Overview in May 2020
When it comes to acoustics, design tasks in the construction sector are often complex, wether it comes to Research and Development (R&D) works or to engineerings tasks.
Until recently, they were based (e.g. for what realtes to the evaluation of performance indicators in terms of sound transmission through walls, sound absorption, limitation of noise propagation in ducts) on technical data possibly coming from two operating modes (and not one more), each one being accompanied by means that are sometimes difficult to mobilize in proper time in the context of given projects, especially if these are of limited financial area or if they are with an overly restrictive implementation schedule:
They are able to provide many and varied performance indicators (in the field of applications mentioned above), (however: with some limitations relating to the tested sample, relating to the frequency spectrum with respect to which the global indices are calculated, which is not necessarily that of interest in the context of a given project, additional transposition calculations being then required e.g. with a spreadsheet).
The results of laboratory tests (basic practice) can be the subject of a more or less broad consensus, for those of tests - not all - carried out according to standardized measurement methods (provided that the reference normative documents are jointly considered by the different parties involved), and from organizations - not all - having the competence and impartiality necessary to give confidence in their services, and being regularly duly evaluated and accredited for this.
However, there are users of this operating mode to consider (knowingly ?) that, in the case of laboratory tests, their preparation is sometimes time-consuming, difficult and delicate, that they must be envisaged over a long term and are often costly, that the configurations tested are - therefore - limited (this is sometimes a headache), and that the results are not always easily transposable to filed realities on the ground (e.g. the standardized measurement the acoustic performance of the silencer is achieved in the laboratory at room temperature, which does not provide any information on the performance of a noise reduction device in a chimney with combustion gases at high temperature).
Mesh simulations (FEM Finite Element Methods, BEM Boundary Element Method, CFD Computational Fluid Dynamics)
They are able to provide many and varied performance indicators (in the field of applications mentioned above), (however: with some limitations depending on the tools used).
Mesh simulations constituted an important evolution of Computer Aided Design (CAD), being then based - including in the field of acoustics - on means (software, computing power and time) very important and which require, to use them, a hyper-specialized (rare?) manpower.
However, there are users of this operating mode to consider (knowingly ?) that it is not ideal to carry out the dimensioning of a non-standard silencer for a (simple) ventilation network or a chimney with - so that the calculations are carried out in an academic manner - a software suite also used for the design of aircrafts or rockets engines, submarine propulsion subsets: complexity of handling, difficulty of formatting input data (e.g. requiring the use of several subroutines and files, with 3D geometric modeling), computation times are commendable (several hours to several days even for models of reduced size, if evaluations are necessary at high frequency, which is often the case for building applications where the octave band of central frequency 4kHz or even 8kHz is regularly considered), even though the models used by such non-specialized tools for taking into account certain parameters e.g. those related to the acoustic behavior of porous materials are sometimes quite rudimentary (and even: dated) or even insufficient with respect to the expected accuracy of the calculations.
Fortunately, even more recent software developments have been made possible by the increase in the power of desktop computers which now allow, in sufficiently short time (a few seconds at most), calculations based on (analytical) simplified methods, e.g. using the evaluation of multiple integrals and the execution of routines intended for resolution, by iterative methods, and with sufficient accuracy, of transcendent equations bringing into play trigonometric functions with complex argument.
These calculations now constitute a third possible operating mode in terms of acoustical design in in the construction sector, being Computer Aided Design (CAD) with regard to the consideration of different phenomena e.g. sound transmission through walls, acoustic absorption, limitation of noise transmission in ducts.
Numerical Simulations by Simplified Methods (NSSM)
They are able to provide many and varied performance indicators (in the field of applications mentioned above), (without the known limitations vis-à-vis other modus operandi mentioned above)
Numerical Simulations by Simplified Methods (NSSM) require only readily available means of calculation (recent workstations with multi-core processor are ideal).
They do not require a hyper-specialized workforce, e.g. when based on Excel.
The use of a single file, the presence of drop-down menus, the provision of a library of materials make, in general, such tools particularly user-friendly for various practitioners in the construction sector e.g. project managers, architects, consultants , technicians and engineers from design offices (acousticians or not) and manufacturers' Research and Development (R&D) centers.
Numerical Simulations by Simplified Methods (NSSM) can be used to (better) prepare and complete laboratory tests and as an alternative to mesh simulations in many contexts where this is possible (and even: preferable).
- Sound transmission through walls (see NF EN ISO 140-3 Acoustics - Measurement of the sound insulation of buildings and of building elements - Part 3: laboratory measurement of airborne sound insulation of building elements)
Numerical Simulations by Simplified Methods (SNMS) allow the calculation of the sounds reduction index of partitions, possibly multilayered: by considering plates (e.g. concrete, plaster, wood, steel, aluminum, glass), possibly joined, and fibrous (mineral wools, e.g. glass, rock, basalt, silicate, polyester wool) or foams. The plates can be flat, orthotropic (i.e. with corrugations), perforated, with or without a bonded viscoelastic damping layer (e.g. septum, PVB): extensional or constrained (between 2 plates: sandwich panels)
Applications are thus possible to assess the airborne sound insulation performance:
- of building envelope elements: walls, slabs, roofs or interior carpentry elements (e.g. metal or wood) and plastering: partitions, ceilings, linings, soundproofing panels, noise barriers) and also components of doors and glazings
- the casing of silencers inserted in the ventilation networks or of chimneys
- Acoustic absorption (cf. NF EN ISO 354 Acoustics - Measurement of acoustic absorption in a reverberation room and also standard ISO 10534-1 Acoustics - Determination of the sound absorption coefficient and impedance in impedance tubes - Part 1: method using standing wave ratio)
Numerical Simulations by Simplified Methods (NSSM) allow the calculation of the acoustic absorption coefficient of linings, possibly multilayered, for the limitation of the reverberation of premises, by considering plates (e.g. plaster, wood, steel, aluminum) possibly perforated, fibrous (mineral wools e.g. glass, rock, basalt, silicates, polyester wool) or foams and surfacing (cloths, fabrics).
- Limitation of noise propagation in ducts (see NF EN ISO 7235 Acoustics - Laboratory measurement procedures for silencers in ducts and terminal units - Insertion loss, flow noise and total pressure loss)
Numerical Simulations by Simplified Methods (NSSM) allow the calculation of the insertion loss, the self noise and the total pressure loss of aeraulic network elements, the impact of which, in the event of juxtaposition, is evaluated:
- silencer either resonant or dissipative (i.e. with absorbent lining, possibly multilayered). The calculations are possible by combining plates (e.g. steel, aluminum) possibly perforated, fibrous (mineral wools e.g. glass, rock, basalt, silicates, polyester wool) or foams and surfacings (cloths, fabrics)
- straight lengths, elbows, nozzles
In general, the components of the air network can be of rectangular or circular section.
Thus, in terms of acoustial design in the construction sector, practices - laboratory tests, mesh simulations, Numerical Simulations by Simplified Methods (NSSM) - are evolving, in relation to developments in Computer Aided Design (CAD).
This last practice constitutes a promising trend, for taking into account different phenomena e.g. sound transmission through walls, sound absorption, limitation of noise propagation in ducts, as a complementary approach and sometimes as a alternative to other approaches.
There are currently only a limited number of players for publishing of Numerical Simulations by Simplified Methods (NSSM) software in the field of acoustics.
However, the possible applications are numerous, in relation to the acoustic comfort of buildings.
The software SILDIS®, developed and marketed by ITS, is a versatile tool in this area.