You must account for symmetry planes in the modeled domain for postprocessing near or far sound pressure field beyond the computational domain.

The HFSYM command accounts for the sound-soft or sound-hard symmetry planes that coincide with the X-Y, Y-Z or Z-X planes of the global or local Cartesian coordinate system. It applies the image principle on the symmetric part of the computational domain to represent the radiation effect of the partial equivalent source beyond the modeled domain so that the radiation from the entire structure is modeled.

If sound-soft or sound-hard symmetry planes exist, issue the HFSYM command before issuing a postprocessing command (PRNEAR, PLNEAR, PRFAR, or PLFAR). Although a sound-hard symmetry plane is a natural boundary condition in a finite element analysis, it must be defined via the HFSYM command.

/post1
hfsym,,shb,shb,ssb ! y-z, x-z plane as sound hard and x-y as sound soft
plfar,pres,sump,,  ! plot far-field pressure in polar coordinate

When calculating radiation parameters, the radiation space can be specified.

The HFANG command defines the radiation space of a sound radiator in terms of the type of radiator. For example, the solid angle of a sound dipole is determined by φϵ[0,360°] and θϵ[0,180°], and the solid angle of a piston above an infinite baffle is associated with φϵ[0,360°] and θϵ[0,90°].

If the sound pressure wave is not radiated into the entire space, issue the HFANG command before issuing a postprocessing command (PRNEAR, PLNEAR, PRFAR, or PLFAR).

The maximum sound pressure can be printed or plotted beyond the FEA computational domain.

To print the near sound pressure field, issue the PRNEAR command.

To plot the sound pressure along a path:

The far sound pressure field and far-field parameters (for example, radiation patterns, directivity, radiated power, radiation efficiency, and target strength) are essential for sound radiation or sound scattering analysis.

The far sound pressure field and far-field parameters can be printed (PRFAR command) or plotted (PLFAR command) beyond the FEA computational domain.

To print or plot the 2D far-field parameters in a 3D computational model, extrude the 2D plane model for a distance Δz in the Z-direction to generate a 3D numerical model. For an axisymmetric model that is equivalent to the y-rotated extrusion 3D model, the far field parameters are calculated on the plane defined in global spherical coordinates (PLFAR or PRFAR with Lab = PROT). The theta angles (input as VAR2B and VAR2E) default to 90 degrees for the 2D extrusion model, so you would typically input only the phi angles (VAR1B and VAR1E) in order to report values on the X-Y plane. The HFSYM,,,SHB,, command may be used for symmetry on the y = 0 plane.

The 2D far-field parameters can also be calculated from a 2D planar or axisymmetric model in which the 2D acoustic elements are utilized without any extrusion. Note that Lab = PROT and Lab = PLAT on PLFAR and PRFAR are invalid when 2D acoustic elements are used to calculate far-field parameters.

A maximum of ten far-field curves can be plotted on a chart (PLFAR) for multiple angles or frequencies. The waterfall diagram for far-field parameters is available with both frequency and angle as variables. The waterfall diagram for radiated sound power level can be generated with variables of frequency and revolutions per minute (RPM), if RPM is defined (see the MRPM command).

A contour of far-field pressure, sound pressure level, weighted sound pressure level, acoustic directivity, scattered pressure, or target strength can be plotted on a defined X-Y, Y-Z, or X-Z plane of the global Cartesian coordinate system or on a defined spherical surface (PLFAR). Alternatively, the same quantities can be printed using PRFAR.

For more information, see Acoustic Output Quantities in the Mechanical APDL Theory Reference.

The far field and far-field parameters at a given frequency can be monitored and stored. To do so, issue this command:

*GET,par,0,ACUS,Item1

To obtain a far field or a far-field parameter with the parameter par via the *GET command, issue the PRFAR or PLFAR command at a specified position prior to issuing the *GET command.

/post1
hfsym,1,ssb,shb,ssb                    ! set symmetric planes
set,1,1
plfar,pres,splp,0,0,1,0,360,5,10,2.e-5 ! spl in polar plot
plfar,pres,splc,0,0,1,0,0,1,10         ! spatial point
*get,par,0,acus,spl                    ! store spl
fini

The far-field SPL or a-weighted SPL over octave bands can be obtained by defining the LogOpt argument on the PRFAR or PLFAR command.

Assuming that the sound waves do not affect the structural motion, the vibration of a flat or slightly curved panel can be solved very efficiently without involving any acoustic mesh. The far field and far-field parameters can be directly calculated from structural results based on the Rayleigh integral in which the modified Green's function is used.

The vibrating surface on the structural model must be identified by the equivalent source surface flag (SF,,MXWF) before the far-field calculation. Use the PRFAR or PLFAR command with Lab = PLAT to perform the far-field calculation for a vibrating panel. The HFSYM command should be issued if planar symmetry is used in the model. Use the PRAS or PLAS command to calculate and print or plot the transmission loss and radiated power of a panel with an incident diffuse sound field.

The HFANG command is invalid for the vibrating panel radiation, since the hemispherical radiation space is assumed.

For more information, see Acoustic Output Quantities in the Mechanical APDL Theory Reference.