All About Sound: The Science of Imaging

When we hear sound, only some of it reaches our ears directly. The rest we hear after it reflects off of room surfaces.

Imagine listening to a saxophone in a club. Direct sound gives us the ability to locate that sax in the center of the stage, for example, in front of the drummer, with a guitar on one side and a bass on the other.

Now imagine listening to a singer in a concert hall. Reflected sound provides us with information about the acoustic environment in which we hear the voice (for example, a large hall with a great deal of ambience).

Speakers have to recreate both direct and reflected sound. Direct sound provides localization cues — it helps our ears identify the location of individual instruments, voices and sound effects within the soundstage. Reflected sound provides spaciousness — a sense of acoustic environment (or of the "virtual" sonic environment created in a recording studio).

Moreover, the balance of direct and reflected sound is critically important Too much direct output results in a narrow soundstage. An over-abundance of reflected sound makes it difficult to localize individual voices, instruments and effects. An accurate balance provides a sonic image so real it can make us forget we're listening to speakers.

Here are the general imaging characteristics of various types of front speakers (assuming typical "living room" acoustics). Keep in mind that speakers with higher levels of coloration will sound worse and image even more poorly.

Wide-Dispersion Speakers (dynamic, multiple-driver "point-source" designs)

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)

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)

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)

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