New Horizons: Noise Control At Outdoor Events
Interesting alternatives to the “usual” PA approach
Editor’s Note: In his August 2003 Ear Sense column, Fred Ampel briefly alluded to Mr. Beale’s unique PA approaches. We were intrigued to know more, and Chris has complied by graciously supplying this article, first presented to the Institute of Acoustics in late 2000. Also note his addendum where he discusses recent deployment of some of the concepts discussed here.
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Large-scale outdoor events are now a major part of the summer music
scene in the U.K. (and, of course, in many areas around the globe)
and are increasing in number throughout the rest of the year. |
Record sales are falling fast in the face of MP3 pirating and the record
companies are looking for live events to provide new opportunities for
promotion of their artists.
Tented festivals, particularly multi-stage dance shows, are now in planning
from April to November, and in the past two years we have seen major outdoor
productions at Christmas and New Year. The proliferation of outdoor productions
presents many problems for those producing and controlling environmental
noise because of the multiplicity of sound sources, the minimal time available
to install and commission the systems and the occasionally combative approach
of the artists and/or their employees.
ALWAYS THIS WAY?
There are many conventions affecting the design and implementation of
audio systems at outdoor events. Some of these are a product of the desire
of the artist to be presented in a manner in which they are familiar,
while others have been established by the sheer inertia of an annual event
and the huge amount of work (both physical and political) necessary to
effect changes.
Most staging systems are designed with PA “wings” to the left and right
of a stage. These can vary in width, but in the case of large-scale events,
the PA stacks are typically about 120 feet (40 meters) apart. By the time
most sound companies are hired, the stage has already been determined,
the PA wing dressings have been ordered and it‘s too late to suggest radical
repositioning of the system.
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Sound engineers tend to place most of the high-energy content of
the mix in mono (i.e., bass instruments, most low-mid information
and vocals) because they are concerned about maximizing power and
coverage. The two PA positions are effectively operating as mono
loudspeakers reproducing identical simultaneous program, developing
a low frequency energy footprint that looks more-or-less like Figure
1. |
The standard left/right configuration is designed primarily to address
the sound engineer and has little to do with presenting a well-resolved
sound field to the audience. For many of the festivalgoers, this arrangement
is akin to placing enormously expensive hi-fi loudspeakers against either
side of their television set and taking a listening seat on the compost
heap at the bottom corner of the garden... In fact, come to think of it,
the smell would be familiar too!
The solution to this basic geometric problem is obvious the main loudspeaker
stacks should be placed far enough offstage to provide effective stereo
imaging for the audience. This will also require a smaller in-fill system
in front of the stage to provide coverage for those out of the field of
the main systems and for the sound engineer.
There is a further advantage here in that the control of sound levels
placed upon the mix engineer relates mainly to the near field system because
the main PA is out of his hearing. In this way higher sound pressure levels
can be allowed at the mixer without compromising off-site noise dosage.
STAGING FACTORS
There are two common types of stage construction: the built-up scaffolding
type with a proprietary flown roof structure; and modular tower construction
that utilizes a number of structural truss towers with a climbing box
type roof structure.
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With a scaffolding type stage, the PA wings are usually integrated
with the stage construction and provide the strength needed to support
the roof. This means that they are inevitably located adjacent to
the stage, and therefore, the width between the PA stacks is dictated
by this factor. And, of course, there is a huge increase in cost
of scaffolding necessary to widen the wings. |
Tower type stage construction suffers from none of these problems because the PA wing is achieved by extending a “goal post” to either side of the stage. The width of the goal post can be extended very cheaply and thus the PA can be positioned optimally. It is clear also that there need be no obstruction to the audio path from the system.
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Another significant issue which promoters are often very reluctant to address is that of the closing of the void between sub-bass cabinets and ground level. Most commonly the deck of the PA wing is at stage level, i.e. between 6 feet (2 meters) and 12 feet (4 meters) above ground level. When sub-bass cabinets are placed upon this deck, they cease to
function in half-space as they would if placed at ground level.
A large amount of energy in the audience area is lost. (Note the
inclusion of a companion piece regarding subwoofer performance in
Backstage Class in this issue.) |
Depending upon the height of the stage, a significant standing wave can
develop between the stage deck and ground level, causing enormous cancellations.
The reaction of the engineer is then to increase the overall energy level
and thus increase the total energy being dissipated by the system, hence
off-site problems. It is very important to fill in the gap between the
PA wing decking and ground level with a substantial acoustic baffle of
at least 18-millimeter-thick timber, reinforced with battens and secured
with strong brackets.
