k-Wave

1. Alexander
   

   Member
   Joined: Jun '17
   Posts: 19

   I was looking at the tvsp_homogeneous_medium_monopole.m example and I moved the sensor to the position of the pressure source. So they are right on top of one another. When this was done and you plot the input pressure signal and then the sensor pressure signal there is a strong dip in the pressure picked up by the sensor. The sensor is reading ~0.6Pa while the source is at 1Pa. I've experienced similar results with other simulations where the pressure will drop from 6Pa to ~1Pa even though the sensor is right on top of the pressure source. This appears to only happen with time varying sources. If I just define some initial pressure distribution, such as ...

   % create initial pressure distribution using makeDisc
   disc_magnitude = 5;           % [Pa]
   disc_x_pos = 0;                % [grid points]
   disc_y_pos = 0;                % [grid points]
   disc_radius = 8;               % [grid points]
   disc_1 = disc_magnitude*makeDisc(Nx, Ny, disc_x_pos, disc_y_pos, disc_radius);
   
   source.p0 = disc_1;

   The pressure readings of the source and sensor both match. So is there a reason for this difference in the pressure reading of the sensor and source with time varying pressure sources?

   Thank you,
   Alex

   Posted 6 years ago #

2. Alexander
   

   Member
   Joined: Jun '17
   Posts: 19

   I also wanted to add that when the tvsp_homogeneous_medium_monopole.m example was run with only the speed of sound defined (so it is a lossless medium) the same drop in pressure amplitude was seen, from 2Pa to ~0.5Pa.

   Posted 6 years ago #

3. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,640

   Hi Alexander,

   By default, a pressure source in k-Wave doesn't specify a boundary condition, but rather the amount of mass that is being injected into the medium. Thus, the input pressure will not be exactly reflected in the output pressure. For example, the pressure added at the previous time step may still be relaxing while the pressure at the next time step is being added. If you want to enforce a particular pressure distribution, you can use source.p_mode = 'dirichlet'. For an initial pressure distribution, the source is specified as an initial condition, so behaves differently.

   Hope that helps,

   Brad.

   Posted 6 years ago #

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