Harmonic Acoustics is Harmonic Response but with extensions that makes it easy to do acoustical things like define a face to apply a harmonic plane wave into a medium, or to define a face where no waves get reflected to simulate an infinite domain on a finite sized solid.

**The Problem of using FEA Acoustic Simulation for Ultrasound Frequencies**

I did a "back of the envelope" calculation to scope out the problem.

What is the maximum frequency, f, of ultrasound you want to use? Typical medical ultrasound goes between 1 and 18 MHz.

Do you know the speed of sound, c, in water? It is about 1500 m/s and ultrasound imaging assumes 1540 m/s.

Let's calculate the wavelength of a 15 MHz sound in water: lambda = c/f = 1500/15e6 = 0.0001 m or 0.1 mm.

At least 6 quadratic elements are needed along one wavelength to adequately represent the wave. That means the maximum element size in the model would be 0.016 mm for 15 MHz. If you take a 10 mm cube of material, and mesh it with 0.5 mm elements, you end up with > 35000 nodes. With the Student license, the limit on nodes + elements is 32000 before it will not solve.

Even if you are on a Research license, a 10 mm cube of material with an element size of 0.1 mm, which is okay for a 2.5 MHz ultrasound, creates 4 million nodes connecting 1 million quadratic elements, so a 0.016 mm element size would create an unreasonably large model on a very tiny sample.

This is why FEA is not a good method for ultrasound. Fortunately the Boundary Element Method (BEM) can cope with ultrasound frequencies as it does not need to fill the volume with elements. Unfortunately, ANSYS does not have a BEM solver.

I recommend you select an acoustic problem with frequencies < 20 kHz through an air medium if you want to use ANSYS.

Best regards

Peter