Steerable ArraysA classic concept becomes practical as technology evolves. 2/16/2007 7:00 AM Eastern
Feb 16, 2007 12:00 PM, By Jack Kontney
A classic concept becomes practical as technology evolves.
If there is one thing we've learned about the loudspeaker market over the past couple of decades, it's that we should never get too comfortable with the current state of the art. Self-powered loudspeakers were once considered impractical for use in any serious installation. Line arrays were thought to be a dated concept with no practical application beyond modestly sized venues. Both are now viable, desirable approaches.
As companion technologies advance, we see time and again that many older ideas have really just been waiting for a combination of creativity, inventiveness, and technology to coalesce. Suddenly, the ridiculous becomes practical, the outrageous becomes commonplace, and a new class of products is born.
Welcome to the world of steerable loudspeaker arrays.
You might see them referred to as steerable column arrays, digitally steerable arrays, beam-steering arrays, or by similar terms. They are cousins to, yet distinctly different from, the hanging line arrays that have proliferated across the industry since the 1990s.
In a standard line array, a group of identical sound sources is stacked vertically. Properly designed and deployed, the line array produces a cylindrical wavefront that is theoretically twice as efficient as the spherical wavefront generated by a point source. As a result, the directivity of a good line array system creates more coherent long throws (up to a critical distance), reducing or eliminating the need for delay speakers. This, in turn, reduces the total number of drivers required to fill a space.
Steerable arrays are more specialized cousins of the basic line array. When the speaker centers of a vertical array are a half-wavelength apart, lobe steering becomes possible. Basic steering involves progressive delay of each array element (or pair of elements in some designs), with the topmost speaker transmitting first. This incremental delay produces an effect similar to that obtained by physically tilting the array. It becomes possible to “steer” the array's output away from highly reflective surfaces, significantly reducing the excitation of the reverberant field and enhancing intelligibility. Each driver is treated individually via a dedicated DSP engine to ensure all output arrives on the intended listening plane on time and in phase.
A key element of successful speaker design in steerable line arrays is correct center-to-center distance of the individual speaker elements. Put simply, this distance must be a maximum of double the shortest wavelength (which has the highest frequency) to be controlled. Since high frequencies have very short wavelengths, the physical size of the driver becomes the limiting factor. Another key factor is array length. Basically, the longer the line array, the narrower its vertical dispersion and the lower the frequencies that will be affected.
Therefore, a steered array could theoretically be controlled across the full 20Hz to 20kHz bandwidth. Unfortunately, practical considerations intervene at both ends of the frequency spectrum. To fully control the long wavelengths of low bass frequencies requires an array to be extremely tall — clearly not practical in many buildings. Conversely, at 20kHz, keeping the centers of the speakers within a half-wavelength of each other would require spacing of roughly 3/8in. Sadly, there is no driver in existence that is that small and able to provide the SPL needed, meaning that high frequencies cannot be effectively steered.
WAITING FOR TECHNOLOGY
Columnar speakers were in commercial use as far back as the 1950s, and systems like the Shure VocalMaster were actually quite popular in the '60s and '70s. As venues grew in size, massive horn-driven systems with delay towers became the fashion, displacing line arrays. But all that changed in the early 1990s when L-Acoustics showed that its vDOSC system, designed by Christian Heil, could produce cohesive, full-range concert sound for medium and larger halls with fewer boxes. Since that time, the proliferation of line arrays has been impressive, with literally dozens of manufacturers producing commercially viable designs. This was not the case with steered arrays.
The steering effect has been well known among researchers for as long as 50 years. Unfortunately, the processing power needed to exert control simply has not existed. Although the physics of lobe steering is relatively straightforward, designing a system that can meet a wide variety of real-world conditions is a much more daunting task, requiring flexible, dedicated DSP algorithms. To date, only three manufacturers — Duran Audio, EAW, and Renkus-Heinz — have gone to market with steerable systems.
