en: Fix some errors and typos

* src/en/sections/music-and-technology-synthesis-rhythm.tex,
src/en/sections/music-and-technology-synthesis-sound.tex,
src/en/sections/music-and-technology-synthesis-speaker.tex: Fix some errors and
typos.

src/en/sections/music-and-technology-synthesis-rhythm.tex

@@ -7,8 +7,8 @@
 \section{Rhythm theory}
 \index{Music!Rhythm}
 
-Our way to the music starts from the discussion of the \emph{rhythm theory}.
-Most of us know that the rhythm is -- in many people it produces the reflex of
+Our way into the music starts from the discussion of the \emph{rhythm theory}.
+Most of us know what the rhythm is -- in many people it produces the reflex of
 head nodding to the rhythm, or mechanical finger tapping on a tabletop to the
 rhythm beats.
 
@@ -33,17 +33,17 @@ \subsection{The \emph{musical bar}}
 impression on the listeners.  So here we are discussing the topic with some
 assumptions.}
 
-On the fig. \ref{fig:music-six-bar} we can see how could a musical composition
+On the fig. \ref{fig:music-six-bar} we can see how can a musical composition
 consisting of six bars look.
 
 \figureMusicSixBars{en}
 
 The length of one bar in time is determined by the tempo of the rhythm, and we
-will discuss this later.  For now, we can imagine that one one bar always takes
-one abstract unit of time.  We can substitute this unit of time with any
-convenient time duration -- such as one second.
+will discuss this later.  For now, we can imagine that one bar always takes one
+abstract unit of time.  We can substitute this unit of time with any convenient
+time duration -- such as one second.
 
-Those bars are divided into smaller parts that used as cells to lay down
+Those bars are divided into smaller parts that are used as cells to lay down
 different sounds.  Great part of music is interweaved with mathematics, and the
 first mathematical thing we face is the simple fraction.  One of the popular
 ways to divide a bar is to use $\frac{4}{4}$ -- or ``four-four'' time.  In this

src/en/sections/music-and-technology-synthesis-sound.tex

@@ -104,8 +104,8 @@ \section{Sound}
 \end{minted}
 
 The first step here is to calculate the wave oscillation period (in
-microseconds) from by dividing a second (specified in microseconds) by the
-specified frequency:
+microseconds) by dividing a second (specified in microseconds) by the specified
+frequency:
 
 \begin{minted}{cpp}
   const long T = 1000000 / f;

src/en/sections/music-and-technology-synthesis-speaker.tex

@@ -14,8 +14,8 @@ \section{Speaker connection}
 article in the Wikipedia.}
 
 The circuit for a regular speaker and a piezoelectric speaker is similar; for
-the our tasks even simple piezoelectric speaker ``for Arduino'' is good enough.
-Or we can use s simple beeper from a PC system board.
+our tasks even simple piezoelectric speaker ``for Arduino'' is good enough. Or
+we can use a simple beeper from a PC system board.
 
 The schematic for the speaker connection is shown on the
 fig. \ref{fig:sound-fig-2}.
@@ -32,21 +32,21 @@ \section{Speaker connection}
 speaker in \texttt{OUTPUT} mode and add a call to our \texttt{play\_tone}
 procedure in the \texttt{loop}.
 
-For the convenience we will specify our port for the speaker as the
+For our convenience we will specify our port for the speaker as the
 \texttt{SPEAKER\_PIN} constant at the top of our program (before the
 \texttt{setup} procedure.)
 
 Now we can generate a simple sound with the specified frequency.  But if we want
 to generate something more interesting (a melody, for example) we have to use
 sounds from the musical range.  In this case we have to get to know at least
-some basics of the musical theory.
+some basics of the music theory.
 
 \subsection{Exercises}
 \begin{enumerate}
 \item Generate a sound with the frequency of 261.63 Hz.
 \item Try to make the sound switch between 261.63 Hz and 349.23 Hz once a
   second.
-\item Modify your program in such a way that to allow to change the sound
+\item Modify your program in such a way that it will allow to change the sound
   frequency with a potentiometer.
 \item Allow to switch the sound on and off by pressing a button connected to an
   Arduino.