Part 2 of A Four-Part Series on Mono – Got half-a-dollar? Want one?

In the first part of this
four-part series, I described how playing mono (also called “monophonic” or
“monaural”) through your stereo system can provide you with tools and tests to
help “dial in” your system and make it sound better than ever before. I also
told you how your system can be set up for mono playback. In this part, I’m
going to tell you about one of those tests and how and why it works.

The first (and
surprisingly most important) test for your system and your room is to just sit
down in your favorite spot, play something in mono, and listen to it. What does it
sound like? How big is it? How wide? How tall? Once you’ve noted these things, change
to a different listening position and listen again, asking all those same
questions. Do it again and again, as often as you need to.

For the theoretically
perfect system playing mono from two identical speakers in the theoretically
perfect listening room, the total sonic “image” presented should be about the
size of a U.S. half dollar coin (3cm), and should be centered at ear-level,
directly in front of you, regardless of where between the two speakers you may
be sitting. Anything other than that – anything wider, higher, lower, or not
directly in front of your listening position indicates something wrong with
your system, your room, or the placement of your speakers.

The reason that a stereo
recording (played back in stereo) has dimensionality — right, left, high and
deep — is because it provides different information from its two recorded
channels. During the recording session, different instruments or performers are
located at different distances from the (at least two) recording microphones and
their sound arrives at those microphones at different times, at different
relative volume levels, and in different phase. When the stereo recording is
played back in stereo, those differences are reproduced by your speakers, and
that’s what allows you to perceive a more or less accurate image of the
locations of the performers and instrument and even the size of the room the
recording was made in. With mono recording or playback, those relative
differences don’t exist:  When played
back, all of the sound from each speaker (assuming identical speakers and
playback electronics for both channels) is identical in time, phase, and volume
to that from the other speaker, and except for the acoustic effects of your
playback listening room, the only differences between what you will hear from each
of the two channels will be differences caused by the distances of your
listening position from both of your two speakers.

That’s why the
“half-dollar” effect: The way our ears work, if we are presented with two
identical sounds arriving at our two ears within
less than about 50 milliseconds of each other, we will hear those two sounds as
being just one (the one arriving first), and we will “locate” and “size” the
source of that one sound based on a combination of the two actual arrival times
and the two relative volume levels at our listening position. Considering that
50 milliseconds at the speed of sound at sea level is equivalent to a distance
of about 55 feet; that the average system’s speakers are placed some six to ten
feet (1.8 to 3 meters) apart; and that the average “sweet spot” listening
position is usually about that same distance (1.8 to 3 meters) from the plane
of the speakers, it’s easy to calculate that delays in arrival time at any
likely listening position will be well under 50 milliseconds and that differences
in relative volume level may be just a small fraction of one decibel.

The result is that — in mono,
in a “perfect” listening room, with “perfect speakers”, perfectly placed — the
differences are so small that we hear everything – even a massed chorale or a
120 piece full symphony orchestra – as coming from one single “point” source, at
ear level, directly in front of wherever we may be sitting, sounding like it’s
about the size of a large coin.

Have
you tried setting up for mono yet? Have you listened? Write-in, please, to let
me know what your system sounds like in mono, and in the next part of this
series I’ll tell you one of the things you can do to make it better.