“I wouldn't say it's easy,” says Gerrit Duran, founder of Duran Audio of the Netherlands. “Steering alone is not magic. But opening-angle control with constant directivity over a large bandwidth is much more difficult.
“DSP is a necessity to achieve a controllable steered array. When we started the development of the concept, DSPs didn't even exist. The appearance of DSPs in the early '90s was very convenient for us.”
COMPLICATED BUT USEFUL
The output in a properly designed and implemented array can be digitally “steered” in the vertical plane. Horizontal dispersion is fixed, and therefore defined by the manufacturer's selection of drivers. Vertical steering, however, allows unprecedented control of floor and ceiling reflections. This capability greatly improves intelligibility, making steerable arrays attractive for use in highly reverberant spaces like transportation terminals and traditional houses of worship.
Another hidden advantage of a steered array is the ability to achieve amazingly consistent decibel levels throughout the listening area. Unlike with a tilted line array, sound radiating to the sides and the rear of a steered array is steered downward at the same angle as the front lobe. Thus, lateral reflections are reflected toward the floor, enhancing the intelligibility of adjacent listening areas. Rear radiation is also steered downward. Thus, when the array is mounted on a wall, these rear reflections assume the same downward angle as the front radiation, rendering their effect on the pattern negligible.
The most obvious advantage of steerable columns is their low profile. Because they are designed to be wall-mounted, steered arrays eliminate the visual distractions inherent in a hanging loudspeaker system. The ability of these systems to blend in with the visual aesthetic of a space is finding great favor with architects and consultants.
Today's systems combine insightful array design with advanced, intuitive software to allow advanced control that solves problems for architects and system designers.
“The potential of digital steering technology is nearly limitless,” says Jeffrey Cox, VP of the EAW brand group. Steered arrays represent a unique design tool that can be applied to a wide range of situations. And, as memory has gotten cheaper and processing speeds faster, steered arrays have become commercially viable.
Because the tall, thin speaker columns are designed to be mounted flush to walls, aesthetic and architectural visions can be realized without sound system intrusion. Speakers can easily blend into the line of the hall. This quality alone makes steered arrays worthy of consideration in many visually driven space designs, especially when the venue is highly reverberant.
Sonically, the greatest benefit that steered arrays provide is noticeably increased intelligibility in the speech range. This makes them an ideal choice for highly reverberant spaces where the spoken word is reproduced. Houses of worship, transportation centers, museums, and other public spaces are excellent examples. Recent advances have extended the steerable frequency range, which in turn makes steered arrays appropriate for a broader range of uses, including music.
“We're seeing an abundance of new designs focusing on steerable arrays,” says David Rahn, national sales manager for Renkus-Heinz. “If the primary challenges of a space are architecture and intelligibility, this is the way to go. However, if rock concert playback level is the primary criterion, that calls for a different solution.”
Of course, might there be other applications where a steerable array might be deployed horizontally? Duran has been there and done that. “Long hallways, like the corridor to the pyramid of the Louvre in Paris, are a good example,” he says. To optimize the direct-to-reverberant ratio, the array there is mounted on the ceiling, with a narrow horizontal dispersion.
The three companies with off-the-shelf steerable array systems all use linear columns of speakers with individual amplifiers and DSP. Form factors and software control vary. The balancing act they perform involves offering all the advantages of steering and beam shaping, while minimizing the inherent compromise in frequency response.
Let's take a look at the three companies currently leading the charge in producing steerable array products.
Duran Audio pioneered the modern steerable array in the mid-90s with its Axys brand Octaray and Octavox products with Digital Directivity Control (DDC). These were followed by the Intellivox 2c, introduced in 1996. With a program of ongoing product improvements, the Axys Intellivox arrays have gained wide acceptance in Europe and a solid foothold in the United States.
