..:: ABOUT LINE ARRAYS ::..

When L-ACOUSTICS® introduced V-DOSC® in 1993, the audio industry was skeptical since at that time, no other manufacturer was using similar principles. Column speaker designs of the '50s were considered outdated since the original work of Beranek and Olsen didn't allow for extended bandwidth operation or coverage over complex audience geometries. Only clear-sighted, adventurous sound engineers and designers considered V-DOSC, listened to it, used it, and embraced the product. Following this, it took years of field experience, extensive training, technical support and memorable tours to establish that V-DOSC had modernized sound reinforcement. Benefits such as predictive, even coverage, longer throw at higher frequencies, precise rigging accuracy and higher SPL were proven over time. It is not surprising that most manufacturers eventually decided to introduce a line array. Not less than 12 line arrays have been released within the past two years and a new fashion movement has been launched!

L-ACOUSTICS' initial R&D efforts focused on analysing the sound field radiated by discrete sound sources [1]. The research goal was to eliminate destructive interference within the main coverage pattern while optimising destructive interference elsewhere so that deviations from an ideal continuous ribbon source were not audible. Objective arrayability criteria were established, however, it was found difficult to make a line array operate properly over the entire audio bandwidth. In "Wavefront Sculpture Technology" [2], five conditions were presented to achieve this goal. We define a "line source" as a full-bandwidth line array that respects these five conditions and therefore successfully approximates an ideal, continuous ribbon source.

To summarize, a line array can be considered as a line source if, and only if, the following conditions are fulfilled:

1. Active Radiating Factor (ARF)
    Each sound source of the array radiates a flat, isophasic wavefront and the area covered by the individual sources is more
    than 80% of the frame area that contains the sources.

2. Frequency/Spacing Dependence
    If the sound sources of the array are neither flat nor isophasic, the spacing between their acoustic centers is less than half the     wavelength of the highest radiated frequency.

3. Allowable Wavefront Curvature
    The maximum deviation of the wavefront radiated by individual sources from flat is less than a quarter wavelength at the
    highest operating frequency (5 mm at 16kHz).

4. Audience Coverage
    Relative tilt angles between enclosures that comprise the line source array should vary in inverse proportion to the listener
    distance.

5. Inter-Element Angle Limits
    Relative tilt angles between adjacent enclosures conform within the following limit:

where is the maximum angle allowed between enclosures, ARF is the active radiating factor, STEP is the vertical size of each enclosure and is the minimum allowed listener distance.

V-DOSC and dV-DOSC® are the physical embodiments of all 5 WST conditions and the patented DOSC waveguide is the core technology that allows for conditions #1& 3 to be satisfied. By defining WST conditions, L-ACOUSTICS has provided guidelines for designing a full-bandwidth line source, predicting results and indicating where the trade-offs are.

There are three important consequences of WST: (1) nearfield extension over the entire audience provides compensation for air absorption losses at high frequencies; (2) cylindrical wave propagation in the nearfield offers reduced SPL attenuation over distance; (3) the designer can smooth the SPL distribution over the audience just by physically shaping the line source.

Cylindrical Wavefronts
All loudspeaker arrays exhibit diverse wave propagation modes which can be characterized as either a nearfield, farfield or a chaotic field (a high interference domain which can't be described in a simple analytical way). Any PA designer aims at eliminating the chaotic field and this is achievable with more or less success by using a line array. A line source exhibits two zones only, nearfield or farfield, with a border that is size/frequency dependant. The farfield of a line source is spherical (6 dB attenuation per doubling of distance) while the nearfield of a flat line source is cylindrical, showing an attenuation of 3 dB per doubling of distance. Denying these facts is a refusal to read what has been published on the subject [1,2] and sound engineers who have experienced the nearfield of a line source know what it is all about.

About the J-Shape
The J-shape designation is a popularisation of the 4th WST condition applied to the case of a flat plane audience. For more complex audience geometries, modelling tools such as L-ACOUSTICS' proprietary ARRAY software should be used to configure the line source in the most suitable shape. A common misconception is that there should be no empty space between adjacent cabinets (gaps at the front). The important issue is the active radiating factor which should be higher than 80% and whether the rigging is front- or rear-pivoting is not significant. In fact, the most restricting factor is the 5th WST condition. This condition specifies a trade-off between enclosure height, the maximum angle allowed between enclosures and the minimum listener distance. For example, it is not relevant to recommend a 10° angle between cabinets containing front-loaded 15'' components (corresponding to a nominal height or STEP of 0.50 m) unless the sound designer is willing to accept audible lobes in the nearfield at mid/high frequencies (>16 dB notches). For such an enclosure, the 5th WST condition specifies a maximum angle of 5° in order to operate the line source from a distance of 10 m and onwards.

V-DOSC Phase II ?
Since the introduction of V-DOSC in 1993 and dV-DOSC in 1999, both systems have benefited from many upgrades in order to obtain the highest levels of performance possible. Recent developments include: precise rigging accessories, powerful lightweight amplifiers, flying amplifier racks, flying subwoofers and improved digital presets. Since both systems respect all 5 WST conditions, they can be considered the most advanced line sources available. All cabinets within the world-wide V-DOSC Network (3,300 V-DOSC, >2,000 dV-DOSC) are compatible with respect to L-ACOUSTICS' specifications. This makes V-DOSC and dV-DOSC the most complete and advanced solution for rental companies while reinforcing the strength of the V-DOSC Network. For these reasons, L-ACOUSTICS is not planning to develop a second generation V-DOSC but will continue to introduce improvements along with complementary loudspeaker designs that serve as strategic sound design tools - not as replacements.

This rigorous process of product/system evolution is complemented by a strong emphasis on technical support in the areas of: training programs, modelling tools for touring applications (ARRAY), advanced modelling tools for permanent installation (custom dlls for CATT & EASE) along with consultancy services for special events and installations. The highly-trained Certified V-DOSC Engineer (CVE) or Qualified V-DOSC Technician (QVT) provide a valuable knowledge base given their training and real-world experience. It is important to understand that today V-DOSC and dV-DOSC are not just speakers - they are part of a complete system which includes the expertise of trained operators - all of which are available on a worldwide basis according to a standard that guarantees a consistently high level of performance to the end user.

To conclude, a "line array" performing as a line source is a very powerful tool which offers many advantages and unparalleled performance. Nonetheless, there are a number of restrictions regarding the use of a line array:

  •  The requested SPL is rarely the issue for a well-designed line array, however, the number of cabinets necessary for a venue depends on its geometry and can't be reduced due to the directional aspects of the enclosures.
 
 •  When multiple arrays are focused in the same direction, nasty interference can result due to coverage pattern overlap unless there is sufficient distance between the arrays. This can occur with any loudspeaker array, however, it is more critical for a line source since it may counteract most of the line source's benefits of interference control within the individual arrays.

  •  The last restriction is psychological: the system or venue are not as responsible for the bad sound of a concert anymore and the onus is more on the designer to work carefully with the technology. WST requires one to change one's habits and ways of working along with adopting a different way of listening.

[1] Heil and Urban, "Sound Fields Radiated by Multiple Sound Source Arrays", AES preprint #3269, 1992
[2] Urban, Heil and Bauman, "Wavefront Sculpture Technology'', AES preprint #5488, 2001.