In the minds of many engineers, Ansys Mechanical can be used solely to address the behavior of solids. However, there is another class of problems that Mechanical can be used for: acoustics, which is exposed with Ansys Mechanical Enterprise licensing.
What is Acoustics?
Acoustics is the study of sound. Sound is created by pressure waves travelling in an elastic medium, typically air or water. Applications of acoustics include the design of concert halls, noise minimization in manufacturing environments, sonar, and speaker design. Typical results of interest are pressure distributions and gradients in the fluid, sound pressure levels, and scattering, diffraction and transmission of acoustic waves. Acoustic analyses can either be coupled, which accounts for the effect of the fluid on the solid bodies, or uncoupled, which models the fluid only.
Fluid Analysis in Mechanical? Why not CFD?
While the behavior of fluids is generally left to CFD solvers, there is a key difference between typical CFD analyses and acoustics problems—in acoustics analysis, the fluid isn’t flowing. Sound waves transport energy through a medium, but not mass. As with all waves, particles travel back and forth without any net movement. This property allows the mathematics and Lagrangian meshing of the Mechanical solver to be leveraged, with some additional governing equations.
Example: Speaker and Plate
To demonstrate, a simplified speaker model was created with a flat plate 2 inches in front of it, partially blocking the pressure waves. We’ll use Ansys Acoustics to evaluate the sound pressure levels behind the plate, as well as the overall sound pressure distribution.
Figure 1: Solid model of speaker and plate
Using the Enclosure tool in Ansys SpaceClaim, we can easily create a fluid domain in the negative space around these solids. In this case, we’ll create two layers of enclosures. The outer enclosure will be defined to allow us to calculate results outside of our meshed fluid domain (otherwise known as “far-field” results). This is a useful way to keep the overall node count reasonable. For numerical accuracy, the outer layer should be at least three elements thick, and be thicker than 1/10th of the longest wavelength being analyzed.
For this analysis, we’re assuming that the solid components are perfectly rigid, so we’ll suppress them from the final model. We’ll use the surfaces that form the face of the speaker to create a mass source excitation—essentially defining the amplitude of our sound wave. We’ll then use the analysis settings in our Harmonic Acoustics analysis to specify a range of frequencies that we’d like to examine.
Within the meshed domain (the “near-field”), we can generate a variety of contour plots showing distributions within the fluid, such as the section view of sound pressure level below.
Figure 3: Sound pressure level at 3000 Hz, in dB
In the far-field, sound pressure levels can be queried by specifying a desired location. Polar plots can be created showing sound pressure at a defined circular boundary—Figure 4 shows the sound pressure level 1 meter away from the model origin in a circle on the XZ plane.
Figure 4: Sound pressure level 1m from origin on XZ plane, in dB
Additionally, as in a structural Harmonic Response analysis, we can sweep a frequency range and store each frequency as a result set in our analysis. Using a far-field mic result, we can pick a location in the far-field and output the sound pressure across the frequency range as shown in Figure 5.
Figure 5: Sound pressure level at (0, 0, 1m) across frequency range
Acoustic analysis is a lesser known category in Ansys Mechanical, but it allows for a wide variety of additional studies to be performed in FEA. Users familiar with the Mechanical graphical interface should find the acoustic modules intuitive, with a little study. The Ansys documentation and the Ansys Learning Hub are great resources for engineers looking to add this analysis type to their toolkit.