http://www.k-wave.org/forum/topic/alpha_coeff-unit-dbmhzcm-or-npmhzcm Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 10:23:13 +0000 http://bbpress.org/?v=1.0.2 q http://www.k-wave.org/forum/search.php http://www.k-wave.org/forum/topic/alpha_coeff-unit-dbmhzcm-or-npmhzcm#post-5489 Tue, 03 May 2016 12:58:19 +0000 Bradley Treeby 5489@http://www.k-wave.org/forum/ <p>Hi Charlotte,</p> <p>The attenuation coefficient <code>medium.alpha_coeff</code> is given in units of dB/(MHz^y cm). There are some more details in the k-Wave Manual, or in the <a href="http://www.k-wave.org/documentation/example_na_modelling_absorption.php">Modelling Power Law Absorption Example</a> (you can also find these examples in the MATLAB help browser).</p> <p>Brad. </p> http://www.k-wave.org/forum/topic/alpha_coeff-unit-dbmhzcm-or-npmhzcm#post-5488 Tue, 03 May 2016 11:24:11 +0000 charlotte.constans 5488@http://www.k-wave.org/forum/ <p>Hi,</p> <p>I am trying to take into account absorption effects on ultrasound propagation through a skull.<br /> I am wondering whether alpha_max should be in Np/MHz/cm or dB/MHz/cm.<br /> I am considering a heterogeneous absorption map, linearly related to density with a maximum value alpha_max=3.5 and minimum 0 in water around the skull. I understand that the unit is supposed to be dB/MHz/cm; however, with a bone thickness of about 3 mm at 800kHz, I obtained a transmission reduction of 66% compared to the same simulation with alpha_max=0. This result would match a mean alpha_coeff of 1.7 Np/MHz/cm = 15 dB.MHz/cm; as the modelled skull is very porous, the mean absorption coefficient is about half the max coefficient so it seems that the software is actually considering that alpha_max=3.5 Np/MHz/cm instead of 3.5 dB/MHz/cm.</p> <p>When searching for more information on the website, I found this code (<a href="http://www.k-wave.org/documentation/example_us_bmode_linear_transducer.php" rel="nofollow">http://www.k-wave.org/documentation/example_us_bmode_linear_transducer.php</a>) where again, it looks like the absorption coefficient unit is Np/MHz/cm and not dB/MHz/cm in the equation "p=p0 exp(-alpha*f*d)<br /> where alpha is the attenuation coefficient in dB/(MHz cm)":</p> <p>"[p=p0 exp(-alpha*f*d)]<br /> where alpha is the attenuation coefficient in dB/(MHz cm), f is the frequency in MHz (chosen here as the transmit frequency), and d is the total (round-trip) distance travelled in cm. Here the distance variable is created assuming that t = 0 corresponds to the middle of the transmitted tone burst.<br /> % create radius variable assuming that t0 corresponds to the middle of the<br /> % input signal<br /> t0 = length(input_signal)*kgrid.dt/2;<br /> r = c0*( (1:length(kgrid.t_array))*kgrid.dt/2 - t0); % [m]</p> <p>% create time gain compensation function based on attenuation value,<br /> % transmit frequency, and round trip distance<br /> tgc_alpha = 0.25; % [dB/(MHz cm)]<br /> tgc = exp(2*tgc_alpha*tone_burst_freq/1e6*r*100);"</p> <p>Could you please clarify this?</p> <p>Thanks a lot! </p>