Acoustics is certainly a field where what you don’t know will hurt you. I have worked in the field of audio for nearly 20 years. Over that period of time, I have had the good fortune to lead the engineering of projects ranging from stadiums to concert halls to meeting rooms and everything in between. In all of that time, the biggest lesson I have learned is that the audio designer’s best asset is a great acoustician.
With that in mind, I will spare you the rest of this article and tell you that the best possible acoustical solution is… a competent acoustician. Unlike audio systems, which are only inconvenient to incorporate into a finished building, proper acoustics are downright impossible to achieve after completion of construction. This is because proper acoustical design involves review of HVAC systems, wall construction, material specification, room modeling, and much more. It is not the practice of hanging some “acoustical panels” on the walls.
For all but the simplest project, we always recommend that a professional acoustician be a part of the design team, whether that person is provided by our firm or as an independent consultant or sub-consultant from another firm. Acoustical solutions are often a matter of providing proper criteria but a good acoustician will often see problems early in the process, as this is an area where the solution is only as good as the weakest link and nuance is everything.
I have seen many rooms with a very expensive wall construction where the sound bleeds right over an improperly designed ceiling or door threshold. With this in mind, here are several areas that a professional acoustician will review in a basic meeting room or collaboration space to provide a proper acoustical solution.
In general, meeting spaces should be based around an STC rating of 55 and an NC rating of 35 dBa or less. In the construction of high STC structures, keep in mind that this is very much a situation where the chain is only as strong as the weakest link. With this in mind, pay special attention to the doors, windows, and electrical outlets. You also need to make sure that it is extremely clear that these walls must extend to the structural deck above, or the effort will be wasted.
With regard to mechanical noise, use large ducts / low velocities. You will also want to use acoustical duct liner, which will reduce inner area, requiring further upsizing. Additionally, use diffusers with an NC<25 and do not locate VAV boxes and fan-coil units above the space.
If you have an acoustician involved in the project, he will most likely specify ASHRAE criteria associated with the NC rating of the space. If not, we assume this is being done directly by the architectural firm. In either case, if addressed properly, the noisiest element in the room is likely to be the projector (if one is being installed in that particular space). In the case that there is no acoustician on the project, this is something you should take a minute to review. While most AV designers are well versed in evaluating the necessary brightness and picture quality generated by a projector, they are much less likely to pay significant attention to the noise generated by that projector.
Projector noise can be a tricky issue, because many manufacturers have gotten wise to the fact that this is a design consideration in meeting spaces. As a result, they look for ways to make these numbers look good, while minimizing product cost. In short, what this means is that you can have a projector that measures low for the volume of the noise, but the character of that noise is extremely obnoxious. In other words, if you were to measure, a meter would give you a lower number, but the unit with the lower number would still be more noticeable because it is more whiny, or piercing in the character of the noise.
Since manufacturers do not provide more in-depth specs, the only solution is to try several units and compare. Obviously this isn’t practical for a single meeting space, but in the fairly common circumstance that you were designing a corporate facility with a dozen similar spaces, this would be worth the investment. Again, this is not something that the typical AV consultant or integrator is going to pay attention to, so if you want to be certain to provide a quiet meeting environment, you will need to provide an extra push here.
Absorption and Reflection
Assuredly, the most contentious area in the development of collaborative spaces is acoustics, as it relates to sound absorption. Of course, this is due to the popularity of reflective finishes in high end conference rooms. As these spaces are intended to serve a role in communication and collaboration, it doesn’t make sense to design a space where people must struggle to hear. There is very little pixie dust available here. It is just a matter of compromise. If the floor is to be reflective, then the ceiling should be absorptive and vice-versa. Even furniture can have a major impact.
While there is no free lunch, there are systems out there that utilize a special covering over artwork, with absorption layered behind. The artwork can be custom printed, based on the client’s preferences. Most meeting spaces should target an RT60 of 0.6 to 1 second. Many offices are trying to create a more open collaborate feel with an open floorplan. In this scenario, you must be very careful about the overall reverberation of these open spaces with the deletion of the ACT. A cost effective treatment in this case is often to treat the deck above with inexpensive acoustical duct liner to increase absorption.
Even if reverberation is maintained to this level, the flutter echo from parallel walls in the space may require treatment of one or two walls with NRC 0.5-0.8 materials from roughly 3’-7’ AFF.
This article just scratches the surface, but hopefully provides some insight into the range of design disciplines that are involved in proper acoustical solutions.