Loudspeaker Dispersion: Designing for Optimum Dispersion
In my December 2003 column, I dealt with the challenges associated with loudspeaker dispersion. Desirable because it improves the stereophonic illusion and the depth of the sound throughout the entire room, wide dispersion prevents the development in areas of the room of poor sound. In short, the wider the dispersion, the better the sound quality in a real-room setting. Unfortunately, achieving this ideal state is fraught with difficulties. In this article I discuss how four different loudspeaker manufacturers have employed varying strategies to create loudspeakers that sound great because of their wide-dispersion technology.
The challenge with front-firing speakers: Paradigm and PSB
Creating wide dispersion throughout a speaker's bandwidth is difficult because the higher the frequency, the shorter the wavelength -- and the narrower the dispersion becomes. Sound quality suffers the most when the speaker driver reaches a critical frequency and the dispersion narrows so that the sound beams. This causes a problem with midrange drivers mostly, but not so much with woofers if they are crossed over at a low enough frequency, since woofers reproduce low frequencies that have naturally wide dispersion due to the length of the waves, or tweeters, because their beaming is beyond human perception. If woofers are not crossed over low enough, however, the same dispersion problems occur that affect midrange drivers.
In traditional three-way-loudspeaker design, the woofer, midrange, and tweeter are all mounted on the front of the speaker in ascending order. Convinced that superior performance can only be achieved by radical and eclectic speaker designs, some audiophiles have concluded that these front-firing, conventionally arrayed speakers are bound to suffer from narrow dispersion. Paradigm's new Reference Signature S8 speakers contradict this assumption. Since dispersion is mostly determined by -- and is a function of -- driver radiation, Paradigm launched an all-out engineering effort to deliver a totally new design for the Signature S8's driver modules. To reduce the dispersion problem, the speaker's crossover frequency from the midrange to the tweeter is lowered, thus preventing the beaming that occurs from the midrange as it reproduces the higher frequencies. However, in order to achieve this low crossover point, the tweeter must be robust enough to handle the frequency range it is now required to reproduce, and that can be expensive to make. However, Paradigm obviously believes that the benefits of wide dispersion outweigh the cost -- they designed a new tweeter for the Signature series as well.
Another front-firing speaker with superb dispersion is PSB's new Platinum T-8. It, too, uses high-quality drivers, but it also uses a slightly different driver arrangement, where a midrange, tweeter and a second midrange all sit above the woofer. "This MTM array provides an unusually wide dispersion pattern at higher frequencies," says Paul Barton, the founder of PSB.
Apart from beaming, another dispersion problem that designers such as Barton work to prevent is lobing, which occurs when interference arises between two sources, when two drivers interfere in the transition region.
"This phenomenon occurs if the acoustic centers are not in the same place, meaning the tweeter is above or below the woofer on the baffle, making them non-coincident," says Barton. "In order to minimize the negative effects on dispersion, you must use small-enough drivers that are arranged close to each other and crossed over at a low-enough frequency. This makes the drivers appears to have the same acoustic centers."
"In this way," he concludes, "no matter where you are, the drivers appear to be the same distance from you; therefore, you will minimize lobing, and the result will be more uniform dispersion, which is a good thing."
Mirage speakers have approached the dispersion challenge by utilizing revolutionary new designs in speaker-driver placement. The first to design bipolar speakers where the drive units are located in the front and back and therefore disperse sound 360 degrees around the cabinet, Mirage then went on to refine the technology, creating the first Omnipolar speakers -- resulting in even wider dispersion in the troublesome higher frequencies.
Mirage design innovation did not stop there. After two years of hard work, Mirage's design engineer Andrew Welker developed the OmniGuide module, now available in Mirage's Omni series.
"OmniGuide technology was born out of the desire to create an Omnipolar-radiating loudspeaker in a smaller, more compact package that could be mounted in any position," says Welker.
