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.