POSITIONING ISSUES
It’s long been the practice to place the next position on the center line,
120 feet to 150 feet (40 to 50 meters) from the downstage edge. The scale
and construction of mix positions has grown to gargantuan proportions,
effectively obscuring the view to the stage from a large part of the premium
audience viewing area.
This is bad enough in itself; however, the negative effect upon sound
mix and noise control is considerable. When assessed with the factors
outlined earlier in this article, it is clear that the mix position is
designed to place the engineer in the “sweet spot.” Therefore, by default,
the engineer is hearing a superior sound image to the bulk of the audience.
Consider also that the site line obstruction of the mix tower forces the
audience off the center line, into the areas of interference illustrated
in Figure 1.
SSE has been involved in developing several techniques to assist in the
management of outdoor systems to the benefit not only of those affected
outside the site but also the paying audience. Highly effective baffles
can be created using a product by the Eve Company known as TrakWay, normally
used for creating temporary roadways.
Each panel measures approximately 10 feet (3 meters) by 8 feet (2.5 meters)
and weighs 720 pounds (330 kilograms). Three or four of these panels can
be erected in vertical orientation behind a cluster of bass cabinets,
effectively causing the array to radiate in quarter space (þR2) and thus
increasing the sound pressure in front of the system. (See Figure 2)
This technique proved highly effective at Creamfields 2000 where, on one
of the stages equipped with TrakWay baffling, the amount of power required
to achieve acceptable bass within the tent was a fraction of that needed
in a similar tent without baffles. (See Figure 3 for a look at
this baffling technique.)
Some years ago at a dance event in Oxfordshire, SSE used a number of TrakWay
panels to create a giant horn some 30 feet (10 meters) deep by 24 feet
(8 meters) wide by 9 feet (3 meters) high. The horn contained 20 high-power
18-inch loudspeakers and while using very small amounts of power, shaking
a production “Port-akabin” some 900 feet (300 meters) away. With a high-pass
filter at 20 Hz, the horn was virtually inaudible from behind or to the
sides.
SUB-BASS CONTROL LOOP
In some cases, it’s useful to “lead” the engineer into thinking he’s achieving
his aims. An excellent example of this approach is the sub-bass loop technique
outlined in Figure 4. A single sub-bass cabinet is concealed below
the mix position prior to the show. The signal to this cabinet is derived
from the main mix output but passes through a low pass filter at 80 Hz
and through a digital delay to enable the output to be aligned with the
output from the main system.
During a performance the output of the concealed bass cabinet can be gently
increased to excite the mix riser, causing the engineer to reduce the
amount of low frequency content of the main PA mix. This is usually accompanied
by praise from the visiting engineer for the performance of the PA!
At a well-known festival, we employed this technique so successfully that
not one of the 30 or so visiting engineers was aware of the secret sub-bass
cabinet. We became quite adept at anticipating the required level in advance
of each artist.
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The design of multi-stage events requires careful positioning of stages with regard to the proximity to sensitive areas and the directivity of the systems employed. However, once the event is under way, the problem is to define which of the stages is producing the most significant sound energy at a particular monitoring position. |
It is very difficult to establish this simply by measuring sound levels
at the mix position of each stage, because the off site influence of that
stage will be related to program content, the local meteorological conditions
and many other factors. The only sure way to establish this is to provide
an effective reference to the program that is being generated by each
stage at that moment, and to compare the reference with the airborne noise
characteristic.
MONITORING IDEAS
The set-up shown in Figure 5 employs a switching device that receives
an audio input from each of the stages and passes the switched output
via a signal delay to a high power wireless link transmitter. The delay
is adjusted to a nominal figure relating to the approximate range from
the festival site to the optimized monitoring positions. The switching
device identifies each stage in turn, playing a user variable period of
program in each case, and cycles continuously through the stages for the
duration of the event.
Each person monitoring noise levels off-site listens to the output of
the transmitter via a high-quality receiver. The sequence of program output
is compared with the airborne noise and within a very short time it is
possible to identify precisely the program that is most audible. The monitoring
technician can then report to the offending stage directly so that immediate
action can be taken to attenuate the source.
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A recent development in multi-stage dance events has been the call for audio from all stages to be connected to a central point for the purpose of Internet broadcasting. This provides the facility the signals to be shared by the switcher
and enables the cost of the exercise to be greatly reduced. |
New developments, including rapidly deployable rigging systems and affordable spread spectrum wireless signal links, are making distributed systems more feasible outdoors, enabling designers to reduce the size of main left/right systems and to achieve better intelligibility in more areas of the audience.
