Discussion Forum

Hello,

I am modeling a PMUT idealised as a circular membrane that consist of AlN in between two Al electrodes. I am interested in the maximum pressure difference at a distance of the PMUT, when the membrane is at resonance. By following a tutorial I have managed to simulate the Absolute pressure in the air surrounding the membrane. In the first mode of the membrane, just above the center of the membrane I got 798.4 [Pa] which is 6 times lower than expected. (The same comparison, if done for air does not give such discrepancy.)

When I do Frequency analysis, the frequency versus displacement plot gives me displacement amplitude of 0.29[nm] at 1.808[MHz] in water. Then, by using the planar wave approximation
p'=diplacement*freqency*rho*speed_of_sound=0.29e-9*1.808e+6*2*pi*1000*1484=4888.9 [Pa].
If I plot this with COMSOL as acpr.c*acpr.rho*acpr.vz I got pretty similar value, however if I plot the absolute pressure I got 6 times smaller value.

Water is much different than air, hence I dig out equations for the spherical source approximation which gave me 5766.7 [Pa] at 10 micro meters from the source (and growing closer to the source).

My small matlab code for computing the pressure with spherical wave at distance r is here:
drive.google.com/file/d/0B6mlnZcmKMS1SFVTS0szNzhCNUU/view?usp=sharing

Hence, my question is what is wrong? Do I miss something in the model?
You can download the COMSOL file from here:
drive.google.com/file/d/0B6mlnZcmKMS1czhlNDhyclFuaWM/view?usp=sharing

___________
REFFERENCES for the Spherical wave approximation:

Finn Jacobsen, Peter Juhl - Technical University of Denmark
- Page 34, Equation (5.20)
web-files.ait.dtu.dk/fjac/p_home_page/notes/Radiation.pdf

Shahab, Gray, Erturk - Georgia Institute of Technology, USA
- Page3, Column1, line 28
www.ssdsl.gatech.edu/wp-content/uploads/2015/03/JAP_2015.pdf