📷I have question about the distance measurement by sonar wave. I try to use the ray tracing concept to explain how does the sound signal to estimate the distance. Now regard the acoustic transducer produces a beam（a group of rays）rather than a sound wave as figure below shown (the transducer will produce a beam in each scan). We know when a ray hit a plane it will be reflected, and there are 2 types of reflection: specular reflection and diffuse reflection. In the Figure below, the BLACK double arrow represent the incident ray and its diffuse reflected ray that return to the transducer; the RED double arrow represent the normal incident and reflection ray (the normal reflection also return to the transducer, this is belong to the specular reflection); the green arrow represent the specular reflected rays, which are return to the transducer. Based on the statement above, one scan will led the transducer receive multiple signals (the normal reflection, the diffuse reflections). The first received signal is the normal reflection because the normal reflection has the shortest TOF （time of flight), so we use the TOF of the first received signal to calculate the distance and then the depth. Is this distance measurement is right? We use the first received signal to calculate the distance?
Hi Dear Vinay K. Trivedi
thanks for your answer.
that is means, In this underwater depth measurment case, we can use the energy impulse response of time domain to estimate the depth (find the peak intensity signal should be the first pulse (the red one) if ignore the noise)?
yibin
THANKS Vinay K. Trivedi
it answered my question, i am trying to model the depth measurement by ray tracing and its impulse response. In each scan ( as i mentioned in the figure), it will produce a cone beam, and on the impulse response of time domain, the heightest intensity pulse shoud be the red double arrow (first received if not consider noise), hence the TOF of this red signal (pulse) can be used to estimate the depth at this scanning position.
yibin
easy question, you had the answer, echo sounders work on the first return (but if a fish school is passing mid-water, you will get it first of all! So you have to do some integration etc)
Dear Yibin Peng,
Ray tracing assumes that the medium is stationary, uniform and isotropic (equal propagation velocity in all directions). Propagation paths can be bent (refracted) by, for example, temperature gradients (see A. D. Pierce's "Acoustics: An Introduction to Its Physical Principles and Applications" for an explanation of this). The mathematical theory that models these effects is called Hamiltonian Optics. It uses first order differential equations rather than linear geometry to predict propagation paths (Born and Wolf's "Optics" I think has a brief summary of this theory; a more detailed explanation is in "An introduction to Hamiltonian optics" by Hans Buchdahl.) If you want to estimate distance by acoustic measurement, you need to know the propagation velocities in all the media that the wave passes through. In the context of underwater sonar, for short distances, (tens of metres) you can probably assume that the medium is uniform and isotropic, although it may not be stationary (there may be tidal currents or other currents that could have refractive effects). Over longer distances you may need to also consider thermal gradients and salinity gradients.
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