Take Me to Your LEDR!
Wouldn't it be nice if we had a test to set up and check a stereo system for correct imaging? Yes, Virginia, there is an imaging test—your LEDR is here (footnote 2). LEDR, which stands for "Listening Environment Diagnostic Recording," is available on a test CD from Prosonus, the Studio Reference Disc (footnote 3). The CD is rather expensive (about $50), but there is no other official test for imaging. [Since this article was published, a LEDR track was included on the first Chesky Records Jazz Sampler & Audiophile Test CD (JD37).—Ed.]
To use LEDR, you only need one set of test instruments: your ears. LEDR consists of a series of computer-generated sounds that are intended to move in a predefined way between a pair of loudspeakers. A sampled cabasa (a percussion instrument often used in Latin music) is electronically manipulated to move through three different paths. The paths are Up, Over, and Lateral. The differences between what these signals are designed to do, and what your system actually produces, represent a measure of the imaging accuracy of the stereo speakers, electronics, and the room environment.
Playing LEDR on several systems, including a bargain-basement bookshelf model, is an ear-opener. LEDR unfailingly reveals the astounding variations in imaging accuracy among the systems. More importantly, LEDR is an objective tool for testing and adjusting loudspeakers and room acoustics that requires no test instrumentation. If a stereo system corrupts or distorts one or more of the LEDR paths, that system cannot be considered accurate. While listeners may like the sound of such a system, they may not pronounce that sound to be accurate. But don't be scared; your audiophile-quality system will probably pass the LEDR test. There is positive correlation between LEDR results and systems already known to produce good sound.
Of course, there is more to good sound than imaging accuracy. Recently a friend reported he had to give a "LEDR grade" of about 50 to his system, consisting of Quad loudspeakers, two Janis woofers, and electronic crossover; that grade improved to about 90 when playing the Quads alone (a testimony to the resolving power of LEDR). But he preferred the superior tonal balance and dynamic range of the bi-amplified system over the basic Quads. It's a matter of choosing your poisons. By the way, bi-amplifying does not necessarily degrade imaging, it is just more likely without careful attention to phase and amplitude response, crossover and driver design, and electrical and physical alignment.
Who's behind our LEDR?
The LEDR signals were generated by Northwestern University's Spatial Reverberator, a software program running on an RISC mainframe computer. The spatial reverberator was designed by researchers Gary Kendall and Bill Martens of the Computer Music Department, while the trademarked LEDR test signal is the brainchild of acoustician Doug Jones. These psychoacousticians have been researching what are called "pinna transforms," the way in which the shape of the head and outer ear permit us to hear signal direction. By programming the filter characteristics of the pinna transforms into the spatial reverberator, the Chicagoans can literally move sound around the room, even behind the listener, using only a single pair of loudspeakers placed in front of the listener (footnote 4).
Grading the LEDR paths
The first path, Up, will amaze your friends and quiet your enemies. It's hard to believe that a sound can appear to travel from a loudspeaker up to 6' above the speaker! This path is generated first in the left speaker, then in the right. The sound should begin at about eye level and then travel as straight as possible up in the air about 6'. You should grade the system on how vertically straight the path is, how high the image goes, if it is continuous (unbroken), whether it approaches or recedes from the listener (it should not), and if the left and right paths are symmetrical.
I like to call the second path the "Rainbow," but Doug officially calls it the Over path. The sound should begin at one speaker and travel in a smooth arc to the other speaker, from left to right and then returning. The top of the rainbow should be as high as the previous Up signal (about 6' above eye level). Judge the Over path by how smooth, continuous, symmetrical, and rainbow-shaped the arc is.
The last path, Lateral, tests left-to-right stereo imaging. This consists of four elements.
First, the sound moves from left to right, between the acoustic centers of the speakers. Since a speaker's acoustic center may not be its physical center, you should use the first Lateral test to adjust your speakers until the sound traverses a 60 degrees angle from the listener's point of view. Second, the sound moves from beyond the right loudspeaker to beyond the left (about 1' out from acoustic center). The next two signals are the mirror image of the above; third, from right to left speaker, and fourth, from beyond the left to beyond the right.
Again, grade your system by how straight, continuous, and symmetrical this path is. Grade the beyond path by how far out from the speaker it appears to go (about 1' to the left or right of the requisite speaker, according to Doug Jones), and that it does not approach or recede from the listener.
Footnote 1: Metaphorically speaking, we are what we hear.
Footnote 2: LEDR, pronounced "leader," is a trademark of Electro-Acoustic Systems, Inc.
Footnote 3: LEDR is track 51 on the Prosonus CD. Most of the other tracks on the disk are designed for professional studio purposes, but a couple of the other tests are suitable for home use without instrumentation. Track 50, the ASC "Music Articulation Test," by the people who make Tube Traps, tests for low-frequency resonances and rattles. Track 52 tests for inter-speaker polarity, and track 48's filtered pink noise can be used to set up a subwoofer system.
Footnote 4: See Peter Mitchell's "Industry Update" column, July 1989, p.55.