Designer Notebook: MLA Compact, A Lower Profile Approach From Martin Audio
An in-depth report on the new MLA Compact by Martin Audio director of R&D Jason Baird

May 10, 2012, by Jason Baird

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In 2010, Martin Audio brought a radical new approach to touring sound with the Multi-cellular Loudspeaker Array (MLA), an integrated system that is self-powered with six channels of amplification and DSP, networked with both audio and control on the same cable.

Furthering the package is a combination of cellular drive and fast, automated optimization software. Cellular technology helps achieve phase-coherent summation across the respective coverage area, holding both frequency response and SPL within a very tight, user-specified window.

The new MLA Compact widens the scope of applications, designed for situations not requiring the full power and throw of the full-size MLA, or where a smaller, lighter system is called for. It is scalable, and can meet the needs medium-scale touring and fixed installations such as concert halls, theatres and churches.

The footprint of an individual cabinet is 31 (w) x 11 (h) x 19.7 (d) inches, and weight is 109 pounds. As a point of comparison, a full-size MLA box measures 44.7 (w) x 14.6 (h) x 26.5 (d) inches and weighs 193 pounds.

In The Box
Compact loudspeakers often use direct radiators for the lows and mids because more efficient acoustic technologies are hard to adapt to small cabinet volumes. The 3-way MLA Compact, however, applies new slot and horn-loading techniques to the low and mid sections.

Like it’s bigger brother, MLA Compact presents an integrated approach. (click to enlarge)

Further, it deliberately avoids the use of coaxial, co-entrant or cross-firing midrange/HF driver arrangements. which introduce acoustic discontinuities that can affect the on- and off-axis frequency response of both MF and HF sections. In MLA Compact, the MF and HF horns are completely separate in order to produce consistent, true 100-degree horizontal constant directivity coverage.

The LF section utilizes 2 x 10-inch (2.5-inch voice coil) neodymium drive units in a unique Hybrid configuration that fosters acoustic efficiency and enhances “punch.” Each driver is slot-loaded into a truncated horn with a low flare rate, to give a high sensitivity of 103 dB @ 1m/2.83V. The slot-loading allows the twin LF drivers to be optimally spaced within the enclosure.

A look at the slot- and horn-loading approaches inside MLA Compact. (click to enlarge)

This double-source arrangement significantly improves the directivity control of the LF section maintaining the 100-degree beamwidth down to 250 Hz and reducing mid-bass output at the sides and rear of the array. The LF drivers are very high excursion with vented poles to reduce power compression and eliminate turbulent air noise.

MLA Compact has separate MF and HF horns with horizontal constant directivity characteristics, so the horizontal off-axis response tracks the on-axis response exactly.

The MF horn utilizes 2 x 5-inch (1.5-inch voice coil) neodymium drivers to produce 109 dB @ 1m/2.83V, which represents a significant gain on the typical efficiency of 102 dB found in cross-firing direct radiator designs in comparably sized systems. It’s possible due to acoustic and thermal designs that utilize forced-air cooling and a thermally conductive aluminum housing.

The I/O capability on the back of each cabinet. (click to enlarge)

The HF section includes 4 x 0.7-inch-exit neodymium compression drivers that feed separate horns providing 100-degree horizontal constant directivity. The driver combination replaces the more traditionally used large-format compression driver and has less distortion, as well as having a more extended HF response.

Instead of adopting flat wavefronts as advocated by some of the early proponents of touring line arrays, sophisticated in-house BEM (Boundary Element Method) modeling techniques have shown that slightly curved wavefronts deliver much more consistent SPL to the audience where the array is curved – as in most practical, real-world applications.

Placing a kite-shaped “wedge” part-way down the horn enables a specific, desired curvature to be achieved – depending on the shape of this wedge.

In the case of MLA Compact, the HF wavefront is curved to provide a balance between optimal summation over distance and summation at the maximum inter-cabinet splay angle of 10 degrees.

Power & Control

Each MLA Compact has 5-channel Class D amplification, DSP and network electronics that enables the individual cells within each enclosure to be driven with the exact signal determined by DISPLAY 2.1 optimization software for independent control of each of the 24 LF cells, 48 MF cells and 48 HF cells in a 24-box array.

The amplification package provides a total of 2.1kW continuous and 4.2kW peak output. One channel powers both LF drivers in parallel and two channels drive each mid independently. For the HF section, two channels drive the four HF drivers in parallel pairs, making a total of five independently powered acoustic cells per enclosure.

Switched-mode power supplies auto-range to global mains voltages from 100 to 240 volts, 50/60 Hz, while Power Factor Correction smooths out the mains current draw over the whole of the AC waveform. Amplifier monitoring via U-NET includes input signal, output signal at the drive unit terminals, limiter status, heatsink temperatures and driver fault conditions.

