Audio Pro

There are some who will use any excuse to bang a nail into the point source coffin, but there are also many who believe it to be, at the very least, a viable alternative to the ubiquitous line array. Here, UK manufacturer Turbosound’s R&D team enters the debate, outlining its philosophy as well as the design characteristics of the Aspect virtual point source systems.

In 2000, British loudspeaker and audio solution manufacturer, Turbosound began the process of updating its large format touring systems. A decision was taken to push the research forward on virtual point source arrays rather than following the well-trodden path of line array based systems.

Virtual point source and line array systems – representing the two common schools of thought in large format sound reinforcement – have been the subject of heated debate in the press and on web forums for many years. Much of the discussion has been misguidedly focused on the core differences in design approach, rather than specific product limitations or the way that a PA can be deployed in a given circumstance.

The reality is that, just like any system design, all modern loudspeaker designs still include an element of compromise, having to balance different physical and acoustical constraints whilst trying to get the best sound possible.

What follows is a discussion of the key differences between point source as applied in the Turbosound Aspect system, and general line array theory, offering simple and clear explanations of how Aspect works and some of the reasons behind Turbosound’s focus on the spherical wavefront of a virtual point source system.

On paper both virtual point source arrays (VPS) and line sources are valid approaches to sound reinforcement. The major advantage of a VPS is that control of the cluster dispersion is available to the user in both the horizontal and the vertical planes. However, the VPS systems of the early 1990’s suffered from disadvantages ranging from large physical array size to poor consistency of coverage over a wide area.
Over the last decade or so most loudspeaker manufacturers chose to develop line-based systems for concert touring, mostly fuelled by the growing success of Christian Heil.

Practical size limitations meant that earlier point source cluster systems, particularly very narrow dispersion products, exhibited poor frequency consistency across the listening area due to destructive interference between adjacent cabinets. Additionally, loudspeaker enclosures were power limited due to the limitations of drive unit materials available at the time. Only relatively recently have techniques and materials been developed to address these problems.

The majority of line-based systems went a long way to addressing these issues. For example a single wide dispersion horizontal source provided much better frequency response consistency in the horizontal plane. At the same time, improved wavefront consistency in the vertical direction allows the catenation of elements to produce near-perfect cylindrical wavefronts.

Why did Turbosound not produce a line array?

The answer is: Refusal to compromise. To date current line array designs only go part of the way to solving the issues just discussed, but for the most part they allow the user to adjust only vertical coverage, and life is not two dimensional.

The wavefront

A virtual point source creates part of a spherical wavefront, the shape and output of which can be readily adjusted by the system designer to evenly match a given area.

The system is simple to rig and offers very tight control in a notoriously reverberant space, which aids in producing a clean image.

A line array creates a plane wave in the vertical, whereas the (by default wide) horizontal coverage is dictated by the dispersion of the individual waveguides and driver spacing. With conflicting polar characteristics, most users deploy a ‘J’ shape hang, the top half of which behaves as a line (planar) source while the bottom section behaves in a similar manner to a virtual point source (spherical).

An unfortunate, and mostly insurmountable, problem arises when the fixed horizontal coverage (typically 90-degrees) of a line array is too wide or narrow for the audience area.
A virtual point source array is intuitively projected into the coverage area. Adapting the audio coverage to a non-uniform audience space simply involves muting enclosures that would otherwise be firing into dead space (i.e. unsold seats). The source always emanates from the same virtual point, so the time and frequency information always arrives at the listener together.

Array shape and size

There are many situations in which a long single drop of PA can look elegant, but there is no logical reason why a shallower, wider cluster cannot be designed into a set, or indeed offer improved sightlines over a thin line.

Weight and Space

When first introduced in the mid 1990s, the modern line array offered considerable improvements to the user in terms of weight, ease of trucking, and integral flying. However, modern virtual point source systems offer very similar physical advantages, simply applying the same technology and design principles in order to offer users a tight-packing, light-weight and quick-flying PA system.

The problems with exponential horns

This is an area in which significant improvements were required to truly address the poor consistency given by other VPS systems whose waveguides are principally based on exponential horns. An example of this is Aspect’s predecessor, the Turbosound Flashlight.

The variable curvature of a wavefront generated by a typical exponential horn leads to a number of unwanted effects. These range from the comb filtering at the lower end of its frequency range, where there is a considerable overlap between the horns, to the on-axis ‘corridors’ at high frequencies where the horns begin to beam – preventing all the seats in an auditorium from receiving a consistent frequency response.

Generating a phase-coherent wavefront

The principle itself is straightforward, simply dividing the exponential horn flare into multiple tapered waveguides, while ensuring that the path length of each micro-horn is equal from the surface of the driver diaphragm to the horn mouth.

This ensures that all frequencies from all parts of the diaphragm arrive at the horn mouth together, and provides the wavefront with uniformity of phase. A further benefit of the Polyhorn geometry is that the sound wave does not suffer from edge-diffraction effects which have a tendency to confuse the directionality of the sound source.

Dispersion angle vs. array angle

Dispersion from conventional exponential horns is not the same at all frequencies.

