(23) |

We will use the Fourier transform of the pressure field along the time axis:

(24) |

The following property of the Fourier transform will be needed:

(25) |

The spatial frequency is defined as:

(26) |

Let be a spatial average of *v* in the medium, a known constant
that does not depend on or *z*. We also define

(27) |

and the function

(28) |

The index notation for derivatives will be used from now on. The symbol will denote the phrase ``for all.''

6/8/2002