Ultrasound Reflection & Transmission At Water-Gold Boundary Strange Behavior
Posted Jan 12, 2021, 2:15 PM EST Acoustics & Vibrations Version 5.6 3 Replies
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I am trying to simulate ultrasound propagation from a low intensity pulsed ultrasound (LIPUS) transducer in a water tank with a slab of solid gold placed in the middle of the tank. The goal is to develop an understanding of what the ultrasound pressure field should look like when I introduce large pieces of gold & eventually gold nanoparticles into the simulation. Based on the acoustic impedance mismatch of gold vs water, I have calculated that there should be ~95.5% reflection of the ultrasound waves at the water-gold interface, however in my results I am consistently seeing high transmission of the ultrasound waves into the gold block as well as some strange reverberation artifacts inside the gold.
I am using the frequency domain pressure acoustics module & defining my LIPUS transducer as a flat line with normal displacement. I defined the water & gold mediums in the pressure acoustics module with a user defined attenuation (COMSOL mph file attached as well). From my understanding, the pressure acoustics module should have the ability to compare acoustic impedance of the gold and water material domains and define reflection / transmission coefficients based on the acoustic impedance mismatch, however that seems to be failing in this simulation.
What's strange, is that if I swap the gold and water mediums with each other (slab of water in a gold tank), I have almost full reflection of the ultrasound waves at the gold-to-water boundry. It looks like for some reason I have the expected reflection only when travelling from domains of high acoustic impedance to low acousic impedance, but not low acoustic impedance to high acoustic impedance.
Does anyone know what is going on here?