The velocity of longitudinal wave changes

Like the following code,I set the velocity of L-wave of the material of my model  to 5900m/s.

c Materials
type elas
wvsp on
prop steel 7900. 5900. 3200.

/* Create tx and rx elements
do loopI I 1 $ele_numx 1
do loopJ J 1 $ele_numy 1
prop etx$I/$J 0. 0. 0.
end$ loopJ
end$ loopI

c Assign materials to grid
/* Make all elements void
regn void

/* Create plate
regn steel $i1 $indgrd $j1 $jndgrd $k1 $k3

/* Create tx array elements
do loopI I 1 $ele_numx 1
symb iend = 2 * $I + 1
symb ibeg = $iend - 1
do loopJ J 1 $ele_numy 1
symb jend = 2 * $J + 1
symb jbeg = $jend - 1
regn etx$I/$J $i$ibeg $i$iend $j$jbeg $j$jend $k3 $k4
end$ loopJ
end$ loopI

/* Defect
sphr void $def_x $def_y $def_z $def_rad


Then I extract the aprs of tow points along the path and direction of L-wave.The distance of  them is 29.5mm.  

c form matlab h1 /* uncomment if Matlab output format is desired
hist func
histname avrg * all
hist xyz aprs 1.8e-3 1.8e-3 1 2.4e-3 2.4e-3 1 10e-3 10e-3 1
hist xyz aprs 1.8e-3 1.8e-3 1 2.4e-3 2.4e-3 1 39.5e-3 39.5e-3 1

Then I plot the A scan signal of the two points in time domain using Matlab and extract the corresponding time of two points.The velocity of the wave from the first point to the second point is calculated as 5917.8m/s approximately.Why?How to calibrate?



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