Innovations In Audio (written for Pro Sound News)
Developing the V-DOSC™ Speaker System
by Dr. Christian Heil, Managing Director, Heil Acoustics
The common trend in sound systems today is to increase both the actual sound pressure level during concerts as well as the size of the audience to be covered. This inevitably leads to an increase in the number of loudspeakers. A common practice is to stack many loudspeakers in arrays or clusters in order to achieve the required SPL. The problem this creates for most traditional systems is that the sound waves radiated by individual loudspeakers do not couple properly and therefore interfere uncontrollably. This creates problems in terms of coverage, consistency, directivity control, intelligibility and overall sonic quality. The chaotic sound fields created by these overlapping sound sources also waste acoustic energy, thus requiring more total power than that needed to power a single, coherent source in achieving the desired SPL.
V-DOSC Is Unique: A Continuous Ribbon of Sound
The V-DOSC system is the one speaker system whose arrays share exactly the same acoustical structure as a single isophasic sound source. This continuous ribbon generates a fully predictable and perfectly defined wavefield. A V-DOSC array is comprised of a number of cabinets vertically attached to each other, forming what is known as a line array. Great care should be taken in the use of this generic term. Line array signifies only that the boxes are vertically assembled, which does not by itself necessarily translate to an improved end result.
In fact, until now it has been almost impossible to make a line array operate properly because of the interference produced by multiple sound sources operating in the same field. V-DOSC matches the ideal conditions of a single sound source by applying specific designs in the speaker arrangement and by loading the high-frequency drivers with a patented “DOSC” waveguide.
As an illustration of this principle, imagine throwing some pebbles in a pool of water. If we throw one pebble into the water, we will see circular waves expanding concentrically from the point where it entered the water. If we throw a handful of pebbles, we can observe the equivalent of a chaotic wavefield. If we throw in a single larger stone, having a total size and weight equal to the handful of pebbles, then we again see circular waves as in the case of the single pebble-only now with a larger amplitude.
A Single Sound Source From Many Speakers
This illustrates the thinking behind the development of V-DOSC. If we can construct a single sound source emanating from many speakers (which can then be separated for ease of handling and transport), then we have achieved the goal of providing a totally coherent, predictable wavefield.
The initial specification of the V-DOSC research and development program was the design of a single acoustic source that is completely modular and adjustable. In 1988, an early Heil Acoustics system called “DOSC” proved the project’s feasibility. Based on this experimental concept, Professor Marcel Urban and Dr. Christian Heil began theoretical research and presented their findings at the 92nd AES Convention in Vienna in 1992 (Preprint # 3269). The theory defines the acoustic coupling conditions for successfully arraying individual sound sources, including wavelength, the shape of each source, their surface areas and their relative distance. Conversely, from the actual measurements of an existing array, the frequency range of acoustic coupling can be deduced. Briefly, the coupling conditions can be summarized as follows:
An assembly of individual sound sources arrayed following a regular step distance on a planar or curved continuous surface is equivalent to a single sound source having the same dimensions as the total assembly if one of the following two conditions is fulfilled:
1. Frequency: The step distance (the distance between the acoustic centers of individual sources) is smaller than the wavelength.
2. Shape: The wavefronts generated by individual sources are planar and together fill at least 80 percent of the total radiating surface area.
Heil Acoustics defines the practical implications of these criteria as Wavefront Sculpture Technology™ (WST). The V-DOSC system is the first exact embodiment of these principles. Individual transducers are arranged in each enclosure in a manner which meets the WST criteria when enclosures are arrayed together, frequency band by frequency band.
Low and mid-frequency transducers, crossing over at 200 Hz and 1.3kHz respectively, meet the first criterion. In order to meet the second criterion (the first one obviously cannot be met at high frequencies, where the wavelength becomes smaller than the size of the drivers), a specific waveguide called DOSC is used to shape the wavefront produced by a two-inch format compression driver. The DOSC Waveguide is protected by international patents.
A V-DOSC array is designed as a system of variable size, built from identical individual elements. The assembly, depending on its size and shape, determines the shape of the wavefront it generates. Individual elements cannot be characterized acoustically, except for their horizontal coverage of 90 degrees. As a result, any V-DOSC array has a horizontal coverage of 90 degrees, with other characteristics determined by the overall design of the array.
Wavefront Sculpture Technology allows the systems engineer to position a “single” (ribbon-type) loudspeaker, combined with a vertical coverage, wavefront shape, available SPL and geometrical distribution of energy which exactly meets the geometry of audience seating area. A V-DOSC array provides directivity control at frequencies where it does not occur with traditional systems.