One shortcoming is that distributed loudspeaker positions tend to be
small and therefore lack directional control at low frequencies. One must
therefore be careful to use high-pass filtering for small, distributed
cabinets to prevent the development of complex low-frequency patterns
that can produce highly unpredictable propagation results.
Level and equalization management for a multiplicity of delay stations
can be a major task as progressive summing of midrange energy between
delays requires different equalization at each position. It is therefore
only in situations where budgets allow for long set up periods that distributed
solutions can be used effectively.
SO WHERE ARE WE?
There are two very distinct processes that are necessary to achieve good
environmental noise control: informed sound design at an early stage in
the planning of the event, and implementation of effective monitoring
and control during the event.
The key is to design for the lowest energy input to the system in order
to achieve the required sound pressure level in the audience area. By
application of some or all of the techniques described, it is possible
to reduce the total energy dissipated by an order of magnitude compared
with a badly designed system.
In tented arenas, the use of TrakWay baffles reduces the effect where
there appears to be more low frequency energy outside the tent than within.
This occurs because the output from omni-directional bass speakers within
the tent is integrated by the diaphragm effect of the fabric, effectively
turning the panels into passive radiators with large surface areas. The
baffles enable the low frequency energy to be focused into the audience
area.
If the total energy dissipated can be reduced, then it is clear that the
total energy leaving the site will be reduced and therefore more readily
controlled. A very interesting licensing technique would be a limit of
permitted electrical power consumption based upon the capacity of the
audience. This limit would force system designers to think carefully at
the design stage and would even please promoters, as they would only be
paying for the equipment on the basis of need.
If a limit of, say 100 kVA were imposed at a 50,000 capacity outdoor site,
it would be essential to design a directive and efficient system with
carefully distributed delays otherwise it would not be possible to achieve
acceptable listening levels. There is a good analogy to be drawn with
Formula One racing cars where physical limits are placed upon cubic capacity
yet extremely focused power output is achieved.
ONE MORE THING
There is a third and equally important process that is not directly related
to the physics of noise management, but which I feel is important to acknowledge.
This is the effective public participation and information exchange prior
to and during the event. While engaged in many outdoor events in many
different countries, I have noticed that only in the U.K. does one experience
extreme objection to outdoor musical performances.
By contrast, European festivals seem to be welcomed by the local population
due to perhaps the income they provide and also the publicity they offer
to the region. The Torhout festival in Belgium and the Ballon d’Alsace
festival in France are examples.
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I believe that the best way to address the ‘not in my backyard’ reaction of many British localities is to ensure that they are properly informed and allowed to share in some of the benefits of events held close to their homes. It is not enough to install a legislature that can provide remedies for noise nuisance after the fact, as I believe that those solutions are often exploited at the expense of a reasonable acceptance of public events. In any case, it’s not much use in the heat of the moment when the problem is actually occurring. |
The current trend is for artists who project aggressive music, often
coupled with profanities that are unacceptable to many people who are
unwilling listeners to the performance. I believe that we’re not at the
moment applying enough effort at the point of specification and design
of these systems to insure against potentially uncontrollable situations.
It is usually the case that, where sold out shows are under way, the noise
levels cannot be attenuated enough to completely rectify environmental
problems due to the compromise to the performance and the possible reaction
of the audience.
It is clear that the promoters must be approached with realistic solutions
now in order that they can make cost effective production decisions and
ensure that the correct hardware is available during the event season.
I believe that it is in the hands of designers and suppliers to provide
that input.
ADDENDUM
Since I wrote this more than two years ago, I find that I haven’t changed
my mind over much of what I said. In fact, I implemented most of it (except
the sub-bass loop thing) in my recent design for the Glastonbury Pyramid
system. Far field and near-field systems can clearly be seen in Figure
6.
It worked very well indeed for the 120,000-plus people in the audience,
but it was depressing to be told by one or two engineers that the mix
position needed to be further forward (it was 180 feet from the stage
in a site 1200 feet deep, for heaven’s sake) or that the system should
have been closer together because it would have been louder for them (in
their opinion). It’s a good thing that I’m not the only one who disagrees
with them or I’d be out of a job!
Chris Beale is group director of SSE Audio Group and can be reached at Chris@sse-hire.com
September 2003 Live Sound International