# Block with two inputs in pgf/TikZ

I am trying to draw the following block diagram using pgf/TikZ. Actually I am struck at getting the two inputs to the first block horizontal.

What is the best way to draw the above diagram using pgf/TikZ.

-

One possibility:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[box/.style={draw,rounded corners,text width=2.5cm,align=center}]
\node[box] (a) {Psychoacustic Model};
\node[left=of a.170,font=\bfseries] (aux1) {Reference};