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In the listening room set up we have a pair of speakers whose centers are between 6 and 8’ apart.  The listening position is between 8 and 10’ back from the plane of the speakers.  This is called the listening footprint.  The speakers are elevated so the “acoustic center” of the speaker is the same height off the floor as the listener’s head height.

The speakers are toed-in to align onto a point that is located one foot behind the listener’s head.  Because of their directivity, the speakers send a widespread signal into all parts of the room.  Only one narrow beam of this, about 0.0001% of the whole sphere, the “direct signal”, impacts the listener’s ears.  The rest illuminates the listening room.

The right speaker primarily illuminates the right ear while the left speaker illuminates the left ear.  The differences between these two signals determine the imaging and creates the sound stage and the images within it.  The sound stage sweeps between the speakers and down both sides almost as far as the listening position.  If both speakers play exactly the same track, a mono stereo signal, the sound stage becomes a single spot, front and center.

Here is what the speakers look like from the listening position and the large center dot indicates the height and center position of the sound stage. It is at ear level, as is the acoustic center of the speakers.

If the speakers are located too far apart the sound stage widens and seems to gather itself around each speaker, leaving a ”hole” or less bright center stage image.  Facing this setup the listener might want to scoot their chair forward to get closer to the center sound stage but that would only create an even deader center mage.  As the listening position moves back the sound stage begins to take on the desired form.

 

If the speakers are set up too close together the sound stage seems to collapse in the middle, more like a large center stage image.  The listener might want to scoot their chair back to get further back to see a wider sound stage, but that would only make the center stage image even more centralized.  Moving the listening position closer to the speakers will widen the sound stage.  It’s not so much about close or far, seeing the sound stage is about the angles between the listener’s head and the speakers.

When the angles are correct the sound stage of a typical small musical group will be well distributed across the stage, similar to what you would see at a concert of the group.

Recording engineers mix their tracks to create the sound stage.  There are certain arrangements of microphones that give a great stereo image, typical for this is a Blumlein pair, two ribbon mics held in line with each other except one is rotated a quarter turn relative to the other.  This mic has a figure 8 polar pattern.  A center stage image is created when the same signal is delivered to both speakers.  This is essentially when a mono signal is split and sent to both speakers.

Left of center is created by sending the same signal to both speakers except the left speaker is set a little louder than the right speaker.  The louder the left speaker is the farther the center image is moved to the left.  Naturally, if the right speaker is louder then the image moves to the right.  This is called loudness panning of the image.

Delay panning is another way to move the image.  Keep the volumes the same but add a short delay, ½ ms, to right speaker.  This means the left speaker is heard first and whichever signal arrives first is where the sound is coming from, according to our ears.

Here’s an interesting image position trick.  Split the incoming signal.  Use one part to create a center image.  Take the other line, add a 6 ms time delay to it and adjust the level down by 2 or 3 dB.  Add that back into the main signal path.  The center image will move back.

Let’s use all of these image positioning devices to  move a center image stage left and a few steps back.  We have a center image to start with so let’s move it straight back by mixing with it a 6ms time delay that is 3 dB down.  Now center image moved back a few steps.

Next we want to move things to the right a bit.  We increase the loudness of the right speaker or reduce the loudness of the left speaker or a little of both to scoot the image off center and to the right.  Or we could have added a short 1/2ms delay to the left speaker so the right speaker gets heard first and that means the images is coming from a direction to the right of center.