acoustics simulation

Hi,

I am trying to run a acoustic wave propagation simulation in Abaqus. The model is a block surrounded by water. I got the following error message:

AN ACOUSTIC MEDIUM IS DEFINED FOR ELEMENT 47 INSTANCE WATER-1. ONLY ACOUSTIC ELEMENTS MAY HAVE THIS MATERIAL PROPERTY.

I researched on the web but nothing really states this problem. Any ideas?

Thanks.

Bo