The Digitally Steerable Array ("DSA") is a type of loudspeaker that is in its relative infancy compared with other sound reinforcement technology. This unique audio animal is primarily intended for small-to-medium size commercial and house-of-worship applications where priorities include intelligibility, controlled dispersion and aesthetics. DSAs present an external appearance much like traditional column-type loudspeakers, but there is way more going on internally than meets the eye. Let's take a look at the concepts behind the DSA and the technology that makes it possible.
Wanted: Direct Sound
One of the critical requirements for any sound system is control of its acoustic output – a concept that was recognized early on by the pioneers of sound reinforcement and HiFi reproduction, resulting in the infinite baffle. The way to increase the feeling of intimate communication between an artist or orator and the audience is by ensuring that listeners hear a maximum of direct sound and a minimum of reflected sound (early reflections or reverb). When a listener is bombarded with a high percentage of reflections, their ability to understand the message is significantly reduced. In an ideal world, listeners would hear only direct sound and no reflections at all. I don't believe that this is possible in any way other than wearing sealed headphones.
The column loudspeaker (really, the first "line array") was designed with speech intelligibility as a priority. A column contains multiple drivers vertically aligned, all identical (often 8 inches or smaller), all producing the same audio signal. These drivers create patterns of constructive and destructive interference resulting in a tight beam of sound that reduces spill and possibility of feedback. The vertical dispersion of a column can be as narrow as 10 degrees (while horizontal coverage can remain as wide as 120 degrees), so even if you are seated directly underneath the column, you won't get blasted. Columns also produce higher output at distances farther than that achieved by larger, single-driver speakers. A column's beam of sound can be aimed, given the rigging hardware required to suspend the cabinet in an aimed manner. In other words, you can aim a column where you want, but it's going to look coyote ugly hanging in the air like a flying junkyard. And if you wish to modify the aim, you have to get up off ‘yer duff and re-orient the mounting of the cabinet. Thus the raison d'étre for the DSA.
Two Key Advantages
Digitally Steerable Arrays are intended for use in installations where column type loudspeakers would traditionally have been used, such as the aforementioned house-of-worship, airport or train station terminals, theaters, theme parks and retail venues. In such applications, speech intelligibility is the primary concern (followed by minimal visual distraction and full-range audio reproduction), but the nature of such structures – reflective surfaces and large physical dimensions – inherently produces excessive reverberation time and reduced intelligibility. Traditional columns can help alleviate these issues, but come with the limitations previously mentioned. A Digitally Steerable Array allows the installer to achieve two major goals which work toward increased intelligibility and reduced room reflections: (1) the coverage angle of the array can be adjusted, and, (2) the acoustic output of the array may be aimed at a particular area of the room without need to change the orientation of the cabinet. Thus the array can be mounted vertically against (or even flush to) a wall, while the acoustic output is directed (for example) downward, or even to the left or right of the cabinet. The Steerable part of DSA refers to the column's acoustic output, which is digitally manipulated to a target area.
This is in stark contrast to simply angling the cabinet which, as shown in Fig. 1, causes the edges of the coverage to spill up the side walls, resulting in an increase of reflections.
Steerable Sound
Right now there are Digitally Steerable Arrays from a variety of manufacturers. Of course, each manufacturer has their own take on solving the issue, but they do share some common traits. Typically, each driver in the DSA has a dedicated power amp and its own DSP. The DSP allows adjustments in phase and frequency response so that the acoustic output of the column array may be steered, almost completely independently of the array's axis (think of it like your head is facing forward and is stationary, but your eyes are shifting up and down and back and forth to focus your vision on different areas of a room).
The steering is generally accomplished by using small amounts of delay between each driver (or pairs of drivers). The result of these small delays is akin to that of physically tilting the array, enabling a designer to focus the acoustic output toward the listeners and away from reflective surfaces, reducing the early reflections and reverberation which would otherwise impede intelligibility. Fortunately, you won't have to manually calculate delay times and frequency response modifications for each driver; manufacturers provide software to manage the arrays, defining the coverage or "opening" angle, elevation and focus distance – in some cases, for multiple acoustic beams – all while the enclosures remain fixed.
In addition to the steering benefits, DSAs retain the traditional column's ability to throw sound for distance, in some cases past 200 feet. This is an important characteristic, since intelligibility becomes worse as a listener's distance from the source (and thus the reverberant field) increases. In cases where there are aesthetic, architectural or legal constraints for a minimum mounting height, the column may be moved to the desired physical location, while the beam is steered to the desired acoustic location. Ditto for rooms with low ceilings. Some DSAs even allow the acoustic beam to be split to avoid sound bouncing off obstructions – such as the face of a balcony – that would cause unwanted room reflections. Most systems offer network capability for control and, in some cases, distribution of audio.
One of the benefits of a line array is that as the length of the array is increased, pattern control is maintained to lower frequencies. The fact that most commercial installations are concerned with audio above roughly 250 Hz complements the fact that a smaller array can maintain pattern control through the low-mids, but a few manufacturers offer LF cabinets as an option, enabling the array to extend low-frequency response in situations requiring full-range reproduction.
