Auditoriums: understanding acoustic criteria
Regarding acoustics in auditoriums we considered the subjective characteristics specific to concert halls that can add complexity to sound installs.
Here we looks at when a sound system should interact with the room acoustics and when this should be avoided, as well as revealing the added issues that come with retrofitting solutions to older venues.
Compromise also comes into play between architects and interior designers, who tend to favour the pleasing aesthetics of shiny, polished materials: beautiful to look at, yes, but less than ideal for controlling late reflections, which are a concert hall’s number one nuisance. This is not to say that the visual professionals go out of their way to make life difficult for the acoustic consultant, it’s simply the reality that – generally speaking – shiny, hard surfaces look nicer than softer ones.
The panels simply have an important job to do, and in a large auditorium we use absorption to treat the space; that’s the main approach to it.
Let’s not forget that loudspeakers are just as visible – if not more so – than acoustic panels. A concert hall has high aesthetic standards, so there’s a need for audio systems that are small in size and with a certain aesthetic in their design to be taken into account. There’s still a need for high SPL, but they must be small systems, and there’s a battle or race between manufacturers to respond to this.
Sound system designers also need to get a little bit more education in acoustic criteria because sometimes it’s worth using the benefits of the room acoustics, and sometimes it’s not. When they don’t have the information about the room, the safe bet is to avoid an interaction with the room by having sharp directivity on the audience.
Line arrays deliver very precise control of directivity in the vertical plane in order to guarantee a good acoustic coupling between each source, and to limit interference. Let’s say 95% of line array systems do that quite well. Very curiously, nobody cares about the horizontal control of directivity.
When it comes time to improving the acoustic performance of an existing auditorium, the temptation is to assume an older sound system is the problem, and that a newer one will make everything better. Not so!
We really have to work out what the problem is first, which has nothing to do with technology; it’s more about listening to the client. A lot of jobs come down to that; you listen to what they think their problem is, and you try and work out if (technology) is really what the problem is, or whether it’s actually something else.
Often, the problem is the room itself. The hope with a newly built auditorium is that it will sound great from the start. In Europe especially, many concert halls were built long before it was ever possible to model a space and treat it accordingly.
The former is accomplished much more readily than the latter: Retrofitting a space is always a bigger challenge because sometimes that space isn’t necessarily well-thought for the function it serves. For example, if the space has a concave surface or some kind of design feature that creates echoes, you need to find ways to solve those issues.
The issues that acoustic consultants face in a retrofit are largely the same as those of a new venue: striking a balance between the optimal acoustic treatment and the visual impact it will have on the space. It’s the visuals that tend to win, especially when the auditorium in question is of any significant heritage.
Even if a new sound system is exactly what’s required to give an older space a new sound, installing it won’t necessarily be straightforward: Retrofitting challenge is really about upgrading equipment to have network capabilities, and concern about weight limits, especially in concert halls where there’s a will to keep it the way it has been in the past. The retrofitting challenges are not really on the acoustic side. Any concert hall has its own personal acoustic challenge. The challenge is whether or not to take advantage of that acoustic identity.
Here technology must step in as best as it can, and keep the interaction between sound and space to a bare minimum if necessary. It is important for the system designer to use directional point sources or arrays with horizontal and vertical control, or beam-steerable arrays where vertical sound beams can be tight and aimed only to areas with listeners.