Can I prevent setting up standing waves in my COMSOL model?

[pitchtwit]

Can I prevent setting up standing waves in my COMSOL model?

I'm using integration over two opposing faces to get the pressure at the 'outlet' - but it seems that no matter what I do to prevent the standing waves, they won't go away.

Any help would be great,

Thanks

 

[Greg Bernhardt]

I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?

 

[pitchtwit]

Acoustic metamaterial - locally resonant.

Yes of course. I'm trying to recreate a model that has already been created in COMSOL by a few researchers - one of which is Christina J Naify (she does a presentation on this here). It's for my Masters project. I plan to try out a few tweeks of my own and write it up. I've already recreated the model analytically in Matlab, but I need to do it on COMSOL to make any further changes.

Here's one of the key papers on this - Transmission loss and dynamic response of membrane-type locally resonant acoustic metamaterials

And here's the plot I'm aiming to get -
https://dl.dropboxusercontent.com/u/11341635/Screen%20Shot%202014-08-29%20at%2013.29.08.png

The basic premise is that a small square membrane - stretched across a square plastic rigid frame - has a small mass added to the centre. This setup - being dubbed an 'acoustic metamaterial' - provides a much enhanced acoustic transmission loss - with a very high peak at a given frequency. Obviously, by changing key variables, the acoustic transmission loss (in dB) over the frequency range can be manipulated. Also, multiple layers can be used to create a more broadband effect.

Here's an image of the setup: -
https://dl.dropboxusercontent.com/u/11341635/Screen%20Shot%202014-08-29%20at%2013.48.54.png

I've been working from a COMSOL template called 'Acoustic Transmission Loss through Periodic Elastic Structures', which is public access.

My problem is that this model is a 2D model - whereas mine needs to be 3D because the modes across the face of the membrane are important. Also, this model works only at one frequency, and changes the angle of the plane wave radiation, whereas my model needs to change the frequency of the plane wave radiation, with only a normal incidence. They use Floquet periodicity (via a periodic condition) to bypass the effects of standing waves - setting the length of the virtual impedance tube sections so that standing wave effects are not an issue.

I was hoping that I could use either matched boundaries, or PMLs surrounding the virtual impedance tube so that reflections were not an issue, but putting this into practice has not proved to be successful.

I find that the error messages I get are not that helpful - I try using them in Google searches, but it doesn't get me anywhere. This is the usual method when you get stuck with a program like Matlab, but here it doesn't seem to work. The help documentation also seems very unhelpful.

I find the equations bit totally confusing and I'm close to giving up on this.