In reality, a nominal 25-degree dispersion horn gets narrow at high frequencies and might typically array at only 15-degrees or so for correct results. The important distinction is that because of the Polyhorn design’s sharp cut-off, its array angle can in practice be taken as being the same as the dispersion angle. Therefore the array angle is narrow enough to generate good SPL at a distance, but wide enough to require only a sensible number of cabinets for good coverage of a typical room.

In practical terms the acoustic behaviour of the Polyhorn concept is easily extrapolated across multiple cabinets, with each Polyhorn and each cabinet contributing to the generation of a single, cohesive, and more or less continuous wavefront without noticeable comb filtering effects. In addition, the Polyhorn design offers the possibility of locating the acoustic centre well behind the motor system and behind the enclosure. The wavefront radii are arranged to coincide with the array curvature, forming a single virtual point source.

The Polyhorn achieves a well-defined 25-degree slice of horizontal coverage, while in the vertical plane similarly organised 15-degree dispersion results. This predictable coverage gives sound designers a more intuitive and flexible approach to system design – building in modular blocks of 25-degrees is an easy way to configure the various sized rooms encountered on a typical tour.

Delivering consistent frequency response into an audience area at high SPL dictates the physical constraints of the boxes, and is achieved with sensible numbers of relatively small, lightweight and identically shaped boxes to give an unobtrusive cluster profile, while keeping the number of motors and rigging points to a minimum.

The performance of the Polyhorn designs contributes in turn to Aspect’s scalability, equally usable for 200 people as for a quarter of a million with only one type of mid-high box. Aspect adapts to varying acoustic and audience-coverage requirements ranging from corporate events, theatres and clubs, to large-scale indoor or open-air concerts and festivals.

Outdoor propagation over large distances

People often question the ability of a VPS system to throw.

Line array systems are marketed as falling off at only 3 dB with doubling of distance, rather than the 6 dB of a single point source. Potentially, in the case of an infinite line, this could yield the much publicised ‘cheating of the inverse square law’. However, in practice, the line is of finite length and is almost always deployed with a varying degree of curvature.

So vertical pattern control is lost for wavelengths comparable to the line length, and so the line approximates to a point.

And for a curved line, the 3 dB-fall-per-doubling rule holds true only while the distance to the vertical acoustic centre is appreciably greater. In typical hangs the radius of curvature towards the lower end will be around five metres and typically this end of the line will be dangling at least five metres from the nearest listener. The change in sound level as you move between five metres and ten metres from the source will be about five dB. In contrast, a typical point source array – as a result of its compact format – will be hanging about eight metres from the nearest listener and the level change as you move the same distance will be just 3.5 dB.

Aspect enclosures have very high output at mid and high frequencies, and consequently by narrowing the array angle between enclosures, an Aspect system is capable of throwing very large distances at high output.

By attenuating individual cabinets within the array it is also easy to improve the SPL coverage over distance, as can be seen below. It is true that this causes combing and breaks our optimum array angle, however at these distances it is presumed the listener is far enough from the cluster for both enclosures to behave as one.

You may have heard people say that they have had a single outdoor PA cluster throwing 150 metres or more under the right atmospheric conditions. While this can be true, HF loss over distance is a rule that applies equally to point source or line array.

Delays are another important issue.

Today, most outdoor shows are subject to noise limits, so hanging an enormous PA at the front and trying to fire it hundreds of metres is simply no longer a viable option.
Outdoor shows can be approached like large indoor distributed systems. Aspect systems normally require delays at 75 metres. Delays simply give better control over the entire audience area, especially valuable when local councils are involved in noise limitation.

An Aspect system is designed in three dimensions and consequently it is a very simple task to create hot areas. In extreme cases, where noise limits are restrictive, we can manipulate a loud zone between FOH and the stage whilst attenuating outer areas to allow a reasonable level for the rest of the audience area.

One further area of concern with outdoor sound is the effect of wind. Subjectively it appears that sound from a line array is more affected by wind than typical VPS clusters, but no rigorous work has yet been done to establish the cause and Finite Element Modelling of the problem is extremely complex.

A possible explanation is that it is not the bulk movement of air which is the cause of the problem, but the effect of vortices caused by turbulent flow. These have a lensing effect which cause focusing and shadowing (often called ‘phasing’). These effects are more dramatic the closer the vortex is to the source.

Aspect’s point source technology gives the flexibility to cope with a huge variety of variable conditions – adverse wind, oddly shaped rooms, stringent noise restrictions, etc – because it is essentially intuitive to deploy and is not reliant on the use of computer programs to optimise coverage. Large scale shows are best served with a point source system because they are inherently more flexible and offer clear advantages outdoors in terms of control.

FOH engineers who have the luxury of choice should simply demand the best sounding system, and the many Aspect rental companies around the world offer the opportunity to have that experience at first hand. Although on paper both line array and point source systems can offer good results, there is a lot to be said for trusting your ears and straying from the comfort zone once in a while.

The only true judgement comes through listening.

For further information on Aspect and other point source products, contact Turbosound on +44 (0)1403 711447 or point your browser here.

For a full pdf of this article with all the associated EASE diagrams and other graphics, please contact API's editor via: andy.wood@intentmedia.co.uk