-By Brian Kubicki Acoustical Design Group, Inc., Mission, KS (adgkc.com)

You asked for it. Now that our original 2006 article “Acoustical Mythology” has returned from an international tour including Asia, Scandinavia and the sub-continents (Europe, the Middle East, Greenland and India, according to Answers.com), the world has pleaded in a clear and omnipresent voice, “Where is the sequel? Please don’t leave us hanging…”

It probably should go without saying (as I said last time) that a subject like sound that is not widely understood is going to be copiously laden with myth and legend resulting from untested (though often “sort of” reasoned) observation. But as human intelligence evolves, we will come to understand that good sound isn’t all that difficult to achieve, provided we can separate truth from fiction.

Here are some more of the common acoustical myths we have encountered in our consulting work with the cinema community:

Myth #1: Sound “bounces around” a room.
You often hear people speak of sound moving through a space, as if it’s a handball that ricochets around a room crazily, guided by an invisible hand playing a game that nobody is watching. I even recall someone who was in a position of superior knowledge over me many years ago describing the movement of sound through three-dimensional space as similar to the way it appears on an oscilloscope. I almost believed that, until I reminded myself that an oscilloscope generally presents voltage as it varies over time—and that’s an engineering tool, not life in three-dimensional space.

While sound waves do indeed reflect off, and get absorbed by, surfaces and materials in a room, it is probably more accurate to analogize the motion of sound as a person blowing soap bubbles in a room. Depending on the frequency of the sound being emanated, those soap bubbles can be very large in diameter or very small, but they are emitted at varying power levels by the source. After those “bubbles” leave the source, they travel through the air and once they hit varying surfaces in the space, they either get transmitted through the barriers of the room, get reflected by the surface, or get absorbed by it. All these things happen at varying rates of application.

If a handball really did what sound waves do, part of the ball would pass through a wall and continue flying through the room next door, another portion of the ball would dissolve in the wall, and/or part of the ball would get reflected off the wall or broken into numerous similar-sized but lower-powered handballs reflecting off in various directions. Sound doesn’t bounce; it either reflects, diffuses, is absorbed or is transmitted.

Myth #2: Resilient channels are necessary for separating adjacent cinema auditoria.
Everyone in the building design industry has likely seen some version of a resilient channel, because there seem to be a thousand different iterations. They are intended for stud and drywall construction, and are designed to serve as a resilient support for a layer of drywall, as opposed to screwing or nailing the drywall to the wall’s supporting studs. Resilient channels are found most often in single stud walls or supporting drywall ceilings.

The intended design goal is to improve the sound-isolating ability of a given wall or ceiling construction, or the impact isolation of a floor-ceiling construction. If properly installed (and often, that is a big if), resilient channels offer considerable improvement in sound-isolating abilities and, depending on the particular channel, varying degrees of impact isolation improvement. In a single-stud wall construction, the resilient channels may be the only resilient element in the wall, so they can result in a lot of improvement in isolation, especially in speech frequencies. For this reason, they find frequent use in residential and office construction.

You don’t often see resilient channels in cinema construction, but sometimes they are seen inserted into a double-stud cinema demising wall. The thought is (probably) that if they cause considerable improvement in a single-stud wall, they should be lights-out good in a double-stud cinema wall. But that is usually not the case.

A double-stud cinema demising wall is already designed to offer extreme performance across a wide frequency range, including in the speech frequencies. Most cinema demising walls are already capable of isolating extremely elevated sound levels. So adding resilient channels to a wall such as this is kind of like wearing a belt and suspenders when visiting a nudist camp. They’re not hurting anything, but nobody’s going to notice that you’re wearing them.

Myth #3: The ideal demising wall between adjacent cinema auditoria uses four layers of gypsum board on one side and three layers on the other side.
There are a few cinema construction “bibles” floating around out there that tell exhibitors and designers to unbalance the numbers of layers of drywall on either side of a given wall because there is some magical resonance that makes such a wall better than walls with the same number of layers on each side. The facts are that there is simply no evidence to indicate that such is the case. Is a wall with four layers on one side and three layers on the other better or worse than a wall with four layers on both sides? The 4/4 wall performs better, just like a 3/2 wall will perform better than a 2/2 wall.

Now, in the odd case where you have a cavity wall on an upper floor that shares the common stud cavity with the space on the floor below, one can make the case for placing at least two or three layers on the cinema side of that wall because low-frequency sound from the cinema speakers can readily fill that cavity and will go where that cavity goes, down to the apartment below. Now that I’ve completely confused you, let’s move on.

Myth #4: Laminated gypsum board is a useful solution in cinema construction.
Laminated gypsum board has been one of the more intriguing developments of recent years. Several companies (Quiet Solution’s QuietRock™, National Gypsum’s Soundbreak™ and—believe it or not—Mason Wyatt’s Qwyatt Rock) have stepped forward with composite drywall products that mimic what Monsanto did years ago with laminated glass. Essentially, the product is made by laminating two relatively thin layers of drywall together, bound by a bonding mechanism (they all call it a “proprietary interlayer”) that works as a damping mechanism which makes the drywall somewhat heavier (more mass, which is always good in acoustics) and also works to eliminate the “coincidence dip.” Coincidence dip is the reduction in transmission loss seen by all material to varying degrees, but more severely in particularly rigid materials like drywall.

The advertised advantages of laminated drywall are that it can make landlords or developers money by offering their tenants more rentable/leasable square footage when the system replaces wider wall assemblies. This appears to be a valid consideration, especially as the number of living units on a floor increases, but this benefit needs to be balanced with the increased costs of both the actual product and its installation. Where traditional drywall is cut in the field with the time-tested score-and-snap method, many of the laminated drywall systems require saw-cutting or at minimum scoring both sides of a given layer before snapping. Estimates we have come across place the premium for installation alone at 15 to 40% higher than traditional drywall.

Another obstacle is more relevant to the acoustician. While laminated drywall is heavier than equivalent thicknesses of standard Type X drywall, the difference is not so great that it will overcome the mass offered by a six-layer partition system (three layers of 5/8-inch Type X gypsum board on both sides of a double-row stud arrangement), especially in the extended low-frequency (rumbles and booms from cinema sound systems) range of sound. Also, while reducing the coincidence dip is never a bad thing, at the high frequencies where it occurs in drywall (2000-5000 Hz range), 60 dB and more of transmission loss is already provided. Most noise transmission from cinema to cinema is effectively controlled with that degree of noise reduction.

While it has benefits and challenges, time will tell if laminated drywall will find wide acceptance in the building construction world. It doesn’t seem to make either acoustical or financial sense in cinema construction.

Hopefully, none of this information will send you scurrying back to your drawings for emergency revisions. Rather, we hope it will instead give you insight into why things are the way they are when acoustics is on the table.