\begin{table}%t3
%\centering
\par
\caption {\label{tab:boundary_layer}Total acoustic emission from the flux sheet into the
ambient medium for different boundary layer widths.}
\begin{tabular}{lrr}
\hline\hline
Boundary-layer width & \multicolumn{2}{c}{Total acoustic emission\rule{0pt}{9pt}}\\
& \multicolumn{2}{c}{(10$^{16}$ erg~cm$^{-1}$)}\\
\hline
20~km (sharp)  &\rule{30pt}{0pt} 23.40 &\rule{0pt}{9pt}\\
40~km (medium) & 13.22 &\\
80~km (wide)   &  8.13 &\\
\hline
\end{tabular}
\end{table}