The input data useful for the design of a silencer are mainly the type of fluid (and its degree of physicochemical aggressiveness with respect to the envisaged silencer), its thermodynamic state (flow rate, pressure, temperature, density), the level of acoustic performance required (insertion loss, remaining sound power), the level of aerodynamic performance required (total pressure loss often called pressure drop), and the space available (dimensions of the upstream network, space available for the silencer, downstream interfacing if applicable). In very numerous cases, the reaction to fire of considered materials is also taken into account.
In the case of realizations in the food industry, in pharmaceuticals, or in hospitals, specific constraints related to hygiene are involved and may necessitate the use of particular absorbing fillings (with waterproof washable linings) or the use of stainless steels.
Other parameters may be involved such as the authorized mass, the longevity required or the available budget.
In a concrete and exhaustive way, the sizing of a dissipative silencer (i.e. a noise limiting device whose efficiency has to do with the presence of a sound-absorbing material) requires knowledge of the following input data (listed in the order of their input for a calculation with the SILDIS® software Modules 1 / 1+ / 1A Prediction of acoustic and aeraulic (aerodynamic) performance of silencers, considering for the acoustic structure of the sound-absorbing lining an assembly made up at most of a porous medium with surfacing and perforated protection, for the fluid of clean and dry air, and without change of cross section neither at the inlet nor at the outlet of the silencer other than that, linked - where applicable - to the presence of splitters with a sound-absorbing filling i.e. baffles, transverse or concentric):
- temperature
- pressure
- (in case of the presence of a porous medium):
- (as a minimum) resistivity, porosity, (and, if known) tortuosity, thermal and viscous characteristic lengths (or else designation of the nature - e.g. rock, glass, basalt, polyester, ceramic wool or foam to be precised -, trade name and density)
- thickness
- (in the event of the presence of a surfacing, if it is not modeled as a porous medium):
- superficial air flow resistance
- weight per unit area
- thickness
- (in the event of the presence of a perforated protection, if it is not modeled as a porous medium):
- designation of the geometry of the holes (e.g. circular, square, slot-shaped perforation) and their arrangement (e.g. square, hexagonal, staggered)
- diameter or side of holes / width of slots
- center distance (for slots only)
- perforation rate
- thickness
- sound power level in 1/3 octave or 1/1 octave bands at silencer inlet
- (if there is a flow):
- mass flow
- direction of fluid circulation (with respect to the direction of sound propagation: identical - if it is a discharge - or opposite - if it is a suction -
- imposed speed limit (where applicable)
- (in the case of a duct with a rectangular section):
- width [1]
- height [1]
- (in case of square section sheath):
- dimension [1]
- (in case of circular section duct):
- diameter [1]
- length of the dissipative section [2]
- (if there is a flow, with a view to calculating the total pressure loss):
- geometry (shape, dimensions) of the upstream end of the dissipative section
- geometry (shape, dimensions) of the downstream end of the dissipative section
- (if there are splitters with sound-absorbing filling i.e. baffles, transverse or concentric):
- geometry (shape, dimensions) [3]
- number and arrangement [3]
- targeted sound power level at silencer outlet (where applicable)
- acceptable total pressure loss
In the event that it is not clean dry air, the additional following input data (for gas at silencer operating conditions) would be required (in case of several components: for the resulting mixture):
- individual constant
- adiabatic constant
- density
- speed of sound
- dynamic viscosity
- specific heat at constant pressure
- thermal conductivity
This is the list of useful input data for the design of a silencer in linear regime. In the nonlinear regime, the characteristics of the acoustic excitation (nature, sound pressure levels per frequency band) must also be considered.
[1] corresponding to the internal cross section (i.e. delimited by surfaces in contact with the fluid); for a calculation with Module 1 of the SILDIS® software, this dimension is assumed to be equal, for what delimits the space occupied by the fluid, to what it is on the one hand upstream of the silencer, and on the other hand downstream of the silencer ; otherwise, i.e. in the case of a change in the dissipative section compared to upstream or/and downstream, a calculation is possible with Module 1B of the SILDIS® software: a sketch (with dimensions) of the muffler cross section is then desirable
[2] i.e. dimension without counting the upstream and downstream ends (e.g. with aerodynamic profile) not lined with sound-absorbing material; a sketch (with dimensions) of the longitudinal section of the silencer is desirable
[3] a sketch (with dimensions) of the cross section and the longitudinal section of the silencer is desirable