Onboard DSP performs all crossover and EQ functions via a combination of IIR and advanced FIR filtering. Fast VanishingPoint FIR filters provide the ability physically separate the MF and HF horns so they do not compromise each other’s constant directivity dispersion pattern, yet achieve the spatial performance of a single device.

MLA Compact incorporates an automated optimization process. (click to enlarge)

Arrays can be remotely controlled over U-NET from a PC or wireless tablet running VU-NET control software with its intuitive graphical interface. VU-NET also enables the user to switch on enclosure identification LEDs with automatic identification of neighboring enclosures and connectivity confirmation.

Dialing It In

DISPLAY2.1 is the “brain” of MLA Compact, providing a virtual environment within which arrays can be configured and optimized, providing highly accurate prediction of the direct sound produced over the audience and also and over areas where sound is to be avoided. It also bolsters array design and deployment, generating accurate spot frequency responses and comprehensive rigging information, including mechanical load safety analysis.

DISPLAY2.1 interacts with MLA Compact’s onboard DSP to deliver consistent sound throughout a venue. It calculates the filter parameters for each enclosure, down to the resolution of individual drive units, and uploads them to the enclosure via the U-NET digital network. The link between DISPLAY2.1 and an individual MLA Compact enclosure is live and bidirectional.

DISPLAY2.1 reverses the sequence of array design software. Starting with a specified SPL and response over the audience floor, the software works backwards to configure an array that will give the required result.

BEM plots of a horn in an array (left) and of a single horn on its own. (click to enlarge)

Since it is a practical impossibility to measure every possible array configuration with different combinations of enclosure numbers, splay angles and drive signals, an accurate acoustic model is essential.

The previously noted BEM (Boundary Element Method) models enable hundreds of virtual array configurations to be investigated in fine detail in a virtual 3D environment. This level of research has transformed our understanding of how arrays really work and shown that the acoustic interactions between array elements are much more complex than originally thought.

An important factor and industry first is the inclusion in the model of the previously ignored effects of adjacent enclosures. If these are not incorporated into the model, prediction errors can be over 8 dB in the midrange. Including the effect of adjacent cabinets is key to the accuracy of the optimization process and increases the accuracy of the acoustic model of MLA Compact.

Completing The Package

The loudspeaker rigging system allows up to 24 enclosures to be suspended via its 2-point-lift flybar, and the same hardware can also be used for single point lifting of up to 12 cabinets, as well as ground stacking up to six high. Inter-cabinet connections utilize custom quick-release pins.

All loads are borne by the integral metalwork and release pins - not the enclosure. Accompanying software determines the safe limits and tilt angles of a specific array, with BGV C1 safety calculations done on the fly.

The enclosure is made of birch and poplar ply construction and finished with a thick polyurea coating. The enclosure sides are fitted with replaceable, steel-reinforced rubber moldings with integral interlocking skids, and an ergonomic bar-handle facilitates rigging and general handling. MLA Compact is supplied in flight-cased pairs, with the base doubling as the wheelboard for the pair.

The companion DSX subwoofer offers is a dual-18-inch (4-inch voice coil) ferrite drivers in reflex-loaded enclosure with four flared ports, designed to improve linear airflow. Each driver is housed in its own separate chamber to move any cabinet resonances out of band. The onboard Class D amplifier module can deliver up to 6 kW of peak power. DSX sub arrays can be designed with specific directional properties and DSP settings uploaded via the VU-NET network.

DSX Subs can be flown alongside or at the top of arrays, as well as groundstacked.(click to enlarge)

A flying version, the DSX-F Sub, can be flown alongside or at the top of MLA Compact arrays, as well as being ground stacked. A maximum of 15 DSX-F can be suspended from the MLA flying frame and symmetrical rigging allows flown DSX-F cabinets to face backwards—enabling directional flown as well as directional ground stacked arrays to be configured. The ground-stack DSX can be upgraded to the DSX–F by the addition of an easy-to-fit accessory kit.

What all of this adds up to is a compact, highly flexible package that provides startlingly consistent frequency response when listening off-axis and “walking the field.” In both mobile and permanent applications for audiences ranging up to 5,000, this is no small feat.

Jason Baird is research & development director at Martin Audio. After completing a degree in Electronic Engineering at Leeds University in England in 1991, he worked in the production industry until he was employed by Wharfedale Loudspeakers in ‘94. After starting his career in hi-fi, he spent 18 months at Fane Acoustics, designing pro audio transducers, subsequently joining Martin Audio in 1998.

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Designer Notebook: MLA Compact, A Lower Profile Approach From Martin Audio