For 2006 VMPS introduces a new breed of loudspeakers: planar hybrids featuring Constant Directivity with Frequency full-range.

Why is Constant Directivity (CD) important? What effect does it have on listening quality?
Both questions are equally important. Loudspeakers without horns radiate in a narrowing pattern as they transverse higher frequency ranges—this is a function of their effective radiating width. In most multiways, and planars in particular, each driver is much larger in diameter than the wavelength of the top frequency it must reproduce. As drivers approach this cutoff, response suffers from an undesirable phenomenon called “roughness” and their directivity narrows to the point that, once their width/diameter is larger than the wavelength of the upper frequencies, they more or less radiate in a straight forward beam, like a headlight. This reduces the “listening window” to on-axis and makes it difficult for listeners sitting away from the “sweet spot” to hear all of the music. 

The problem worsens in the crossover regions of a multiway where a large diameter driver and the smaller one above it differ in phase and dispersion causing frequency response protrusions and suckouts collectively known as “lobing”. While this effect can be reduced with increasingly higher-order crossover filter slopes, there are practical limits to implementing passive high-order networks due to poor transient response, group delay, and reduced dynamics caused by losses through the filter elements. Speakers like ours with first order filters (chosen for their optimum listening quality, not for smooth dispersion) suffer from the “lobing” phenomenon more than their steep-sloped cousins. 

What is needed for CD is for all drivers in a multiway to be the same width, and that width needs to be smaller than the wavelength of the frequency you want to hear with good or constant directivity. If you want to hear 20kHz well off axis, that means a speaker no more than 2/3” wide, down to as low in frequency as possible, where it can mate with larger diameter woofers which are already working into 2 pi (180 degree) space and are about to transition to omnidirectional or 4 pi space. In the past speakers have been made with long and narrow drivers. These tend to be tweeters, however, and virtually all exceed 1” in width, which means they will beam at around 13 kHz or lower.

All our planar speakers use a 2.5” wide midrange panel working from about 280Hz up to 7 kHz, handing off to a 7/16” wide tweeter operating to above 20 kHz. While their horizontal dispersion is very good, there is lobing in the crossover region and some wave interference in the vertical plane (vertical dispersion of long ribbons is poor, usually not much beyond the length of the driver itself). If we want Constant Directivity type dispersion then whole speaker would have to be made drastically skinnier, impossible to do without sacrificing LF extension and sensitivity. For this reason older designs have not attempted to achieve CD coverage without horn loading.

CD horns (mostly tweeters, which are the most directional) have been used in pro sound applications for at least 30 years, in order to provide high frequencies to audience members seated off-axis. A characteristic of CD horns is the 6dB/oct HF rolloff caused by dispersing the same amount of treble energy over a much wider angle than it does unaided. Therefore, much pro sound gear includes a “CD” or “Horn” filter that boosts trebles to compensate. This is known as “CD Equalization”.

The key to the new VMPS CD planars is a Constant Directivity Wave Guide (patent pending) incorporating a damped, 2/3” aperture or “diffraction slot” that takes advantage of a peculiarity of large-diaphragm planars suspended at their edges, in that they have most excursion and output in the center of the panel. Instead of pistonic motion, these planars act more like plucked strings when set in motion. We found that about 75% of the diaphragm can be masked off (with suitable absorption to prevent a lot of sound energy being reflected back into the diaphragm), leaving the 2/3” wide center to generate output. Since our tweeter is already narrower than 2/3”, it is equipped with a smaller waveguide of its own that somewhat widens it acoustically. Result: a true line source, two-thirds of an inch wide from 280Hz up with 180 degrees of dispersion.

The wave guide (which snaps on over the front baffle) eliminates lobing and line-source-to-point-source discrepancies in phase angle and wave interference. Off-axis and on-axis output is the same, and after 180 degrees horizontal spread the signal above 280Hz drops off with a sharp skirt. The only penalties are a approx. 1.5 dB SPL loss in midrange sensitivity (from the absorptive masking of most of the diaphragm) and the CD high frequency rolloff that requires some treble boost, either from the speaker level potentiometer, a 6dB/oct “treble control” found on many older preamps and receivers, or outboard active equalization from devices such as equalizers, digital speaker or room correction. Many modern preamp-processors like the Ampzilla are already so equipped.

Skeptical? Try this experiment: stand between the CD speakers (VMPS model RM 30C, $3500pr, shown with a pair of New Original Subwoofers with Amplifier, $949ea) at a spot where you would normally listen, and make a mental note of the sound quality. Then walk forward until you are almost directly between the speakers. You’ll note the tonal balance of the music changes little (what change there is comes from differences between amounts of direct vs. reflected sound where you’re standing). Finally, walk just past the plane of the speakers: sound above 280 Hz simply disappears!

How does full-range Constant Directivity sound? Glorious! Music reproduction is exceptionally smooth, clear, widely dispersed (in a manner much like live instruments, few of which are at all directional), three-dimensional, surpassingly natural and true to source. Our line of CD planars starts at $1598pr (626R), up to $16,000pr (RM/X with TRT caps).
As stereotimes.com said of the original RM30: “There’s a new sheriff in town!”.

Brian Cheney (2006)