k-Wave

1. yrpnova
   

   Member
   Joined: Dec '12
   Posts: 1

   Hi. I want to change the frequency of the US source in the bmode_linear_transducer example from 1.5 to 3MHz. When I changed the value of tone_burst_freq to 3e6, it seems that I get a signal that is not correct.

   http://imageshack.us/scaled/landing/443/54888411.png
   which is clearly not centered around 3MHz.

   Even the bmode image looks very off

   http://img201.imageshack.us/img201/2748/31561673.png

   Is there anything else I'm supposed to change in the code to accommodate the new frequency?

   I tried playing around with the defining_transducer example to see if I could find out what the problem was but as soon as I changed tone_burst_freq to 3e6, MATLAB would crash immediately after the program finished running. O_O Now it will crash even after setting the value back to 0.5e6. I tried reinstalling matlab too but the problem still persists.

   Please help (I need help with bmode example more than defining_transducer example).

   Posted 11 years ago #

2. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   When you set the source frequency for the simulation, you also need to consider the spacing of the grid points. The maximum frequency that can be represented on the spatial grid is the Nyquist limit of c_min/(2*dx) where dx is the grid spacing. In this particular example, the grid spacing is 180 um which means the maximum supported frequency is around 4 MHz. If you set your source frequency to 3 MHz and run a nonlinear simulation, the high-frequency harmonics that are generated will not be supported by the grid, and will instead be aliased to lower frequencies making your results incorrect.

   Essentially, if you want to use higher frequencies in the simulation, you need to decrease the grid spacing.

   Unfortunately I don't have any suggestions for MATLAB crashing - I hope you can find a solution!

   Brad.

   Posted 11 years ago #

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