Gradient Revolution loudspeaker
The perceptual effects of early reflections and standing waves in small rooms are still being actively investigated. For example, the Archimedes project of the late 1980s—which involved a partnership between the Acoustics Laboratory of the Technical University of Denmark, KEF of England, and Bang and Olufsen of Denmark—was primarily concerned with evaluating the influence of room acoustics on the quality of reproduced sound. Inevitably, we hear the room's response function (or, should I say, colorations) superimposed over the loudspeaker's anechoic response.
The next revolution in loudspeaker design would then center around the use of such research to optimize loudspeaker performance for domestic listening environments. I'm sure you've seen anechoic-response measurements touted by various manufacturers as proof of excellence. Unfortunately, a flat anechoic response is almost always wrong for the average audiophile listening room—especially in the bass, where room gain is a given. In defense of designers, it's fair to say that most have little control over speaker placement, the type of room in which their product will be used, choice of listening seat, and acoustical treatment. However, it is possible to facilitate the integration of the loudspeaker into the customer's room by properly controlling its directivity. Gradient's Jorma Salmi strived to do just that with the Revolution.
Salmi's goal was to design a loudspeaker that was less room-dependent than ordinary speakers are. Careful attention was paid to the origin of standing waves in small rooms, with the idea of minimizing the generation of standing waves. He also sought a wide listening sweet spot that was void of early reflections.
Frequencies above 200Hz are reproduced by a concentric midrange/tweeter driver designed and made in cooperation with SEAS in Norway. The concentricity of the drivers approximates a single point source of sound, and ensures excellent coherency under nearfield monitoring conditions. A 1" aluminum-dome tweeter is mounted at the apex of a 6.5"-diameter fiberglass cone, so the midrange diaphragm acts as a wave guide to control treble dispersion. Gradient points out that an inevitable drawback of this arrangement is on-axis roughness in the treble response.
The Revolution's mid- and treble drivers are housed in a separate triangular enclosure that sits atop the bass unit, connected to it by an umbilical cord and 2-pin XLR connector. Depending on which way the XLR is plugged in, it's possible to connect the woofers in- or out-of-phase to the midrange. The proper polarity is determined by the orientation of the woofers. With the woofers aimed toward the listener, the correct connection is in-phase.
A cardioid radiation pattern was selected as a design goal for the mid/treble drive unit. Such a pattern has almost no rear radiation, and limited side radiation, so it's ideal from the standpoint of reducing the levels of early rear and side-wall reflections. Such a controlled directivity is achieved through the use of an "acoustic resistance" enclosure. Resistively loaded slots in the sides of the enclosure are designed to allow some interference between the front and back waves of the driver to yield the desired contouring of the radiation pattern. The resultant cardioid pattern's front-to-back attenuation is 20dB.
In small rooms, bass response below 200Hz is dominated by standing waves, or stationary sound-pressure waves established between opposing room boundaries—ie, between the floor and ceiling, and between walls. The frequencies at which standing waves occur (known as room modes) are determined by the dimensions of the room. At these frequencies, at certain locations in the room, sound-pressure peaks and dips of 20dB or more may occur. Standing waves are also slow to decay after the music signal has stopped. The end result may be an uneven response, bass boom, and a loss of clarity.
Ordinary box speakers, whether closed or vented, radiate bass information evenly in all directions. In other words, they exhibit an omnidirectional radiation pattern in the bass region, and hence excite all available room modes in three dimensions. To control standing waves, the Gradient's woofer system uses a dipole radiation pattern. Two 12" long-throw woofers (the same drivers as those used in Gradient's SW-63 subwoofer for the Quad ESL-63) are connected in parallel, and are mounted on an open-baffle bass enclosure. The resultant figure-eight sound pattern, which is essentially bi-directional, minimizes the formation of standing waves between side walls and between the floor and ceiling.