In addition to vertical steering, a well-designed Intellivox system can achieve extremely even levels over large areas. Using new DDC2.0 control, an Intellivox-DC500 (a 16ft. column) shows SPL measurements within 1dB of each other at distances of 16ft. and 160ft. The array's audio beam can be steered up to 16 degrees up or down from the acoustic center, the opening angle can be adjusted between four and 10 degrees, and focus distance can also be controlled. The smallest DDC column is slightly taller than 3ft.
An extremely interesting development from Duran is the Intellivox DDS (Digital Directivity Synthesis) array series. Using a new optimization algorithm, DDS lets installers specify the desired beam output, then sets up DSP array control to create that distribution. DDS products are offered in the same range of sizes as the DDC range.
The two primary Intellivox lines, DDC and DDS, are suited to differing needs. DDC arrays are best suited to flat audience planes like traditional churches, while DDS systems are best suited to more complex audience areas, like theaters and concert halls.
Duran Audio also offers 3D acoustic-modeling software called DDA (Digital Directivity Analysis), at no charge. DDA allows the designer to predict the performance of an Intellivox array system. During installation, Intellivox systems are configured using Duran Audio's proprietary WinControl software. Parameters include volume control, four-band parametric EQ, and delay. Once the system has been configured, the PC can be removed, with settings stored in non-volatile memory. A RISC processor then handles ongoing system monitoring every 20ms.
The Iconyx series is a digitally controlled columnar loudspeaker array that employs a modular approach designed to make the system easy to specify and install. The core module is the IC8 array column. Approximately 3ft. tall, the IC8 holds eight 4in. speakers in a tight vertical array. Each speaker has its own Class D amplifiers and DSP engine by D2 Audio, developed specifically for this application. The Iconyx provides an option for PCM digital inputs to each module to create a fully digital signal path.
The other side of the system is Renkus-Heinz's BeamWare 2.0, introduced at AES. Using BeamWare, the system designer specifies the descriptions of both the system and the audience area to be covered. The software simulates the shape and aiming of the array's output, and also provides a reading of the SPL from the front to the rear of the coverage area.
Once the design is finalized, BeamWare calculates all the control parameters and downloads them to the installed Iconyx array modules. The Iconyx system also incorporates a feature called Vertically Adjustable Acoustic Center (VAAC) that allows the installer to make adjustments in situations where the array cannot be mounted at the acoustically optimal height (typically about 7ft. or 8ft. above the floor).
The physical form of the Iconyx, the IC8, is an architecturally friendly column measuring just 6"×6". Each column module is 3ft. tall, and up to four modules can be combined. A full IC32 system is 13ft. tall, providing effective steering control down to roughly 100Hz.
EASTERN ACOUSTIC WORKS (EAW)
EAW's Digitally Steered Array (DSA) attempts to address bandwidth issues. In the two-way DSA250, low-frequency and high-frequency arrays coexist within a single columnar module. The high-frequency section consists of a linear array of eight 0.98in. soft-dome tweeters spaced just 1.5in. apart. To minimize the physical size of the enclosure, the low-frequency array of eight 4in. cone transducers is staggered. Additional low-frequency DSA230 modules can be combined with two-way DSA250 modules to extend steering control down into lower frequency ranges.
Like all steered arrays, each DSA enclosure contains separate amplifiers (40W for LF elements, 20W for HF) and DSP engines for each speaker, employing a proprietary algorithm for system control and implementation. EAW describes its DSP engine as employing a combination of traditional and proprietary digital filters. The steerable frequency range for a single module is roughly 300Hz, while the full frequency range of the system is specified as 100Hz to 16kHz.
Software communication is handled via an integrated RS-485 transceiver connected to a personal computer. To ensure that outputs of the high- and low-frequency arrays of the DSA are coherent across the listening area, EAW software specifies system coverage using the audience coverage method. In addition to using vertical beamwidth, steering angle, and focal distance to define the coverage area, the audience coverage method adds secondary measurements to help control polar response, which can help optimize the system for spectral consistency (enhanced intelligibility), maximum projection (enhanced musicality), or a compromise as needed. System designers can make this choice using a simple slider control in the software domain.
