The Power of Two

The requirements for horn drivers used in professional loudspeaker systems (and line arrays, in particular) typically include high efficiency, high power-handling capacity, extended high-frequency range, low irregularity of the frequency response and low nonlinear distortion.
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JBL's new D2 high-frequency
compression driver
JBL D2 Dual-Diaphragm Dual- Voice-Coil Compression Driver

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The requirements for horn drivers used in professional loudspeaker systems (and line arrays, in particular) typically include high efficiency, high power-handling capacity, extended high-frequency range, low irregularity of the frequency response and low nonlinear distortion. In addition, the overall diameter and physical dimensions of the drivers must be as small as possible to maximize acoustic power density in high-performance line arrays. This requirement is especially critical for line array applications where typically several discrete drivers must be positioned next to each other in a compact manner.

Some of these requirements conflict with each other; for example, high power handling and low thermal compression of each compression driver presumes a massive voice coil having good heat dissipation. However, a large and heavy voice coil prevents the efficient reproduction of the high-frequency signal. Thus, traditional horn drivers have several characteristics that ultimately limit the high-frequency range. The most significant factors are:

• The diaphragm assembly’s moving mass that causes 6 dB/octave low-pass roll-off above a certain frequency.

• Compliance of air in the compression chamber that also causes another 6 dB/octave low-pass roll-off above certain frequency.

• High-frequency air resonances in the compression chamber: These can cause irregularity of the high-frequency response.

• The voice coil’s inductance typically attenuates high-frequency response with a slope of approximately 3 dB/octave above a certain frequency that adds to the aforementioned roll-offs.

One of the methods employed to increase the level of high-frequency signal in horn drivers is to use the full-metal (typically titanium) dome diaphragm and surround (typically having a “diamond” or a “tangential” 3-dimensional pattern). This combination is characterized by the high-frequency resonances (breakups) that produce “boost” of the high-frequency response level. However, this “amplified” high-frequency response is characterized by severe irregularities, i.e. by the sharp peaks and dips that are evidenced in the frequency response.

In addition, due to the nonlinear nature of these high-frequency resonances, their existence is accompanied by a strong nonlinear behavior. In the case of an input sinusoidal signal, this particular nonlinearity generates high harmonic distortion and, to make matters worse, it also generates subharmonics whose level may easily equal or even exceed the level of the fundamental input tone. In the presence of musical or speech signals, the nonlinearity associated with breakup modes adversely affects subjective sound quality and manifests itself in the form of harshness of the reproduced sound that may be very irritating to the listener.

At Winter NAMM 2012, JBL Professional introduced its next-generation VTX Line Array Series, which builds on the Vertec series’ sound quality with highly advanced sound reinforcement technology and support. The VTX Series represents a breakthrough in loudspeaker design innovation and technology development, best exemplified by the revolutionary new D2 Dual-Diaphragm Dual-Voice-Coil Compression Driver, which makes the VTX Series’ dramatic improvements in high-frequency sound and performance possible.

An exploded view of the JBL D2 Driver, which merges two compression drivers, each with its own voice coil, into a single, compact transducer with a single acoustical output.The D2 Driver overcomes the limitations of conventional compression- driver technology: limited highfrequency extension due to mass of the diaphragm and voice coil, and distortion characteristics that arise due to dome breakup modes. Merging two compression drivers into a single, compact transducer with a single acoustical output, the D2 Dual Driver utilizes two voice coils, each with its own lightweight, polymer, annular diaphragm, its own magnet assembly, and its own specialized phasing plug. Also unique to the D2 Driver is the use of magnesium for the fabrication of front and rear phase plugs and the front adapter. Magnesium provides extreme precision in the manufacture of cast parts combined with significant weight reductions.

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The approach to developing the D2 was dramatically different from the traditional way of designing compression drivers that are based upon a metallic dome diaphragm and surround. Instead of the large and “heavy” single voice coil metal dome diaphragm of conventional compression drivers, D2 is driven by two light but strong voice coils, two motive “forces” instead of one, acoustically connected to a single exit chamber.

The two annular polymer diaphragms combined have the same effective radiating area as a conventional single dome, while overall output and power handling are dramatically increased due to the lower moving mass and enhanced heat transfer obtained with two separate voice coils. The result is a compact compression driver with dramatically improved efficiency, power handling and smoother, extended high-frequency response with significantly lower levels of nonlinear distortion.

In addition, both pole pieces have precisely engineered copper rings that dramatically decrease alternating magnetic flux in the voice coil gaps. Essentially, this eliminates alternating flux and its modulation by the voice coil current and makes the high-frequency impedance of the driver resistive. At higher frequencies, this resistive impedance eliminates the 3 dB/ octave roll-off that attenuates highfrequency response in simpler, less sophisticated designs.

Well validated by objective measurements, D2’s performance has also gone through exhaustive subjective listening tests, demonstrating its superior sound quality compared to traditional metallic, mono structure drivers.

D2 provides an extreme output advantage over conventional systems, allowing significantly higher array power density. Compared to a conventional system, the D2 has double the number of voice coils and more than double the power handling, but with a 30-percent reduction in weight. This results in a dramatic increase in pure, high-frequency sound pressure levels in the same physical footprint.

The first product in the VTX Series, the V25, features three D2 drivers mounted on a third-generation waveguide and patented RBI Radiation Boundary Integrator assembly. The power handling of the three D2 drivers used in the VTX V25 is each 200 watts per AES standard. Overall VTX V25 high-frequency section power handling is therefore 600W (AES 2 hour) and 2400W peak, allowing for maximum SPL of 144 dB continuous (150 dB peak). With two smaller diaphragms and voice coils working on the same acoustical load, the VTX Line Array Series provides high efficiency and SPL, plus increased extreme high-frequency reproduction due to the smaller moving mass of each diaphragm and voice coil.

The configuration of the D2 driver is scalable for different size driver formats. Additional flexibility is provided by a possibility of series or parallel connection of the voice coils to optimize the overall load to suit various applications. Taking the D2’s scalability into account, coupled with its impact on significantly improved loudspeaker performance and sound quality, we expect to see the D2 play an integral part in JBL Professional’s future product development. The D2 Driver is a radical departure from traditional compression drivers and represents a significant breakthrough in loudspeaker system design.

Paul Bauman is senior manager, tour sound; and Alexander Voishvillo is senior manager, transducer engineering for JBL Professional.

JBL Professional
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