This sounds like a terrible solution as it will cancel the sound that you want to hear. However, if the speakers are arranged in one of a series of proper configurations, the sound coming off the back of the speakers will be cancelled out and the sound going forward will still be present. This approach can simple enough to do with sub bass, but requires extra bass speakers, usually as a ratio of one rear facing for every two forward facing subs.
The caveat mentioned above is that, although this is all true, the size of a waveform is related to its frequency, so the cardioid method mentioned above will only work properly for a limited range of frequencies. If the sub is cancelled at the rear of the speakers effectively at say 38Hz, it will not be completely cancelled out at 90Hz, because the cancellation effect only works based on the physical distance of the speakers from each other, and/or the time alignment delay used to set them up.
A similar effect can be employed for the low part of the mid high section of the system, but its method really depends on the type of system. For a point source system which, if very large, will be a cluster of multiple boxes wide by multiple boxes high, it would be difficult to configure a low frequency array which would effectively cancel everything coming off the back of the system, due to the size of the cluster being many more times than the size of the wavelength of the frequencies in question.
With a line array type arrangement, another vertically arranged line of bass cabinets can be flown behind the mid high section and appropriate delay and phase shifts can be applied to effectively remove the noise going backwards. Some manufacturers have gone further with great effect by incorporating all of this into the mid high elements themselves, so for example, in a box that would have two low frequency drivers facing forward, there would be another facing backwards in separate chamber which would be fed a different signal to produce the cancellation effect. This is easier to deploy and more repeatable in its effects, because the distances between the forward and rear facing drivers, are always the same and the possibility of user error only really comes down to the correct settings being used in the audio processing. Any time that cardioid cancellation is employed, there will be some degradation to the front facing signal, however in most outdoor applications, the advantages of greatly increased output, totally outweigh the disadvantages.
It is worth pointing out that off-site noise issues aren’t the only ones to consider here. If operating audio at an event where there are multiple stages, the last thing that each stage wants to hear is noise from another stage. While it may be possible to eliminate noise getting off site, that will invariably lead to it being pointed towards another stage area.
Track Down The Noise
I have mentioned tools available to operators to mitigate noise issues, more in terms of actual devices and methods of stopping noise either being created or removing it once it exists. One of the best things an audio engineer can have at their disposal to predict where issues may happen or where they have already happened, are noise measuring devices. In both a festival setting or a music venue (or indeed a noisy factory situation), having measurements of what frequencies are present in multiple locations, will allow an engineer to determine where the noise is coming from.
At a festival, knowing exactly which stage is the source of noise nuisance for potential complainants can be difficult especially with changing weather conditions such as wind direction and also as the acts themselves change, their stage’s overall noise emissions change. One example here may be moving from a band who use in ear monitors, to a band who use wedge monitors and have them very loud, or a folk band to a dance music act etc.
The role of any competent festival noise team is to minimise the impact on local residents while maintaining the necessary noise levels required to make the show work properly for the audience.
By using sound measuring devices in multiple positions around the site, the technical team can monitor noise levels and create a framework to allow for the most on-site noise with the least impact to the festival, alongside continual communication with stage managers and the festival’s site teams to minimise sound leakage to potential complainants’ properties or other stages. If a complaint has been made about noise when the levels are within agreed limits, or if a measuring device shows that there was no noise at a complainant’s property when it was alleged, it is much easier to argue licences for the next event with proof to back up good noise management.
For a music venue, the building is fixed and can’t move. For many venues, noise complaints are a real problem and while they may be responsible, sometimes it is actually a different venue which is the real source of the noise. Again, by using multiple units, it is possible to triangulate noise and prove that the venue wasn’t the real source. On the other hand, if it is that venue, the multiple positions will show an engineer exactly where to pin point the noise leakage and prevent it getting out in the first place.
All of these methods, techniques and pieces of equipment are just tools which are available and must be used alongside experience, knowledge and some common sense to get the most out of them. There is no magic bullet and every festival site or venue is completely different so a careful, sensible approach is what will win on the day.