k-Wave

1. lio2it
   

   Member
   Joined: Apr '12
   Posts: 2

   Hi all,

   I’m new to this simulation tool. I’m currently having some issues during the simulation of two ultrasound transducers.
   The room that I’m trying to simulate consist of:
   - A 3D space (40x40x40cm) with a resolution of 0.5mm filled with air (sound_speed=340m/s, density=1.18Kg/m^3, alpha_power=0.99, alpha_coeff=1.62)
   - An external PML of 10points (5mm) for each of the 3 dimensions, with attenuation of 4 Np/(m*rad/s).
   - One source transducer positioned at [0,Ny/2,Nz/2] (330kHz, 24 pulses generated by toneBurst, 10 active elements)
   - One sensor transducer positioned at [Nx,Ny/2,Nz/2]
   - Nothing between the two ultrasound transducer

   If I try to simulate this situation, with CFL=0.3, for about 70us, looking at both the standard plane plot and a 3D representation of the presence of sound within the room I observe that there are some sound contribution around the source traducer that reach instantaneously the sensor transducer and that are present during the entire simulation. Figures can explain better than me:

   http://i.imgur.com/kjhff.png

   Is this a bug of k-Wave or am I making any mistakes?

   Thanks,
   Daniele

   Posted 12 years ago #

2. bencox
   

   Developer
   Joined: Mar '10
   Posts: 292

   Hi Daniele,

   How many points have you used in your grid? You will need at least 2 or 3 points per wavelength for an accurate simulation. For a source of 330 kHz in air, the wavelength is about 1 mm, so you need the grid spacing (dx, dy, dz) to be about 0.3 mm or less.

   Ben

   Posted 12 years ago #

3. lio2it
   

   Member
   Joined: Apr '12
   Posts: 2

   Thanks for your answer Ben. Grid spacing that I've used is 0.3mm, but also with 0.1mm the result was the same.

   I forgot to say that strange sound contributions appear only after a few hundred of nano seconds. So it seems that sound travels at a speed of about 400km/s.

   Daniele

   Posted 12 years ago #

4. bencox
   

   Developer
   Joined: Mar '10
   Posts: 292

   Hi Daniele,

   Without seeing the code it's difficult to say whether you've uncovered a bug or whether it's something in your code. k-Wave uses a spectral method, which can lead to artefacts such as this if there is something which changes too quickly spatially and therefore cannot be described accurately by the limited number of Fourier coefficients available. There are two ways around it: (a) made the mesh finer and finer (ie. more and more points), or (b) make everything smoother. The `smooth' function is included for that purpose.

   We've not used it ourselves for modelling propagation in air, and the abrupt interface with the wall may be what is causing the trouble. We designed it for use on soft tissue where the changes in sound speed and density are < 10% or so.

   Hope that helps,

   Ben

   Posted 12 years ago #

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