Some Audio
Fundamentals
While most of the language of audio is quite specific, one
area where vagueness reigns is the way we refer to the whole subject. Practically
everybody uses such words as "sound," "audio," "stereo," and
"high fidelity" (or "hi-fi") more or less interchangeably. In fact,
all these terms have very specific meanings, although they all apply to virtually every
component on the market today.
Sound properly exists only in air, as a series of
pressure differences. We can hear these directly, as our eardrums move in step with the
compressions and rarefactions. The various devices used to store, retrieve, alter,
reproduce, or move from one point to another what started out as sound is audio
equipment, whether in the recording studio or in our listening rooms. It includes purely
electronic devices such as amplifiers and FM tuners, electrodynamic transducers such as
microphones and speakers, mechanical turntables and tape transports, and computer-derived
compact disc players. The word audio comes from a Latin root meaning "to hear."
The Greek word for "solid" gives us stereo (or
stereophonic). In our grandparents' day, "stereoscopic" photographs were
popular, which produced three-dimensional images by means of a pair of pictures taken from
slightly different angles, just as our eyes would see the original. This gave an illusion
of depth and solidity, and the term was borrowed by the audio world when multichannel
sound reproduction became a reality because a similar sort of sonic effect was created.
For most people, stereo has come to mean two-channel sound;
in reality, any number of audio channels (other than one) is stereo. Single-channel audio
is termed monophonic (more commonly, simply mono) or, less correctly, monaural.
A system with more than two channels is usually called multichannel (although two
is really "multi" as well); because multichannel systems usually include at
least one signal designed to be heard from beside or behind the listener, it is normally
referred to as surround sound.
In common early days, stereo was often thought of as the
improved version of high fidelity (or hi-fi). Actually, this term refers
mainly to the quality level of the system, compared to earlier equipment. It is entirely
possible to have a system that is stereo but not high fidelity, just as a mono signal may
be hi-fi. The two terms usually do go hand-in-hand, however: virtually all of today's
audio equipment is both stereo and hi-fi.
Every musical instrument produces sound made up of
two parts: the fundamental, determining its pitch (the note it is playing), and a
series of mathematically related harmonics or "overtones," which give the
instrument its particular character or timbre. In a live environment, this complex
wave radiates outward, bouncing off or being absorbed by anything that gets in its way,
until its energy is finally used up.
An orchestra consists of many such complex sources, each
radiating its own sound pattern, each with a different point of origin. As all these sound
waves meet, they influence each other: where two compressions cross they reinforce each
other to make a larger compression; where a compression meets a rarefaction, they cancel
each other partially or totally, depending on their respective amplitudes. The interaction
of all these waves, both direct and reflected, is extraordinarily complex, but at any
given point within the acoustic space, at any moment, there is only one pressure level:
the net total of all the sound waves reaching it at that instant.
The combination of reinforcement and cancellation of the
different frequencies at that point produces an extremely complicated pressure pattern
over time, but if an eardrum happens to intercept it, the human brain has no trouble
sorting out all the elements into individual instruments. A microphone placed at the same
point will convert the pressure variations into a constantly changing electrical signal;
when this is reconverted into pressure waves by a loudspeaker, we are still able to
identify violins, oboes, human voices and so forth, even though they have all been
combined into a single electrical signal.
Nor need this combination of information from many sources
happen acoustically at the location of the microphone. Signals from two or more
microphones or other audio sources can be mixed electrically with the same result: when we
hear the finished product we are still able to pick out the individual instruments.
In a real acoustic space, the pattern of sound is never the
same at any two points; the various waves arrive at slightly different times, and with
slightly different intensities. Because our ears are spaced a few inches apart, we can use
these differences both to hear a spacious, three-dimensional sound, and to locate specific
sound sources. The use of two or more independent channels in a stereo system allows some
of this information to be heard in the home.
...Ian G. Masters
ian@mastersonaudio.com
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