k-Wave

1. winebra
   

   Member
   Joined: May '12
   Posts: 1

   Hi,
   I wish to know what are absorption factors of air.
   alpha_power and alpha_coeff

   Thanks.

   Posted 12 years ago #

2. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   Hi winebra,

   What frequency range are you trying to model?

   Brad.

   Posted 12 years ago #

3. shacklepants
   

   Member
   Joined: Nov '12
   Posts: 2

   I'm having much the same issue i need to know the alpha power and coefficients for air, Im using a 40 KHz signal source trying to replicate 40khz ultrasonic emitters. Perhaps its my understanding of the equation which is flawed?

   Many thanks

   Chris

   Posted 12 years ago #

4. bencox
   

   Developer
   Joined: Mar '10
   Posts: 292

   Hi Chris,

   Is the problem that you don't know the absorption coefficient in air at 40 kHz, or that you don't know how to put that into the model?

   If you don't know the absorption coefficient in air at 40 kHz, can you get hold of this paper: H. E. Bass and F. Douglas Shields, "Absorption of sound in air: High‐frequency measurements" J. Acoust. Soc. Am., Volume 62, Issue 3, pp. 571-576 (1977) ? Note that it depends significantly on the humidity.

   Once you know what absorption coefficient you want, and if you are really working at a single frequency (so using a source which is a continuous tone), then you can choose alpha_power and alpha_coeff to be whatever you like so long as the combination alpha_coeff * frequency^alpha_power equals your known absorption coefficient.

   If you are using a tone burst or even more broadband signal then you will want to ensure that the variation with frequency of the absorption coefficient over the bandwidth of your source matches the known values. For that you will need to choose alpha_coeff and alpha_power so that alpha_coeff * frequency^alpha_power is a good fit to the known values over the frequency range.

   If you don't have absorption that varies with frequency to some power, even approximately, then k-Wave can't help at the moment, I'm afraid.

   Hope that helps,

   Ben

   Posted 12 years ago #

5. shacklepants
   

   Member
   Joined: Nov '12
   Posts: 2

   Ben,
   Thank you for that post I will look the paper up. I was hoping to use K-wave to model the Ultra sound sensors used in vehicles (which means ultrasound in air) I wanted to be able to match what we see now with simulation in order then then move forward and postulate what else could be done with the technology. However I'm relatively new to simulating ultra sound.

   Chris

   Posted 12 years ago #

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