Beating waves

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% Author: Izaak Neutelings (December 2020)
\documentclass[border=3pt,tikz]{standalone}
\usepackage{amsmath}
\usepackage{etoolbox} % ifthen
\usepackage{tikz}
\usetikzlibrary{arrows.meta} % for arrow size
\tikzset{>=latex} % for LaTeX arrow head
\colorlet{xcol}{blue!70!black}
\colorlet{vcol}{green!60!black}
\colorlet{myred}{red!80!black}
\colorlet{myblue}{blue!80!black}
\colorlet{mypurple}{blue!50!red!80}
\colorlet{metalcol}{blue!25!black!30!white}
\tikzstyle{vvec}=[->,vcol,very thick,line cap=round]
\tikzstyle{node}=[xcol,scale=0.8]
\tikzstyle{metal}=[draw=metalcol!10!black,rounded corners=0.1,
  top color=metalcol,bottom color=metalcol!80!black,shading angle=10]
\tikzstyle{ring}=[metalcol!20!black,double=metalcol!70!black,double distance=1.2,line width=0.3]
\tikzstyle{rope}=[brown!20!black,double=brown!70!black,
  double distance=1.2,line width=0.6] %very thick,line cap=round
\tikzstyle{wood}=[draw=brown!80!black,rounded corners=0.1,
  top color=brown!80,bottom color=brown!80!black!80,shading angle=10]
\def\tick#1#2{\draw[thick] (#1) ++ (#2:0.1) --++ (#2-180:0.2)}
\begin{document}
\def\xmax{6.48}
\def\ymax{0.9}
\def\A{0.78}   % amplitude
\def\s{4.7*\A} % shift
\def\N{400}    % number of points for plot
\def\fa{1.775} % wavelength 1
\def\fb{2.20}  % wavelength 2
\def\waves{
  % TWO WAVES
  \draw[->,thick] (-0.2*\ymax,0) -- (0.4+\xmax,0) node[right=4,below left=1] {$t$};
  \draw[->,thick] (0,-\ymax) -- (0,0.2+\ymax) node[below=2,above left=-3] {$y$};
  \tick{0,\A}{0} node[scale=0.9,left=-1] {$A$};
  \tick{0,-\A}{0} node[scale=0.9,left=-1] {$-A$};
 
 
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