I'm running the "Modelling Nonlinear Wave Propagation Example" in Matlab with a time varying source that produces linear sine sweep centered at 1 MHz and spanning 0.9 MHz. I want to see the higher order harmonics so I set up the time and grid parameters so that I have 674 MHz sampling rate and 101 MHz maximum supported frequency. When I look at the resulting spectrum of the sensor data, I see bunch of frequencies are squished around 95-101 MHz. Please see this link (https://imgur.com/a/71uxjbq ) for the spectrum. To my understanding, aliasing doesn't seem to explain this. I'd still expect a decreasing pattern in higher frequencies even if they were aliased. Am I missing something? What could the reason be?
k-Wave
A MATLAB toolbox for the time-domain
simulation of acoustic wave fields
High frequencies are not aliased but instead compressed together?
(6 posts) (2 voices)-
Posted 4 years ago #
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Hi gk7,
Could you try running your simulation under linear conditions (don't define
medium.BonA) to see if you still see the high frequencies? It could be related to your source definition rather than the nonlinearity. You have a very flat spectrum before the high-frequency peak, which doesn't immediately look like spectral blocking.Brad.
Posted 4 years ago # -
Hi Bradley,
I checked out the linear spectrum. As you suspected, it's flat except the main lobe at 1 MHz without high frequencies. How do I need to define the source to obtain proper results with nonlinear simulations?
Posted 4 years ago # -
Hi gk7,
Just to confirm, the high frequencies >95 MHz appear / disappear when you set / don't set medium.BonA?
Brad.
Posted 4 years ago # -
Yes, high frequencies (> 95 MHz) appear when I set medium.BonA and they disappear when I don't set them.
Posted 4 years ago # -
Hi gk7,
Ok, it is likely related to spectral blocking then. Does your simulation have any absorption? If not, you could try adding some.
Brad.
Posted 4 years ago #
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