The following is a good tutorial model on the use of perfectly matched layers (PMLs): Acoustic Scattering off an Ellipsoid: Application Library path Acoustics_Module/Tutorials,_Pressure_Acoustics/acoustic_scattering
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The behavior of the PMLs at low frequencies is discussed in the following model. Plotting the total radiated power can be a good indicator of possible issues. Lumped Loudspeaker Driver: Application Library path Acoustics_Module/Electroacoustic_Transducers/lumped_loudspeaker_driver
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Infinite Elements, Perfectly Matched Layers, and Absorbing Layers in the COMSOL Multiphysics Reference Manual
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The PMLs damp a certain wavelength existing in the system. The wavelength is deducted from the frequency and a reference wave speed cref. The wave speed is defined in the Typical Wave Speed section. Set cref equal to the speed of sound of the material in the PML.
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Infinite Elements, Perfectly Matched Layers, and Absorbing Layers in the COMSOL Multiphysics Reference Manual
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When a perfectly matched layer (PML) is present in the model do not apply an Incident Pressure Field on its outer boundaries. The PML is applied to absorb waves that move out of the computational domain. Defining an incident field on its boundary will lead to unphysical results.
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In the settings for the Perfectly Matched Layer it is important that if you change the Typical wavelength from option to User defined, then it is not the actual wavelength that should be entered but rather the speed of sound per Hertz. For example, if User defined is selected in a normal air domain, then enter 343[m/s]/1[Hz]. The reason is that in the time domain the PML is not related to wavelength but to speed of sound. Transient signals typically include many Fourier frequency components.
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See Theory for the Perfectly Matched Layers in the Time Domain for details about the implementation.
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