【簡譜兩條下劃綫可能的畫法】

\documentclass[12pt]{article}
\usepackage{tikz}
\usepackage{amsmath}
\begin{document}
\newlength{\musiclen}
\settowidth{\musiclen}{1155}
\noindent
\makebox[0pt][l]{1155}%
\makebox[0pt][l]{\rule[-0.3ex]{\the\musiclen}{0.4pt}}%
\rule[-0.5ex]{\the\musiclen}{0.4pt}
\end{document}