Are You Overcompensating? Taking a practical look at acoustical design measures
I thought about offering another cinema design Q&A forum for this year’s technology edition of FJI, but it seems that one particular issue is coming up again and again in our cinema projects. It’s an often-delicate issue that touches on essentially how confident the designer is in what they know about their discipline and how far into the future they can extend their imagination in trying to arrive at that sweet-spot balance between cost and performance. The key question is: Are we over-designed?
Over-design in structural design is necessary because keeping buildings and structures in their designed place, carrying us where we need to go, is a clear safety issue. Factors of safety, A.K.A. over-design, are a necessity in such situations, to the degree that governments get involved via building codes and inspections. In designing for mechanical, electrical and plumbing, similar concerns exist and regulatory agencies get busy with dictating fresh-air delivery rates, electrical circuit loads, plumbing flow rates, etc.
But in the more subjective pursuits of building design, such as aesthetic form, artistic value and acoustic environment, determining what is appropriate is more a function of personal taste and opinion than life safety. While there are certain elements in sound and acoustics design that do impact life safety, such as emergency audio announcements or fire alarms, most do not, and in designing cinemas there are a number of issues that dance delicately along a cost-vs.-acoustic performance tightrope.
To be fair to those responsible for cost and acoustic performance, these aren’t frivolous exercises. Movie sound levels often seem to be virtually unlimited, especially in the low frequencies. Imagine engineering a bridge with unlimited loading. There are “Maximum Load” signs on roads and bridges for a reason. Some of the more relevant issues that occur in cinemas are presented below.
Auditorium-to-Auditorium Demising Walls
There is probably no element in cinema design that gets more overdesigned than the walls that separate adjacent cinema auditoriums. I have seen triple walls with an eight-inch-thick concrete wall with separate freestanding stud walls installed on each side of the concrete wall, with two and three layers of gypsum board on the room-exposed face and batt insulation filling the cavities. In Asia, poured concrete was cheap, so 30-inch-thick concrete walls were used to separate adjacent cinema auditoriums.
Most cinema designers will use a double-stud wall set in separate floor and head tracks two to four inches apart, but some will install four layers of gypsum board on one side and three layers on the other side, adhering to an unsupported hypothesis that unbalanced layers of drywall will somehow improve sound-isolation performance.
The reality is that structural limitations dictate how much demising wall is needed to properly separate the spaces. Sound flanking, which is the transmission of sound along a common construction element that passes uninterrupted over, under or beyond a demising wall, occurs to different degrees at the roof deck over the auditoriums and at the floor. When the building occurs on-grade, an expansion joint can be installed through the floor slab to limit sound flanking. However, that cannot occur for structural reasons at the roof, so sound flanking is the weak link in the assembly separating auditoriums. So no matter how thick or heavy the demising wall construction is, you are always going to get a degree of sound flanking. Years of field testing have revealed that the point where you get little additional benefit out of a wall system lies at the configuration of two rows of studs and three layers of gypsum board on each side plateau. No point in going any further.
Sound-Absorbing Wall Treatment
Next on the overuse scale of drywall and concrete in demising walls is the application of sound-absorbing wall treatment on the walls of a cinema. The basic acoustic need for sound-absorbing wall treatment in a cinema is for discrete reflection (echo) control in the space, and to a lesser degree some reverberation reduction. The entire area (100%) of the wall behind the screen and the rear wall should receive two-inch-thick fiberglass-based sound-absorbing wall panels. This part is not overdesign. It doesn’t require much untreated sound-reflective wall area to propagate an echo. Sidewall treatment is mainly needed for flutter-echo control, but 100% coverage of both walls is not necessary. Staggering treatment areas such that sound-reflective parallel wall areas do not occur across from each other is sufficient, and the treatment on the sidewalls need only be one inch thick.
Some high-performance cinemas may employ thicker insulation in wall treatments, but in truth, the difference is only seen at low frequencies and it isn’t much in terms of sound absorption. That cost-benefit curve comes back again!
Sound-Absorbing Cavity Insulation
Fiberglass or mineral-fiber insulation blankets are usually in great supply in a movie theatre complex. Some will fill every cavity 100% full across the thickness of both rows of studs, but in reality all that is practically necessary in terms of improvement of the sound-isolating performance of the wall is to fill about half of each wall cavity with a two to three-pcf density fiberglass or mineral-fiber insulation blanket. Anything beyond that and you are not getting much in the way of additional benefit.
Oh, and please stop laying blankets of insulation above the lay-in ceilings in the auditoriums.
Think of it this way: Most of these ceilings by design already absorb 70% and more of incident sound. In practical terms, that’s from only one pass through the ceiling. More sound absorption occurs once the sound that makes it through the ceiling reflects off the roof deck and gets absorbed again from the back side of the ceiling. So in reality, closer to 90% of incident sound gets absorbed. Save your money and put better sound seals on the exit doors.
I sincerely appreciate your time and, as always, welcome your feedback and correspondence at email@example.com, 913.400.3694.
Brian Kubicki, P.E., heads the consulting firm Acoustical Design Kubicki.