Re: [Phys-l] sound waves and beam flexures

[Moses Fayngold <moshfarlan@yahoo.com>]

--- On Tue, 2/23/10, John Denker <jsd@av8n.com> wrote:


From: John Denker <jsd@av8n.com>
Subject: Re: [Phys-l] sound waves and beam flexures
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Date: Tuesday, February 23, 2010, 12:57 PM


> And speaking of plane waves:  for non-planar sound
> waves, such as the spherical waves spreading out from
> a point source, the physics is different.  Specifically,
> it is dispersive (whereas plane waves in the same medium
> would be non-dispersive).  "The wave equation" must be
> reformulated to take this into account.

 
    I do not see how and why the geometric shape of the wave-front can affect the dispersion. Whether a wave (more accvurately, its propagation) is dispersive or not is determined by the properties of the medium and, if we also take into account the non-linear terms, by the wave amplitude, but not by geometry. In this respect I can understand that dispersion may be different for longitudinal and transverse wave since different elastic modulae of the medium would be involved in these two cases. But assuming the both waves are of the same kind (say, both longitudinal to make it more simple) AND neglecting the non-linear effects, the dispersion (as I understand it), must be the same for a plane and for a spherical wave. 
 
  Moses Fayngold,
NJIT