>not being a whiz in the material room I don't know, but is there a way with Math function to divide the amplitude by distance from centre

Sure.
I also added another node Math_Functions_11 to
change the phase of the sin. Just enter something
in value_2

By the way, the renderer seems to have an error.
In the middle of the plane there is always this little
tip, no matter how the phase is.
Do you think I should report this to CuriousLabs?

Ernst