Oct 1, 1995 2:32 PM, By Russ Berger and Tom Rose

SOMETIMES, MORE IS LESS

Have you ever found yourself scouring old copies of Mix for drawings of wall sections in hopes of finding the ultimate partition design for your studio? If, like many readers, you are planning the new construction of a private recording facility, you will be working with a tight budget. Who isn’t? You want the walls to provide the most isolation for the least money. If you are planning a commercial facility, hopefully you will show good judgment and enlist the help of an acoustical consultant. (We had to get that plug in there somewhere.) Well, let us temper your search and selection of partition designs with some not-so-common sense logic and little-known facts about wall construction.

HOW MUCH IS MORE OR LESS
STC (Sound Transmission Class) is used almost universally as a standard in the construction world to describe the sound isolation abilities of all types of partitions. Please note that we said “all types of partitions,” regardless of their intended use. The ASTM E-413 standard states that STC is designed to correlate with subjective impressions of sound isolation from normal sources found in dwellings and offices. This means that the STC rating of construction is not easily applied to music studio construction. STC would probably be better thought of as “Speech Transmission Class” since the testing procedure is weighted heavily to the mid-frequencies centered around the speech range.

Thankfully, it should be noted that at least one industry source has not only recognized this problem, but is actively promoting a new testing and rating system, MTC (Music Transmission Class). This is a valiant attempt to fill the gap between evaluation of high performance walls that need to cover a broad musical spectrum and that of speech noise control problems. However, the only acoustical rating or performance information of studio walls you will usually see published, if at all, is STC. No tests, no calculations, only an occasional STC number probably copied from a third- or fourth-hand source. We would even venture to say that a majority of the walls constructed for studio use are not designed at all but are brought into being by either of two basic methods. One is the “elephant gun” method: A man, when asked why he carried an elephant gun with him everywhere, replied, “You don’t see any elephants around here, do you?” Traditionally these are expensive, over-built walls constructed with large quantities of strange assortments of materials in particular “proprietary” (bizarre) ways. The other method is the FOF technique.

Frequently we hear people exclaim that they have a friend-of-a-friend (FOF) who built this super incredibly great wall and it worked wonderfully. Well, will it really work in your application? Will you get the performance you need and are paying for? There is usually no acoustical rhyme or reason behind assuming someone else’s construction will work in your situation unless you have proof of performance in the form of test results and engineered predictions. Inadequate performance can render your facility useless; too high performance means you paid too much. Having said this, some examples are in order to illuminate one aspect of the complex but predictable mechanics of transmission loss of partitions.

MORE IS LESS?
To demonstrate the acoustical kernel of the article, let’s compare the performance of three gypsum board walls, named A, B and C, which are all commonly used in construction today and have been evaluated by independent acoustical tests. Shown in Figure 1 are the three different wall constructions and their associated STC ratings. Confused by what you see? More is better, right? Wrong! The ratings were not inadvertently reversed; they are correct.

Each wall section is of a double-plate construction with wood studs, insulation and with no cross-cavity bracing. Wall “A,” with a single layer of gyp board on each outside face, rates an STC 56. Since two layers of gyp board is good, two more layers must be better. Right? Wrong! By putting these extra layers inside the cavity on one of the studs, we degrade the wall to STC 53, as shown by tests of wall “B.” Well, according to the law of “more is better,” we continue on and add two more layers of gyp board to the inside face of the other stud. Oops, wall “C” at STC 48 is worse still.

The acoustical kernel of knowledge demonstrated here is that how materials are arranged is more important than the quantity of materials used. A “mass-air cavity mass,” or two-panel wall, is the best (and cheapest) type of construction for obtaining maximum transmission loss. In the example, you can see the STC decreasing as the inner layers of the three-panel walls are added. So, in this case, more is less!

MORE OR LESS?
As mentioned earlier, the STC rating of partitions is inappropriate for studio construction. A preferred method of evaluating the performance of a partition is to analyze the transmission loss/frequency spectrum. Figure 2 shows two walls of standard construction with identical STC ratings. However, one look at the comparison of their individual transmission loss spectra (Figure 3) reveals that the concrete masonry partition outperforms the drywall partition significantly in the lower frequency range. Figure 4 demonstrates the magnitude of difference between speech and music below 250 Hz. Good wall transmission loss performance in this region below 250 Hz is extremely important in studio construction, although expensive to achieve. Only mass stops low frequency sound effectively. When you hear sound coming through a wall, what do you hear—high frequencies or low frequencies? It is the low frequency “boom-boom” that is predominant every time, indicating a low frequency deficiency.

This exemplifies that the single number STC rating alone doesn’t tell the whole story, especially in the design of studios. In selecting a partition to effectively isolate unwanted sound, it is best to use the wall’s transmission loss/frequency spectrum for the evaluation. This is a better way to determine if a wall will perform appropriately in the desired ranges.

LESS IS MORE!
By now you are probably anxious to see the transmission loss spectra of those three walls in the first example. You are probably thinking there is no way that walls with less material will outperform ones with more, especially in the lower frequency range. Would we lead you astray? Look at Figure 5. Breaking up the air cavity between the outer layers greatly affects and derates the performance of the walls, even at low frequencies, and counteracts the benefits of the added mass. In this instance, the spectral performance is in agreement with the STC ratings in meeting an important goal for studio construction: good low frequency performance. You should also notice the addition of another partition type (D), which is common—one constructed of two layers of gyp board on each face. The tested performance results of this wall should further drive home the point that two-panel walls perform better.

So, the next time you’re perusing the pages of Mix looking for some good ideas for wall or window construction, keep in mind the fact that a wall with multiple small cavities will probably not give you a good cost/performance ratio. A “mass/air-cavity/mass” construction will provide predictable, reliable, and cost-effective results. It is as important to consider the way something is constructed, as of what it is constructed.