The coverage of a V-DOSC system is very accurately defined, being uniform in level and bandwidth within the coherent wavefield. Outside of this wavefield the SPL rejection is about 20dB (average, with a minimum of -13.5dB per frequency). The geometry of this soundfield is defined by the bending of the array with the available acoustic pressure defined by the number of elements in the array.
The arrangement of the transducers in a single V-DOSC element is symmetrical with respect to the plane of propagation of the wave, i.e. the plane bisects the horizontal coverage angle. This is Coplanar Symmetry. High-frequency transducers are in the middle, mid-frequency transducers are mounted on both sides of the high-frequency transducers, and low-frequency transducers are positioned on both ends (laterally).
Coplanar Symmetry-equivalent to coaxial arrangements for individual (spherical) sources, but transposed in the cylindrical domain-enables homogeneous coverage in the soundfield at any listening angle. It typically avoids off-axis acoustic cancellations at crossover frequencies.
Bringing Everyone Into the Near Field
One of the most demonstrative applications of WST is to use a planar V-DOSC array with a zero-degree angle between each cabinet. This configuration is used for the long-throw portion of the array, with that portion of the array being equivalent to a flat isophasic ribbon source that provides a particular wavefront, extending the near field. Purely cylindrical in the higher frequencies (near-field), it becomes purely spherical in the lower frequencies (far-field), with the borderline being frequency dependent.
Anyone may experiment with the near field at long distances with V-DOSC. In this domain, the sound pressure level is decreasing proportionately to the distance, as opposed to the square of the distance. This yields a lower high-frequency attenuation rate than the usual inverse square law, enhancing the high-frequency response at very long distances. This compensates for high-frequency attenuation due to air absorption, allowing a wide bandwidth and good tonal balance at distances where other systems are not even providing good intelligibility. With your eyes closed, the perception is that you are far closer to the system than you actually are. This is a great advantage with large-scale audiences since it offers better clarity and greater coverage than traditional systems. It also offers great benefits in highly reverberant venues, since it enables the engineer to very precisely adjust the directivity contour of the array in order to avoid unwanted reflections.
We must also be mindful of this-that in order to guarantee the performance of this system and the total absence of comb filtering, two limitations must be accepted. The first one is that to maintain the correct step distance in the high frequencies, the maximum vertical angle separating any two cabinets is five degrees. The second is that it is not advised to set arrays directly next to each other because it then introduces the same comb filtering, lobing and phase cancellation problems inherent in traditional speaker system arrays.
In situations which require an unusual level of vertical coverage, L-Acoustics has developed a simple, lightweight piece of rigging hardware that mates V-DOSC with ARCS™ cabinets to cover the requirements. The ARCS system is the other DOSC Waveguide product in the L-Acoustics line which meets the conditions defined in Wavefront Sculpture Technology. Trapezoidal in shape, the ARCS enclosure is designed for a lateral mechanical coupling with the next enclosure. To achieve an arc-shaped array, the angle of the enclosure is 22.5 degrees-with four enclosures combining to a total coverage angle of 90 degrees. When arrayed in any number, an ARCS system is equivalent to a single toroidal sound source providing a perfectly defined spherical wavefront. This presents a significant opportunity to more effectively deliver reverb and localization.
V-DOSC’s Rigging System: Simple and Predictable
Unique and variable, the same V-DOSC rigging system is used in all venues. The variations are in the number of cabinets used and the angular arrangement between them, which obviously is related to the coverage area and SPL requirements. The rigging is already integrated into the cabinets so that only a light dedicated bumper and small angular adjustment straps are needed to join and adjust the cabinets’ focus. After the rigging is set up and fixed, the complete array becomes a rigid “one-box” system, fixed in position except for the main site angle which can be modified by adjusting the elevation of the forward motor relative to the rear motor.
A properly configured V-DOSC array is precise, predictable and easy to build. The cabinets are attached to each other on the floor while still on dollies (provided). The bumper is then attached to the top cabinet. The assembly is moved to the motor’s position, which controls the bumper, front and rear. The array is then progressively flown like a caterpillar, while two operators remove the front dollies. Easy!
V-DOSC and ARCS are true arrayable systems, one designed for large audiences and long-throw applications, and the other one dedicated to medium-throw needs, with adjustable azimuthal coverage. Both use the heart of Wavefront Sculpture Technology-the patented DOSC Waveguide-to achieve remarkable results.
V-DOSC™ and ARCS™ are products of L-Acoustics International and are manufactured by:
• Cox Audio Engineering, 709 Cross Avenue, Los Angeles, CA 90065. Phone 213.344.8587
• Heil Acoustics, Rd 40, 91400 Gometz la Ville, France. Phone +33 1 60 12 56 17