Adjusting spacing after \hline in tabul …

Adjusting spacing after \hline in tabular environment

in preamble

\newcommand\tstrut{\rule{0pt}{2.4ex}}
\newcommand\bstrut{\rule[-1.0ex]{0pt}{0pt}}

in content

\begin{tabular}{ccc}
\hline\hline 
Head 1 & Head 2 & Head 3 \tstrut \bstrut \\
\hline
a1 & a2 & a3 \tstrut \\
b1 & b2 & b3 \\
c1 & c2 & c3 \\
d1 & d2 & d3 \bstrut \\
\hline\hline
\end{tabular}

https://www.msu.edu/~harris41/latex_tablespacing.html
http://stackoverflow.com/questions/696157/latex-hline-spacing