A U D I O... P A P E R


The RADIAL Loudspeaker

by Steve Deckert
Jan. 2001


OVERVIEW

The Radial Speaker was originally inspired by the Ohm Walsh bending wave speakers that radiate sound 360 degrees. Having listened to these many times, as well as the German speakers using bladders, and other 360 degree designs I observed that despite their flaws they all did something special.

To understand the Radials you have to understand some basics about listening room acoustics.

On conventional speakers the ultimate goal is to hear only the direct energy from the speaker itself and not the reflections of your room. If this can be achieved, a coherent and believably holographic soundstage will result.

In order for our brain to perceive a focused image there must be both a delay as well as a reduction in volume that distinguishes the direct energy from the reflected energy. If these reflections are not delayed by at least 50 milliseconds AND reduced in volume by at least 15 dB or more, the brain can't tell the difference between the two. This results in a poorly rendered sound stage with less focus.

In a conventional home listening space using conventional speakers these reflections are not delayed or reduced in volume enough. The direct energy passes you ear and hits the back wall of your room where it refracts to the side wall and then reflects off the front wall where it comes past your head a second time. To correct these reflection points properly, a series of quadratic diffusers must be installed on each of the four walls.

These diffusers work by taking a beam of energy such as that emitted by your loudspeaker and phase grating it out to a hemi-disk pattern. This busts the beam and reduces its energy. One second of sound will create over 500 reflections in your room so having diffusion on all four walls compounds (actually squares) the effect.

You can then see why conventional speakers including panel speakers and bipolar designs sound so unpredictable from room to room. We have developed serious room treatment products to correct these problems, and unlike little pillows and tube traps, these products add 2100 lbs of mass to a typical listening room. Believe it or not this is what it takes to completely correct a 15 x 13 room.

You will notice, or probably have already, that in untreated listening rooms the louder you play a system the more these room reflections screw up the imaging and focus, not to mention frequency balance changes caused by nodes of standing waves which result directly from these reflections.

This goes against the tendency for raised listening levels to add more weight, presence and detail to a recording. So you have two things working in direct contrast with each other when you raise the volume. Add to that the fact that nodes in your room are so numerous that moving your head as little as one inch can actually change the frequency balance. It's no wonder you system sounds different from time to time since all of these variables combine to determine the actual frequency balance.

In an untreated room using conventional speakers it is not uncommon to have nodes creates peaks and dips in the frequency balance that are as high as 12 or even 15 dB.

Using a speaker like the Walsh for example these problems are reduced from about 2000 cycles on down by the simple fact that the energy from the speaker is already diffused to a point where room reflections are only a minor problem. However, these and other 360 degree speakers have also been radiating the high frequencies at a 360 degree pattern.

I have found that for holographic imaging you do NOT want frequencies above 2000 cycles to be shot all over the room. Since high frequencies are much narrower and therefore beamy to begin with, they travel around the room with their original pattern or beam intact many times more than lower frequencies. This creates a new problem in untreated rooms that is actually more severe than a conventional speaker. The result from 2000 cycles on up is a soundstage with poor focus and a lack of consistency.

Bi-polar speakers or speakers with rear firing auxiliary tweeters actually create an artificial ambience that many interpret as actual space in the recording. These types of speakers compound reflection issues.

The Radial speaker is designed with an inverted cone that has a tailored dispersion with a natural shelf in the response at 2000 cycles. Therefore it can be mated with a tweeter without the use of any crossover. This alone adds tremendous coherency and dynamics to its performance. Above 2000 cycles is a delicate blend of ambient reflected energy and direct energy from a single point source.

The dispersion pattern of the main driver is also designed to use the ceiling to double the size of radial wave fronts in the mid band frequencies. This projects an image centered between the top of the Radial speaker and the ceiling resulting in the correct image height.

The way these radial wave fronts react with the typical problem reflection points in a room is by eliminating the points. The energy is already diffused in wide hemi-disks when it leaves the walls in almost exactly the same pattern created by our quadratic diffusers.

The actual radial drivers used in this speaker utilize a top cone with less than 6 grams of mass and a very tight compliance. This lightweight cone has incredible speed and detail far exceeding conventional audiophile woofers with the exception of perhaps a Lowther driver. There is also a higher-mass cone working off the same motor assembly that gives the driver a usably low frequency response.

The cylindrical enclosure is diffraction-free making it completely invisible to the ear. Its small size of 30 inches high makes it easy to move. It uses an open bottom design that couples to the floor for enhanced low frequency response and can be fine tuned. Usually placed on carpeting, the weight of the enclosure against the height of the feet create a varo-vent for wide band damping. Adjusting the space between the carpet and the enclosure bottom will tailor the low frequency response to your room acoustics and taste.

The tweeter used is a very small Audax poly dome sputtered with titanium. It has a balanced 3/8" drive where the mass of the diaphragm and voice coil is evenly split. This particular tweeter has incredible spatial qualities and is perhaps the fastest dome tweeter I've ever heard so it mates well with the Radial driver.

Listening tests found these to be far and away the best imaging speakers with the most accurately rendered soundstage we've heard to date. Even placed only 4 feet apart they created a soundstage that exceeded the width of the room itself. Depth is seemingly boundless. The height is wonderful, but unlike some speakers with good height to the sound stage these were true to the recording. Instead of artificial height that works on everything, these speakers accurately portray the height of the original recording space.

By design, these speakers sound better when you're sitting down and at a reasonable distance back. Listening test confirmed this. Usually in smaller rooms standing up creates a more dimensional perspective on the soundstage adding to the depth. This is due in part to the comb filter effect of your floor. With the Radials there is almost zero comb filter effect. All of our other speakers including panel speakers are less than tolerant of the sitting position, and all give the best results when sitting nearfield on the edge of your seat... a less than relaxing posture. These speakers gave the same thrill of being "in the sweet spot" almost anywhere in the room at a distance of 6 feet or more.

Comparisons with our Magnepans found the Radials to have better detail, better top end extension and more spank at lower listening levels. They have none of the characteristic boxy sound of box speakers, so like the panels the frequency balance is very well behaved.  Comparisons with our Acoustat Electrostatic Monitors and the RL-2's we find the frequency balance to be the same, with no apparent seam between woofer/tweeter.  The imaging was actually better doing a better job of making the walls disappear.

With our Zen Triode amplifiers, the Radial speakers resolve more information in the recordings than any other speaker we've tried to date.

Power handling on the motor assembly is very high, however the lightweight delicate cone used to create the radial wave fronts limits the power handling to sane listening levels. A stiffer cone with a more compliant suspension would eliminate this restriction but at the cost of linearity, speed, and intermodulated distortion. We feel strongly that the trade-off is MORE than justified. So remember, these speakers are delicate and dynamic but not meant for excessively loud playback levels in excess of 100 watts.

The end result seems to be a small speaker that is room-friendly and creates a very LARGE lifelike sound stage with about 3 watts.

The Radial project started in 1996 as part of our room acoustic tests that helped developed our Room Acoustic Products. I posted the initial prototype white papers in our "Wild D.I.Y. Projects" section of the web site. You may find more in-depth technical information on how these speakers work in a paper I wrote at that time called "Direct vs. Reflected Energy"

Additional information can be found in these original documents.

 

 

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