Pro Sound News, September 2006: Sound Innovations: Efficiency and Accuracy by Design - Electro-Voice Dvx Woofers
SOUND INNOVATIONS: Electro-Voice DVX Woofers
By Alan Babb, Electro-Voice Transducer Engineer
The DVX Series is the latest generation of high-performance woofers from Electro-Voice (EV). The story begins in spring 2005, when EV launched its ZX5 as the “World’s Most Versatile Loudspeaker“: a two-way portable powerhouse capable of filling large spaces with concert-quality sound, all while weighing in at less than 50 pounds. Word spread quickly.
The motor behind EV’s offer was built into the ZX5’s sleek molded enclosure: the EV DVX-3150-a brand new 15-inch woofer, based on the all-new DVX platform. It’s not often that an experienced team of engineers is given a blank piece of paper and asked to develop a product that represents the very best in all aspects, without compromising on a single detail. With DVX, that was the mission. The DVX-3150 features an all-new cone and surround structure for very low distortion and high excursion, along with an all-new frame and motor structure for high output with very low power compression.
Having impressed everyone who heard it in the groundbreaking ZX5, the DVX concept soon materialized again in the X-Line Very Compact, the latest and smallest of the EV line array family. Shortly after the launch of X-Line Very Compact and its 12-inch DVX-3120 woofer-equipped XS212-LF cabinet (the sub element in the XLVC line), the phone calls began: “The XS212-LF sounds incredible for its size, with tons of power; are these speakers going to be available in the XLC medium sized line array format?“
EV has always listened closely to its customers, and these questions became the impetus for EV’s engineers to put DVX at front and center in their transducer program, the result of which was the development of a brand new 12-inch LF driver, the DVX-3121, and a new mid-bass driver, the DVN 6.5-inch. Which brings us up to date: both speakers made their debut at InfoComm 2006 inside the XLC127DVX three-way, full-bandwidth line array element, based on the world renowned XLC127+. The DVX-loaded XLC has all the sonic signatures that made its predecessor a favorite, but with extraordinary power handling and performance specs that surpass those on many full-sized line array offerings.
So how was all this achieved? Allow me to explain some of the “sound innovations“ behind the DVX concept, at the drawing board stage and beyond: Woofers, like all loudspeakers, are governed by a complex melding of disciplines: mechanical engineering, electrical engineering, physics and acoustics. The DVX line has been optimized in all of these areas using the latest technologies. First of all, the DVX magnet structure, the motor that works with the voice coil to provide the drive force to the woofer, was optimized for strength, stability, travel and linearity. Non-linearity in a speaker’s magnetic field or suspension not only leads to added distortion but also greatly impacts the life expectancy of a speaker.
For example: if you have a speaker motor that is stronger when the cone is moving out of the basket than when moving in, the speaker will tend to offset itself to the outside. Since the suspension of the speaker (the surround and the spider) can only move a finite distance in either direction before being damaged, you are going to stress these components more in one direction than the other. It also makes the speaker limit (stop moving) too quickly in one direction, and that means you lose sensitivity and increase distortion. To further compound this problem, the magnetic field generated by the voice coil can cause the field of the permanent magnet to modulate. This is the equivalent of your car motor losing power many times a second. It would be difficult to have a smooth controlled speed ride.
To solve this problem, the woofers in the DVX line have been optimized via extensive computer FEA (Finite Element Analysis) to have linear magnetic fields that do not overly modulate. With the aid of a laser displacement measurement system we can also optimize the suspension to not only have the right stiffness but also to stop the cone motion in a smooth and controlled fashion when the speaker is pushed to extremes. What does all this mean to the prospective customer? Quite bluntly, it means less distortion under normal conditions and no rude noises when the speaker is overdriven.
Another problem common to woofers is that the voice coil of the speaker, like any coil of wire, has an inductance. Place a hunk of iron inside a coil and you get even larger inductance.
Graph plotting the variation in the strength of the magnetic field in the previous models. In speaker terminology we call that hunk of iron a pole piece: every speaker has one. Most people understand the problems caused by inductance at high frequencies. Engineers take advantage of inductance all the time in crossover design. The problem with inductance in a speaker is as follows: as the cone moves outward it moves out of the magnetic circuit and the pole piece no longer inside as much of the coil as when the cone is moving inward. This asymmetry in the inductance also causes an offset of the speaker’s center position, just like a non-linear magnetic field or suspension. Besides negatively impacting the life expectancy of a speaker, this asymmetry causes distortions that are particularly audible and offensive.
The Klippel measurement system is used to measure the symmetry of the magnetic field, the compliance of the suspension and the inductance. To combat this problem, the DVX woofer uses multiple approaches. First we computer modeled the magnetic structure and optimized the geometry to eliminate asymmetries. After the FEA phase prototyping, the tests and measurements begin. The use of multidimensional measurements using acoustic and laser data further speeds perfecting the design, we can very quickly identify areas to improve and the solutions. This enables a final design that is far superior to using traditional design methods. Unfortunately you cannot simply eliminate the pole piece from the equation, so we did the next best thing. We have put a conductive but non-magnetic ring inside the magnet structure. Sometimes called a Faraday Loop, this shorting ring blocks the field from the coil that would interact with the iron core, without affecting the DC or static field generated by the large ceramic magnet of the motor structure. Another benefit is that is also counteracts the modulation of the static field that can be caused by the field generated by the voice coil.
This rear view of EV’s ZX5 cabinet interior reveals the physical construction of the DVX-3150 woofer At the end of the day, of course, all this adds up to a superior sounding loudspeaker. DVX goes to show there is still room for improvement in the world of loudspeaker design. It’s not just about the greater overall output: in DVX; the level of sonic clarity-even for an arena-size system running extremely high SPLs-is unprecedented.