Recording - January 2018 - 64

STUDIO 101
harmonic content at all; most of a snare's sound is noise
(random frequency content), with only the slightest hint of tonal
content. See Figure 4.
From this, you can see that the content of a sound is quite
complex-especially when we start to consider the elements
that make up its tone. It gets even more complex when we
want to record the sound, and therefore move it into the
electrical domain. Let's examine that process.

Moving sound into signal...
To record and reproduce sound, we need to move it into
a medium that can be stored. It's difficult to accurately store
acoustic information, but electrical data is another matter. To
turn acoustic pressure waves into electrical signals, we will
need a microphone.
You can think of a microphone as a great big mechanical
conversion machine: it has a diaphragm that is sensitive
enough to respond to the pressure waves of a sound, then
uses well-established techniques to turn the diaphragm's
movement into electrical signals. In a future article, we will
be discussing details about microphones that will help us
understand the differences between microphone types, but
they all perform the same function: change the pressure
waves into mechanical movement, then transform the movement into an electrical signal. This is called transduction;
mics are transducers, as are speakers (working in the other
direction-see below).
But what about signals from an electric piano, electric
guitar, or synthesizer? They obviously don't need any conversion,
because they are already electrical. How are they able to
produce sound? Depending on the device, they will either use
an electrical pickup to directly sense vibration (strings, tines,
etc.), or they will generate oscillating waveforms electronically,
or play back digital recordings of sounds (samples).

FIGURE 3

Level (%)

Square Wave Harmonics

Harmonic Number

RECORDING January 2018

FIGURE 4

...and back again
What exactly is the electrical signal that is generated by
a microphone or by a synthesizer? It's a signal whose voltage varies in the same way that the sound's pressure wave
changes. In this way, we have a simulation of the sound in
an electrical circuit. This will become important next month,
when we look at the digital side of things; in the meantime,
just think of sounds as varying waves of electrical voltage
instead of pressure waves in air.
As for turning electricity back into sound waves, it's quite
simple, because a lot of the efforts of electrical engineers focus
on changing electricity into movement. That's the function at
the heart of motors, relays-and speakers. A speaker works
by having a diaphragm move in and out, mirroring the changes
in air pressure that a microphone originally captured.
So now you can see how all this works-but you can also
probably imagine all the inefficiencies of moving sound
to electrical signals and back into sound. Any inconsistent
behavior in a microphone's diaphragm, the electrical storage medium, or a speaker's transducer can translate into
significant alterations of the sound. This process is why we
find some reproduction systems better than others, and why
engineers are constantly striving to improve the technology
behind microphones, signal processors, recorders, amplifiers,
and speakers.

Conclusion
And there we are: we now have a basic understanding of
how sound works, and the process of audio conversion into
an electrical signal. This information will be key to making
good decisions in our recording practice-and to make sense
of some of the technical issues (and documentation details)
we'll confront as we learn more.
What we've learned thus far will help us gain an understanding of microphones and speakers, but there is still the
issue of storing the audio signal-and of digitizing it for use
inside our computers. This is the content of next month's entry,
where we will dive into storage media-from vinyl to SSD-
and the voodoo of digital audio. See you then!
Darwin Grosse (grosse@recordingmag.com) is a recording
engineer, author, educator, music software designer, and
podcast producer. You can listen to his weekly interviews with
electronic music technology professionals at artmusictech.
libsyn.com.

http://www.artmusictech.libsyn.com http://www.artmusictech.libsyn.com

Table of Contents for the Digital Edition of Recording - January 2018