (airflow resistance 30 Nsm-3, mass density 90 g/m2) [111] in the series cloths database of SILDIS® without perforated protection [112].
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Cross section of a circular/annular silencer - mounting C1A - (da=d1 = peripheral sound absorbing lining thickness, 2ha = inner diameter i.e. connection diameter, 2hi = 2di = 2d2 = pod diameter, D1A = overall diameter) |
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 | See 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) | I21;J21 | Select a reference (material a list) | BYOb;BYOb | [107];[7] |
| Resistivity (Nsm-4) | M22 | Enter a real positive number | 16000 | [107];[7] |
| General model (porous medium) | I27;J27 | Select a model (in a list) | M76 | [108];[8] |
| Thickness (porous medium) (m) | I56;J56 | Enter a real positive number | 0.14995;0.19995 | [105]&[110];[5]&[10] |
| Incorporation of the series perforated protection (0/1) | I125;J125 | For NO enter 0, for YES enter 1 | 0;0 | [112];[12] |
| Reference (series cloth) | AA21;AB21 | Select a material (in a list) | BYO;BYO | [111];[11] |
| Airflow resistance (Nsm-3) | AD22 | Entrer a positive real number | 30;30 | [111];[11] |
| Mass density (kg/m2) | AD23 | Enter a positive real number | 90;90 | [111];[11] |
| Incorporation of the series cloth (0/1) | AA26;AB26 | For NO input 0, for YES input 1 | 1;1 | [109];[9] |
| Thickness (series cloth) (m) | AA27;AB27 | Enter a real positive number | 0.00005 | [110];[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 | Input | See placemark / comment |
| e(m) | BS18 | Input a positive real number | 0 | Pre-filled (0 for mounting C1A) |
| hi local (m) | BV18 | Input a positive real number | =BU18 | Pre-filled (hi=di=d2) |
| Mounting C1A to get D1A (m) | BS33 | Input a positive real number<1 | 1.000 | [1'''] |
| Half airway (m) | D34 | Input a positive real number | =BS34 i.e. 0.26587 | [6] |
| Mass flow rate (kg/s) | D66 | Input a real number | 4 | [13] |
| Length L (m) | D74 | Input a positive real number | 1.2 | [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) | ZER | [17] |
| Model of inlet pressure loss | BX96 | Select a model (in a list) | 2081 | [19] |
| Model of lineic pressure loss | BX99 | Select a model (in a list) | 2081 | [19] |
| Model of outlet pressure loss | BX101 | Select a model (in a list) | 2081 | [19] |
| General model for the flow acoustic power | BX106 | Select a model (in a list) | 2081e* | [20] |
| Model of thermodynamic correction | BX107 | Select a model (in a list) | ZER | [21] |
| Model of spectral correction | BX108 | Select a model (in a list) | 2081e | [22] |
Outcome: main results of performance prediction with Module 1 of software SILDIS®
Sound power level with silencer 127.8 dB(A), after consideration of regenerated noise which limits sound attenuation
Silencer insertion loss 9.3 dB(A)
Silencer total pressure loss 102 Pa
Outcome: screenshots of worksheet [in-out CODIS4] (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 prediction of the acoustic performance of a circular/annular 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 prediction of the aeraulic/aerodynamic performance of a circular/annular silencer with software SILDIS® |
Remarks regarding performance of dissipative circular/annular silencer (with a central pod) simulated with Module 1 of software SILDIS®
The input data and the outcome displayed in the previous sections are those of the version of Module 1 of the SILDIS® software usually marketed; some cells (e.g. model selections for different computation stages) are pre-filled by default with robust selections (that the user can change) for the considered silencer mounting. In order to limit questions from users possibly not familiar with some of the input data to be entered (in relation to what user's manual might be helpful), it is possible to restrict the features of the software package by preventing the modification of some settings for different stages of the computations (to an extend varying on foreseen applications), in oder to make sizings with SILDIS® even easier and faster.