It's been more than 10 years since Duran Audio's first steered array was installed in The Netherlands' Amsterdam Schiphol Airport. Since that early success, the growth of steered arrays has been steady, marked by advances in design that allow deployment in an ever-widening set of applications. Where early designs were optimized for voice applications, today's systems offer frequency bandwidth that produces excellent musicality. So, what problems remain? What are the key issues in specifying steerable arrays?
According to Rahn, “It really comes down to qualifying the customer and what their priorities are. Steered arrays can overcome a lot of significant challenges presented by architecture and intelligibility. So look for customers whose primary concerns are in these domains. But if output is the primary criterion, if they want to get really loud, look for a different solution. These are not designed for concert-level playback.”
The bottom line is this: You can play music through a steerable array with good fidelity, but true full-range response is unrealistic. As Duran says, “Good music reproduction requires a larger cone surface than 4in. We all make products for touring sound for those situations. Steered line array products are meant to serve voice applications primarily.”
Putting speakers on the rear wall of a house of worship raises the issue of feedback. But according to Ralph Heinz, senior VP of engineering at Renkus-Heinz, it's not as much of a concern as one might suspect.
“In general, this technology is actually highly resistant to feedback,” he says. “A microphone directly in front of the array is not seeing the coherent output of all drivers at once, just the contribution of one or two.” In addition, the opening angle can be moved up the array in the software domain, to minimize radiation at critical mic positions.
From a technical standpoint, steered array technology is most efficient when it is aimed at a flat audience area and the mounting height of the array is relatively low. But increased processing power and improved design software have made more complex installations feasible. David Gunness, director of R&D for EAW, notes that while the company's digitally steered array provides the user with complete control of the vertical pattern throughout the vocal range, adding modules provides more low-frequency output and extends pattern control to lower frequencies.
He says, “The point is that the sound professional can use the modules in a variety of combinations to meet specific output and directivity needs.”
In venues such as traditional houses of worship and transportation hubs, intelligibility has been a somewhat elusive goal for many system designers. Steered arrays offer the ability to overcome the problems of overly reverberant spaces without resorting to aesthetically objectionable methods such as heavy acoustical treatments or distributed zone speaker systems. For this reason, more and more architects are considering steered array systems when designing new buildings and retrofitting old ones. Today's system designers and integrators would be well served to do the same.
IntelliDisc: Future of Steered Arrays?
Because steered arrays are linear in nature, they work best where the listening area is rectangular in shape and all on one plane. But that is just the starting point. Now Duran Audio has raised the bar with the introduction of its Axys IntelliDisc DS-90, a fully integrated, digitally controlled, two-dimensional loudspeaker array.
The IntelliDisc-DS90 consists of 41 4in. speakers, a 32-channel digital amplifier, and a pair of 32-bit floating-point DSPs, all contained in a wall-mounted octagonal case. IntelliDisc uses a new array control optimization concept to allow manipulation of both the vertical and horizontal dispersion of the array to fit the target area perfectly. The array is capable of providing a constant sound pressure level over a 100ft. throw, even under highly reverberant acoustical conditions. The array measures 33.5 square inches and is less than 8in. deep. It weighs 125lbs.
The secret is Duran Audio's proprietary Digital Directivity Synthesis (DDS) technology, a specially adapted constrained weighted least-squares optimization algorithm. Starting from a desired direct SPL distribution in space, the optimum output filtering for each array channel is calculated. Thus, the desired audio “illumination” of the hall or space is mapped back to the array, rather than the conventional method of adjusting array response to fit the hall.
Well known throughout Europe, Duran Audio is a Dutch firm that pioneered the steered loudspeaker array in the mid-1990s. Its Axys Intellivox brand is distributed by Performance Devices of Torrance, Calif.
For More Information
Jack Kontney is the chief cook and bottle washer at Kontney Communications, a content creation consultancy specializing in the pro audio industry. Contact him at firstname.lastname@example.org.