Outdoor festival production faces many challenges but one of the most difficult to manage is noise pollution. Bass, aka low-frequency noise propagation, generates the most complaints from local residents. Festivals don't tend to invest in the management of these issues due to the temporary nature of their sound output.

It often falls to the individual sound companies to address these issues to the best of their abilities. Long-distance low-frequency sound propagation is a also significant concern for industrial and transportation sectors. Thanks to their extensive resources, software is available to analyze and predict the acoustical paths and potential remedies. An excellent environmental acoustic assessment program that can assist in noise control design and strategy during planning and construction can be found at olivetreelab.com/Terrain

Other software specific to the concert production industry can assist in predicting and illustrating steps that can be taken to minimize a festival's impact on the surrounding residents. These programs can be used to predict propagation over shorter distances but also offer valuable information related to how to orient the noise sources in order to dramatically reduce the noise propagation that tends to disturb local residents.

Two softwares that are free to the end user are Meyer MAPP XT and AFMG Ease. MAPP predicts the performance of Meyer loudspeaker systems but the laws of acoustics apply to all systems equally. MAPP predicts planar response, which is to say 2-dimensional propagation. You can, however, create a vertical and horizontal prediction of each simulation. Ease allows you to create a single 3D simulation but gives you 2 dimensional views of the propagation.

All these programs deal with propagation in air. Propagation of sound system energy through the ground is not considered. The speed of sound in solids is significantly faster than in air but the energy required to move the mass of the ground over large distances is generally greater than can be generated by common loudspeakers. Trucks and trains weighing many tons rolling over bumps generate far more energy into the ground, which is why the transportation industry is a great resource for noise mitigation information.

With regard to primary isolation, in a multi-family residence or shared commercial space where people share walls, floors and ceilings, isolation is critical. Preventing the direct transmission of energy from solid to solid, such as from a wooden speaker box to a concrete floor, makes a huge difference in how much energy reaches the neighbors' spaces. For instance, placing a speaker on a sponge rubber mat allows the mat to absorb the vibrations rather than transmit them directly into the concrete. On the other hand, over greater distances where structures aren't physically coupled to each other, separating speakers from the earth isn't of significant benefit.

It is known that sand and soils dissipate energy when subjected to cyclic loading, so sand has long been used as an excellent low-frequency absorption medium. Putting speakers on sand is an effective de-coupling mechanism on its own. As far as propagation to distant neighbors are concerned, pallets and blankets appear to have no beneficial effect at the offending frequencies.

There are much more effective and relevant options to consider. To illustrate what can be of benefit, I ran simulations in Meyer's MAPP XT of a speaker on the ground and of a speaker elevated 5 feet above the ground. (See below.)

The simulation indicates that the elevated speaker (upper simulation) actually propagates more energy over greater distance than does one directly on the ground. (Bottom simulation.) With the speaker elevated, the sound reflected from the ground acts as a second source.

These simulations cover a an overall length of 1500 feet with the stage facing from left to right and the propagation distance of 1125 feet from the speakers to the farthest (right) edge of the simulation. The patterns will hold true over greater distances but wind and the terrain do have an increasing effect as distance increases.

In looking for ways to provide a more comprehensive and scientific approach to solutions, it's useful to note some additional factors that aren't often considered or understood about sound propagation.

Many people have heard that low end is not directional, which is true when a single radiating source is involved, however when there is more than one source of low end, the interactions produce very significant directional effects.

Image 2 (above) shows the propagation pattern of a stereo pair of subwoofers placed 20' apart. This pattern is viewed from above. It's clearly evident that the interaction between the two sources creates areas of reduced sound propagation, specifically 45 degrees off axis from the direction of orientation.

It's evident that orienting the system(s) has a greater influence over how much energy reaches a particular area than elevating a speaker or any de-coupling concept. A simple plan of orienting systems 45-degrees away from the nearest residences or potential complaint sources would allow the event organizers to minimize the noise levels at the nearest potential complaint points as well as minimize the interference between adjacent systems.

Simply drawing a line on the ground or tying a ribbon between two stakes to show which way a system needs to be oriented allows the participants a guide for system placement without restricting their creativity and all that's required to create sufficient directivity is a simple left-right speaker system.

If a participant wishes to use a large line of speakers, not in a left-right configuration, (shown in image 3, below)  it's still practical to orient such a system so that neighbors both near and far are minimally effected. The area of least radiation for a line of speakers is 90 degrees to their axis but they radiate nearly as much energy backwards as forwards. Pointing a big line of speakers "away from" the neighbors isn't nearly as effective as pointing the speakers crosswise relative to the neighbors. If the organizers know where the neighbors are and what the system configuration is, they can provide the information to the participants about which direction to aim their system, again by providing nothing more complicated than a line on the ground.

Even greater control can be achieved using cardioid programming, (as shown in image 4, below, also viewed from above) but it is expected that the majority of the systems in use will not have the capabilities or expertise to benefit from this technology.

For events that provide the sound systems, and for the audio production companies that serve them, it's worth considering deploying cardioid system configurations to minimize the spill of noise to the surrounding area. Most cardioid configurations are deployed for the benefit of keeping bass levels lower on stage. Even when festival producers specify the need for the control of sound bleeding off site, very few companies are able to utilize these technologies effectively for that purpose. I recommend that anyone interested in growing their production business invest time in learning how to deploy these principles because, as you can see from the information provided in this brief article, even without the application of complex cardioid system setups, it's possible to realize benefits from the naturally occurring interactions between ordinary speakers.