All About Sound: The Science of Imaging

An important part of the concept of dispersion (the lifelike radiation of sound from a source, either an instrument or a speaker) is the idea of "imaging."

Imaging is what we call our aural ability to pinpoint the location of specific elements of a soundscape. When we hear a saxophonist taking a solo on a small stage, we can close our eyes and our ears can locate the sound exactly where it originates, even among the other instruments on stage. When we hear a singer belting it out in a concert hall, we can close our eyes and our ears can sort through the ambient sounds of the expansive hall around us and pick out the location of the singer down there, center stage.

It's important for a loudspeaker to deliver a wide, expansive, ambient-enhanced dispersion pattern. It does this by smoothing the crossover slopes between the midrange and high frequency driver responses, to avoid constricting "beaming" effects.

But it's also important for a loudspeaker to present a clear and detailed high frequency performance, because it is those high frequency details that best cue our ears to a sound's originating location.

The demands of "imaging" are why they say that the hardest working speaker in your home theater is your center channel speaker. It must radiate not just a wide, lifelike dispersion pattern to encompass everything happening on your screen, but it must also deliver precise high frequency detail so that you can locate voices and effects on and off the screen space. It's a critical job to support the illusion of reality.

Another element of imaging is speaker placement. You know that the center channel speaker is placed front-and-center in your home theater, to anchor the image on the screen. But different types of loudspeakers, designed with drivers in various arrays, have different imaging and dispersion characteristics depending upon their placement in a room:

Wide-Dispersion Speakers (dynamic, multiple-driver "point-source" designs)
Placement: Direct and reflected output can be balanced for relatively easy room placement.
Spaciousness: good to excellent
Localization: good to excellent

Highly Directional Speakers (horns, rear-absorbed electrostatics, ribbon panels)
Placement: With more direct than reflected sound placement is usually easier. Electrostatics and ribbons with wider horizontal dispersion require greater care in placement.
Spaciousness: fair to good
Localization: fair to excellent

Multi-Directional Speakers (bipolar, reflecting)
Placement: Because there is more reflected sound than direct, room placement requires more care. Bipolar speakers must be placed well away from listening room walls.
Spaciousness: good to excellent
Localization: fair to good

Limited-Dispersion Dipole Speakers (dynamic, planar or electrostatic dipoles)
Placement: Room placement is critical. These speakers must be specifically positioned so that the reflected rear output does not cancel the front output.
Spaciousness: fair to good
Localization: fair to excellent