We had a discussion recently with a client about our role at open-air concerts and how and our role when it comes to monitoring concert noise. We told her how it was our job to ensure that noise levels from the concert are kept to within a designated noise limit at the nearest noise sensitive property by means monitoring the noise levels and consulting with the touring sound engineer during the show. She jokingly said the following in response:
“ Well, obviously you weren’t doing your job properly! I live near Terenure and I could clearly hear Ed Sheeran playing in Phoenix Park last year. That’s miles away, on the north-side! The music would have needed to be blasting out of the speakers in order to reach my back garden, are people deaf? How come I can hear the sound from a concert when I’m on the other side of the city?”
No doubt, we believed her. These types of remarks are common; many people in south Dublin will hear the din of Longitude festival held at Marley Park every year, whilst residences on the north-side might catch a few songs from a Croke Park gig. What seems to strike people is the sheer distance between the venue and their home, and how sound can travel so far and be so clear, such that the person can make out every work sung by the artist. There are many variables that will influence how far sound will travel through air, here I will try to explain the basic science behind this phenomenon.
Sound propagation, the process by which sound travels from point A to point B, can be subject to interference that can either enhance the sound or detract from it. This interference can come from barriers between the two points, or interference/mixing with other sounds. However, the one we are interested in here (under unobstructed free-field conditions) is how meteorological conditions affect how this sound travels.
Wind causes sound waves to bend (refraction) in the direction of the prevailing wind . Wind closer to the ground moves slower than wind at high altitudes. The difference in velocity creates a wind gradient, causing a sound traveling downwind to bend downwards, sort of like a golf-ball in flight. The prevailing wind will essentially carry sound waves further than they would have travelled without the wind; the significance of this effect depends upon the wind speed. Conversely, is the sound source is traveling against the wind, the sound will bend upwards with reduced propagation/ See the example of the car below.
People will tend to hear noise from concerts more easily at night time due to the inherent cooling of the air closer to the ground, with hotter air at higher altitude. The conditions for sound propagation at night under normal circumstances is shown in diagram (a) below. Much like with wind, sound waves will bend with enhanced propagation from the concert to a noise sensitive receiver. Typical daytime conditions are represented in diagram (b). Sound waves here will tend to bend upwards and away from the receiver, creating a ‘shadow zone’ where the sound energy is greatly reduced.
Music events are dynamic. The sound engineer plays a pivotal role in ensuing that the audience get the full experience of the touring artist by means of mixing the gig. The order of the songs on a setlist are strategically placed such that the more up-tempo, higher energy songs are left until the end of the night, the climax. Of course, the sound engineer will want to push the sound level as far as it will go, hence we can expect that noise levels will increase as the night goes on. In addition, the orientation of the stage plays a huge role in sound propagation, such that the stage for the Ed Sheeran concerts at the Phoenix Park was facing towards the south of the city.
The background noise at night time in typical residential urban areas will decrease by 4-9dB over daytime levels. Any decrease in the level of background noise will allow distant noise sources to be perceived more easily.
It is through a combination of these main factors that we perceive sound from outdoor concerts with enhanced effect during the evening and night time. There are other psychoacoustic elements relating to the nature and characteristics of music that will increase the ease to which it is perceived over steady-state ambient noise, but that’s another kettle-o’-fish!
Yes, of course. Our job is to ensure that we do not exceed a pre-determined noise limit at a set location(s) in the vicinity of the venue, usually this limit is 75dB LAeq,15 mins which is the average A-weighted sound pressure level measured over 15-minutes. We also have a duty of ensuring that people who have payed good money to see the event receive a full experience in terms of sound level from the speakers. Careful planning and good experience of the venue is necessary, and consideration to the surrounding areas and likelihood of complaints from people in those areas is also important.
Of course, everybody does. The reality is that yes, people will of course hear the concert, but the significance of this impact at distances greater than a kilometre, or two, from the venue will be minimal, such that it might just be annoying (if you don’t like the artist). The concert certainly won’t be stopped due to a single compliant from a person living far from the venue. The granting of licences for outdoor concerts by the relevant authority’s is a thoughtful process; they certainly don’t give them out for fun. Hence, you might only expect a few concerts per year from a given outdoor venue. Certain residences in the direct vicinity of the venue can be determined by concert planners as ‘high-risk’, such that the homeowners might be sent notices reminding them of the concert details, curfew times, and their rights to complaint should noise emissions become significant.