直径0.5吋的直探头放在钢板上，钢板后紧贴有丙烯树脂塑料。声束在两种材料中的扩散角约为（ ）。
A 10MHz, 0.5in. dia. , search unit is placed on steel and acrylic plastic in succession. The beam spread in these two materials is approximately.
直径0.5吋的直探头放在钢板上，钢板后紧贴有丙烯树脂塑料。声束在两种材料中的扩散角约为（ ）。
A 10MHz, 0.5in. dia. , search unit is placed on steel and acrylic plastic in succession. The beam spread in these two materials is approximately.
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