 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Natural Science Forum / Physics / Acoustics / November 2005180 degree phase shift on air/water interface
Can anybody help me with a problem. Supposedly, there is a 180 degree phase shift when a hydroacoustic waves encounters the air interface at the underside of the sea surface. First, is this indeed the case? If so, then suppose an earthquake generates a series of LF pressure pulses (hydroacoustic) that strikes the undersurface at a 90 degree angle. What happens to the energy? Suppose the angle is 45 degrees? I have a russian research paper that talks about such a phase shift. In this paper, a positive pressure pulse strikes the undersea air/water interface and experiences a 180 degree phase and becomes a negative pressure pulse generating acoustic cavitation that can be detected and used as a tsunami warning signal. Any truth to this? >Can anybody help me with a problem. Supposedly, there is a 180 degree >phase shift when a hydroacoustic waves encounters the air interface at >the underside of the sea surface. > >First, is this indeed the case? Yes, the 180 degree phase shift - or more properly the inversion - is quite right. >If so, then suppose an earthquake generates a series of LF pressure >pulses (hydroacoustic) that strikes the undersurface at a 90 degree >angle. What happens to the energy? Suppose the angle is 45 degrees? Much of the energy will reflect, much as it would off a mirror. The rest will be translated into surface waves - lateral rather than longitudinal - that will radiate away from the centre of the pressure pulse. A small amount will dissipate into the air. >I have a russian research paper that talks about such a phase shift. >In this paper, a positive pressure pulse strikes the undersea air/water [quoted text clipped - 3 lines] > >Any truth to this? I would imagine that the degree of cavitation would depend largely on the amplitude of the pulse. A small one would not generate any cavitation. d Pearce Consulting http://www.pearce.uk.com On 11/26/05 1:24 PM, in article 1133040247.325337.139440@o13g2000cwo.googlegroups.com, "david" > Can anybody help me with a problem. Supposedly, there is a 180 degree > phase shift when a hydroacoustic waves encounters the air interface at [quoted text clipped - 13 lines] > > Any truth to this? My approach to this kind of problem is usually the same for ANY wave problem in a homogeneous medium. Because my experience is mostly with electromagnetism, I use that to guide me. The key is the mismatch of impedance as a wave travels from one medium into another. For an electrical transmission line, the characteristic impedance is sqrt(L/C) where L and C correspond to the distributed inductance and capacitance (per unit length that is) of the line respectively. The corresponding quantities for an acoustic medium would be density and elastic modulus respectively. That acoustic impedance is sqrt(rho/E). It is also equal to rho*c where c is the speed of propagation. Because I never can remember the condition for which there is a 180° phase shift, I remember that a short circuit corresponds to a extremely low impedance. A voltage wave, corresponding to a pressure wave in acoustics, must reflect with a 180° phase shift so that the sum of incident and reflected waves add up to zero voltage at the short. Thus, if a wave in a medium with impedance rho0*c0 strikes a boundary with an impedance rho*c lower than rho0*c0, the reflected wave will have the 180° shift. Otherwise, there will be no shift. Bill -- Ferme le Bush > Can anybody help me with a problem. Supposedly, there is a 180 degree > phase shift when a hydroacoustic waves encounters the air interface at > the underside of the sea surface. > First, is this indeed the case? sure! > If so, then suppose an earthquake generates a series of LF pressure > pulses (hydroacoustic) that strikes the undersurface at a 90 degree > angle. What happens to the energy? Suppose the angle is 45 degrees? 90 degrees incidence is a negative pressure for sure. 45 degrees wolud probaly amout to square root of two less in amplitude (o.707). > I have a russian research paper that talks about such a phase shift. > In this paper, a positive pressure pulse strikes the undersea air/water > interface and experiences a 180 degree phase and becomes a negative > pressure pulse generating acoustic cavitation that can be detected and > used as a tsunami warning signal. Why not! The pheomenon is vaguely related to the water hammer oft heard in seam pipes. > Any truth to this? That hypothetically the cavitation pelse (water hammer) could occur under certain circumstance, there is no doubt. wheter it can be triggerd by practical (typical) tsunami phenomena is another story. We need some hard data. sounds like a good research proposal when Congress is in a spending mood (not now). Angelo Campanella |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.