The Module 1 of software SILDIS® allows dissipative circular/annular silencer predicted performance:
- not exclusively using english language: french can be used too (also for entering input data)
- possibly in line with the methodology of some engineers associations (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 less than 10 minutes only, including time necessary for data entry in a way similar to the sizing of silencers with a rectangular cross-section, for which a demonstration video is available elsewhere[1]
The Module 1 of software SILDIS® also allows silencer modeling in cases being - sometimes: by far - more demanding than example mentioned above, as some industrial contexts may involve:
- with more complex 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 (in some versions of the software, libraries are available)
- 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 different sound absorbing materials for peripheral lining and central splitter (pod)
- with other geometries e.g. without central pod, without peripheral lining - sometimes making easier noise attenuation by means of sound absorbing cartridges craned in stacks (chimneys) ducts - , with rectangular cross section, with pine-tree shaped splitter baffles; regarding dissipative silencers with a circular cross-section, it is also possible to account geometries with sound absorbing linings in the form of concentric rings
In addition, setting to zero one or the other thickness/admittance of sound absorbers considered above for peripheral sound absorbing lining and pod for mounting C1A allows the computation of (respectively) a single noise reducing cartrige for stack or a simple coaxial (straight-trough) muffler. For such a silencer with a circular cross-section and without central splitter aka pod (i.e. if d2=0 with notation mentioned above), acoustic performance evaluation is possible either with Terminal 2 of the software - relating to mounting C0 - (cf. example 1.2.3 in user's manual) - or with Terminal 4, used for this calculation example, output data of the latter being based on the use of Bessel functions, e.g. for calculating the surface impedance of a curvilinear sound-absorbing lining, just like those of Terminal 3 covering exclusively the geometry of a straight through circular silencer (mounting C0).
With Module 1 of the SILDIS® software, solving the sound propagation equation in the silencer's air passages and in the sound-absorbing lining/pod by means of an analytic method forms the basis for simulating the propagation loss - a fundamental component of acoustic performance -; therefore, none of the limitations often associated with other approaches pose an obstacle. Laboratory tests - allowing parametric regressions - provide the basis for predicting the total pressure loss.
Returning to the C1A assembly, i.e, for a circular/annular silencer, as modeled in the context of this calculation example, the applications are numerous:
- in the building sector, e.g. air conditioning systems, where the ductwork often has a circular cross-section, at least for the downstream sections: such a noise reduction device is easy to install and provides - at a moderate manufacturing cost - a good compromise between aerodynamic performance - without which the efficiency of a Heating Ventilation Air Condtionning - HVAC - system would be negatively impacted - and acoustic performance - without which there is no acoustic comfort in buildings
- in industry (both with regard to objectives of decent working conditions and the absence of noise disturbance for residents):
- in air ducts or similar systems, e.g. exhaust silencers for internal combustion engines, small-capacity gas turbines, soundproofing of chimneys for various processes
- in pressurized fluid networks of all kinds
Thus, Module 1 of the SILDIS® software allows, with a single Excel workbook and a single data entry, the calculation - combining various models - of the aeraulic and acoustic performance of silencers. Besides insertion loss (i.e. transmission loss with infinite input impedance and anechoic termination), the latter also includes a bypass correction (reflecting the fact that the performance of a 4-meter-long silencer is not four times that of a 1-meter-long silencer), in addition to calculating airflow noise (aka regenerated noise).
In fact, the combination of such features makes the SILDIS® software package - at the very least - a rare sizing tool, especially when the simulation capabilities it offers come along with numerous other advantages:
- Module 1 of the SILDIS® software avoids the often tedious (complex, delicate, time-consuming, and costly) tasks associated with other silencer performance prediction strategies, thus offering several stengths (due to the approach based on filling in Excel spreadsheet cells with numerical values and/or selecting models from drop-down menus):
- no specific prerequisites for the user
- light training, as getting started is easy after a very quick initial support session
- no necessity to import or create the silencer geometry, due to programmed parametric configuration
- no necessity to mesh
- the calculation time is not counted in hours, or even minutes (being the same for small silencer or big silencer)
- 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; no additional cost for subsequent years to be foreseen, except advanced support if needed whatever the reason
- the license cost is such that even occasional use is sufficient to get shortly return on investment, all the more so when it comes to a software package providing - for example in case of performance of multiple runs - silencer performance predictions that would be more expensive to obtain otherwise (assuming they are then as complete and reliable ?)
Furthermore, can not be compared to SILDIS® tools 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, rarely distinguishing between presence or absence of cloth, fabric or perforated sheet) for a limited number of geometric configurations and which are only valid for air under laboratory conditions (air speed being in general so low that flow direction - with respect to that of the propagation of sound waves - is not accounted).
The Module 1 of software SILDIS® (based on Excel) has been made possible by 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 aerodynamics by a human ressource basing its approcah on the consideration of both theory and practice measurement results (acquired in laboratories, or in-situ). It is a polyvalent, user friendly and reliable tool for sizing dissipative/resonant silencer in all contexts: from easiests cases (ventilation/air conditioning circuits 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, sales, training & assistance 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 aiming to size silencing devices in the context of sound insulation/noise control projects, for engineering studies as well as for Research & Development (e.g. silencer manufacturers or integrators, engineering companies, acoustic consultants & architects offices).