Knowledge

Understanding NOISE & How To Control It

As musicians, DJs and sound engineers, our jobs are to make noise. For some, making a lot of noise is vital to making the difference between the show being mediocre or mind blowing. Sometimes, this is inside a venue surrounded by walls, and sometimes this is out in a great big field at a festival.

The obvious problem that comes with making noise, is that some people don’t want to hear it. For noise generated inside a building, it will find a way out if there is one. Being able to mitigate this, can mean that a venue which is already at risk from losing its licence from a noise abatement, can continue to operate normally without fear of being shut down.

Most music, especially electronic dance music, has a lot of bass content. Usually, the lower the frequency, the harder it is to stop escaping. The reason is that low frequencies have long wavelengths which find it much easier to get past barriers than high frequency, short wavelengths. This usually means that if noise is a problem, the bass has to be limited more than the rest of the audio, resulting in a less than ideal scenario with probably a lower overall maximum volume and certainly far less bass than should be there for decent, balanced audio.

Music Venues

The primary methods for stopping noise escaping a venue are down to treating the structure of the building. This includes soundproofing walls, ceilings, windows and flooring. The tools available depend on what sort of room layout the venue has and whether it is a stage/DJ booth led type of venue, or whether the sound system can be more distributed. Common sense would suggest that more speakers would make the problem worse. In fact, in many cases, more speakers can mean far less noise gets out to where it isn’t wanted. The reason is, if there are more speakers, each person can’t be very far from one. If the listener is closer, the speaker doesn’t need to be as loud, for it to be loud for the listener, in the same way that someone whispering in your ear is audible to you because it is very close, but is inaudible to someone at the other side of the room. If you only had one loud stack in the corner of a huge room, it would need to be extremely loud by the stack for it to be loud enough for listeners in the opposite far corner.

Image showing audio dispersion from one speaker

Sometimes spreading the speaker placement out is possible, but as mentioned above, sometimes it can’t work because of the way the stage is laid out such as in a live music venue. Delay speakers can be used in this situation and can work especially well in ‘under balcony’ areas where the main sound system just can’t penetrate properly. The delay speakers can’t just be hung up, plugged in and hope for the best. They would require time alignment delay to be assigned to them, so that the sound doesn’t come out of them until it has physically reached them from the main system. For example, if they were 20 metres away from the main system, time delay equivalent to 20 metres would need to be applied to them (roughly 58 milliseconds, but the delay time/distance relationship changes depending on the temperature and humidity of the air). Sometimes, if it is a very long venue, multiple delay systems can be used to maintain the volume from the front the back without there being excessive volume anywhere.

Image showing audio dispersion from 3 speakers.

Festival Sites

If you are making music-based noise outside such as in a field at a live event, you will also face issues which may limit how much noise you can make. The difference here, is that there are no walls to contain the noise. Therefore, the methods available are very much more limited. Usually, the site chosen for the event is a reasonable distance away from built up areas, (although sometimes not) but sound will travel a very long way outside un-impeded, so the factors that must be taken into account to limit noise nuisance off site, are down to where potential complainants are, the type of sound system used and its configuration and quite literally the lay of the land. If the land is arranged in a bowl for example, there is a natural barrier to stop sound travelling in a straight line to a potential problem area. If the stage points out to sea, there won’t be anyone out there to be bothered by the noise. These natural settings aren’t always available to be used, particularly the last one.

The sound system type and its method of deployment are the only tools which are always available to combat noise related issues. This comes partly down to two different types of system, namely the traditional point source system versus the line array system.

There are also different schools of thought on this, which we’ll avoid here, but the differences can present both an advantage and disadvantage. The point source system gets quieter by half every time the listener’s distance from the stage doubles. With a line array system on the other hand, the volume will theoretically fall by half every time the listener quadruples the distance. I say theoretically, because in practice, it is very difficult to get all parts of the audio spectrum to couple as a true line source, and so part of it will likely be point source and part of it, line source. The benefit with a line array. The volume doesn’t vary as much for the audience across the field as it would with a point source system. Secondly, only in very large fields would delay systems need to be employed to get the sound to the back. The downside to this, is the as the sound throws a very long way in the festival’s field, so too will it throw a long way to the nearest place where residents don’t want to hear it. This can be partly mitigated by the fact that the longer the line source, the narrower the dispersion in the vertical plane, so it may be easier to aim it at the audience with less spill to noise sensitive areas.

The other failure that everyone who has been to an event with a large field and a big line array system will know, is that because the sound throws a long way and there are less delay systems used, any wind will seriously affect not only the volume, but also the frequency response (how it sounds) of the audio. With point source systems, this is far less of a problem, simply because there need to be more delay systems and therefore, the overall noise from the mid highs on site will be lowered and the noise that leaves the site will be lowered. The coinciding obvious problem with this approach, is that a lot more equipment is needed, which will take longer to set up and will cost more to both move, and rig as it needs more people working on it for longer.

To get round some of these noise problems with large flown systems, accurate time alignment delay between speaker cabinets can be used to steer the sound away from certain parts of a festival site. This can be very effective, but low frequencies will always be thrown backwards off any cabinet. This will then pose a problem for any noise sensitive area behind the stage. Some systems employ a method where the whole system hangs vertically and each driver’s time alignment is changed to make it behave as though the hang is tilted towards the audience. This is not without its problems as there will always be some amount of time smear caused by the very method by which it works. This time smear will also disrupt any line source coupling resulting in uneven dispersion as the line source benefits with dispersion will be restricted.

Cancellation

Cardioid/end fire arrangements of speakers can almost totally remove these issues and there have been a few different methods employed by different manufacturers and additionally, creative methods used by engineers and touring companies. For low frequencies with long wavelengths, it is possible to use more energy to end up with less energy (it’s possible with all frequencies but only necessary and practical for low frequencies). Let me explain;

As sound is a pressure waveform, it is made up of varying positive and negative air pressure corresponding to the adiabatic compression and rarefaction of the air through which we hear it (known as the medium). If you have two of the same speakers playing the same material next to each other (there is a caveat here which I’ll explain shortly), the sound will add up because the plus parts are made more plus and the minus parts are made more minus. This makes the amplitude of the waveform greater, which we perceive as it being louder.

image of sine wave addition

If one speaker’s position were to be shifted backwards, then the sound from one speaker would arrive at a different time to the sound from the other. If this difference of arrival times is enough so that the two waveforms are exactly 180 degrees out of phase (so that the plus part of one waveform lines up with the minus part of the other waveform), the two will completely cancel each other out for the listener in front and there will be no sound at that frequency.

image of sine wave cancellation

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.

By Pete Rollinson. January 2021
Images by Kegan Rowland
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Understanding Noise & How To Control It
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Understanding Noise & How To Control It
Description
Understanding noise and how to control is all about noise that we want to hear, but that others possibly don’t. We explain how noise escapes and what measures you can take to minimise problem noise without reducing the quality of your event.
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Audioserv Ltd
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