Each OmniGuide has a saucer-shaped deflector suspended over the woofer, with a pure titanium hybrid 1" tweeter mounted upon a sloped surface and another deflector over the top of that. This entire OmniGuide module is, in turn, mounted atop the speaker.
"Sound waves act in a similar manner to light in that they can be reflected off hard surfaces," Welker says. "A convex surface will disperse sound waves in all directions."
One of the problems Welker encountered during the design effort was that very wide dispersion interfered with imaging. The solution? Welker combined a convex reflector with the natural radiation properties of a cone mid-woofer.
"The sloped platform has benefits as well," he says.
"We have biased the dispersion of this lone woofer slightly towards the front of the speaker. The horizontal dispersion will be very similar to a forward-mounted woofer, but with the benefit of greatly improved vertical dispersion, essentially midway between forward-radiating and omnidirectional.
"Now, if we go back to the idea of a convex reflector and place this centrally above the sloped woofer, the frequencies where the woofer's dispersion narrows [beams] will be reflected off of the convex surface and uniformly dispersed 360 degrees around the cabinet. We are actually using the limited dispersion of the woofer at high frequencies to aid in the overall dispersion of the loudspeaker." Welker goes on to explain that the same principal is applied to the tweeter, which has its own convex reflector.
"The end result is a loudspeaker that disperses sound in a nearly spherical fashion, along with a perceived in-room direct-to-reflected ratio that is very close to the ideal 30/70 percent found in reality," he concludes, adding that it also allows the loudspeaker to be placed in almost any position in a listening room without causing sudden shifts in tonal balance or perceived dispersion.
Welker has achieved a minor magic trick with the OmniGuide technology, claiming that it results in a loudspeaker that virtually "disappears acoustically." Furthermore, he says, due to a wide and uniform dispersion pattern, the listener is able to sit in any location relative to the speakers "without affecting the overall soundstage presentation or tonal balance."
Sausalitos Acoustic Lens
Manny LaCarrubba and David Moulton of Sausalito Audio Works have also developed a totally new concept in loudspeaker design that effectively corrects the beaming that occurs at high frequencies and also limits the erratic variations that occur at crossover frequencies. Acoustic Lens Technology was patented by Sausalito and later licensed to Bang & Olufsen for their BeoLab5 speaker. I had the opportunity to ask David Moulton about this new technology.
"Wide high-frequency dispersion from loudspeakers generally yields benefits in terms of having constant timbre laterally over a wide coverage angle (+/-90 degrees), improved stereophonic illusions, and improved depth and envelopment of stereophonic playback," Moulton told me.
As a general rule for home and control-room playback, he says, their research at Sausalito Audio Works has found that the following are desirable:
To achieve this, the sound emitted from the speaker's midrange and tweeter drivers is collected by a lens and redirected, creating a dispersion pattern with a uniform frequency response over a wide horizontal angle.
"The lens works approximately as a kind of reflecting waveguide," Moulton says. "It takes the output from an upward-firing driver and emits it into a free field horizontally. It is based on a section of an ellipse (in rotation), so that the driver is at one focal point and the other focal point becomes a focal ring around 180 degrees in front."
"The result," he says, "is flat frequency response to above 15kHz across 180 degrees horizontally, and a vertical angle of 30 degrees."
Speakers with Acoustic Lens Technology, such as B&Os BeoLab 5, have a much greater depth of field, and an enhanced sense of spaciousness, width and stability of phantom images. They yield a comparatively large sweet spot. The reverberant and ambient detail of a recordings is clear; the listener can hear into the recorded layers with great definition and detail. Not only do the speakers sound better, but also their placement in the room is not critical.
At the end of the day
In today's column, I've discussed four different techniques for dealing with the challenge of wide dispersion; there are, of course, many more, with each design approach yielding its own benefits and unique characteristics. For the consumer, the bottom line is thankfully much more straightforward: No matter what design one chooses, dispersion is key for great-sounding speakers.
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