Simply adding more mass would not close the performance gap economically, as will be more thoroughly discussed in the next section. It is important to note that the conventional wall was heavier than the Green Glue wall. This is due to heavier drywall (drywall varies a bit from board to board).

Effect of adding mass is only incremental. Note how the red and black lines are only slightly higher than the lighter blue wall? This is because the effect of simply adding mass to the common wall is only incremental. Based on a survey of historical data (NRC Canada, IR761 and IR818, USG/RAL data circa 1983, Owens Corning data circa 1972), doubling the number of layers results in about 4-5 dB of overall improvement. In this case, the net improvement was about 4dB regardless of what calculation was used to assess performance.

It is the ratio of mass that is important. Another important thing to remember about adding mass to the common wall is that it is the ratio of the increase in mass that matters. To get as much improvement as you did from going from 1 layer on each side of the wall to 2 layers (doubling the mass), you would have to again double the mass – to 4 layers per side.
So, you see, simply adding mass to the common single-wood-stud wall does not have an enormous effect. Adding mass + damping in the form of Green Glue, however, has a great effect.

Other products, such as Mass Loaded Vinyl (MLV) or pre-fabricated damped panels are often utilized for sound isolation applications. This chart outlines the performance and cost of a Green Glue wall compared to these other options. The assemblies were as follows:

All the assemblies featured the same stud and insulation configuration as above.

Chart 3 – Green Glue against other commercial products

Green Glue easily outperforms these other products, both in absolute performance and in cost/performance. The prefab damped panels easily outperform the limp mass materials in most calculations as well. Green Glue is certainly not the only quality sound isolation product, but it is a very good one, and among the most economical options available.

Appendix 1 - Estimating performance of even heavier walls.
The test series was stopped at 2 layers of drywall per side. However, we can estimate the performance of a wall with 4 layers of 5/8” (16mm) drywall on each side by adding the gains that were observed when moving from one layer of drywall per side to 2 layers per side. Add the gains that resulted from doubling the mass, and a very good estimate of the performance of a wall with 4 layers of drywall per side can be attained.

Working from this type of estimation, we offer the following table of performance.

* - A detailed discussion of the curves utilized for these calculations is found in the appendix
** - This wall was considerably flanking limited, see above and the test report for details

Even doubling the number of conventional layers (relative to a Green Glue wall) will not match the performance of a Green Glue assembly on the common wall

Here we will take a look at the cost of each of the assemblies that feature product on both sides.

* - In some situations it is necessary to compensate for floor space consumed by the wall, as the consumed floor space represents lost saleable value. In this calculation we assume a 9-foot high demising wall, and floor space valued at $200 per square foot. We normalize to a 4.75” deep wall. Labor for saw-cut panels assumed to be 15% higher than for scored drywall.

About these costs: All costs based on national average material and labor rates taken from the National Construction Estimator, a Craftsman product. Rates will vary considerably depending on location, time of year, and other factors. Labor will vary the most. You can browse some of the data presented in the National Construction Estimator at www.get-a-quote.net

Results: Looking at the costs, it seems that in situations where space is a relevant factor Green Glue is overwhelmingly cost effective. When considering that 4 layers of conventional drywall on each side is very unlikely to match the performance of 2 layers with Green Glue, we can see that GG is easily cost effective from a materials + labor standpoint, or even from a just-materials standpoint (important to the DIYer).
It can be interesting to look at the economic performance of Green Glue in terms of improvement per money spent. This can be done by comparing the cost of any given improvement relative to the base wall (just one layer of conventional drywall per side) divided by the improvement in performance. One can select any of the performance ratings listed above, or any other performance rating that you wish to consider.

Cost per square foot per improvement in STC
When compared to a standard wall with a single layer of drywall on each side.

Green Glue is notably cost effective in this analysis. Only materials and labor are included, if floor space was accounted for, the margin would be immense. The flanking limit of the lab may make this comparison somewhat unfavorable to Green Glue, as the STC of the Green Glue assembly may in fact be higher.

Cost per square foot per improvement in the full-band flat noise rating

The sound isolation behavior of the common wood stud wall was explained fantastically in JASA, 1977 by Linn and Garrelick. Their paper was:
Sound Transmission through periodically frame parallel plates JASA, vol. 61, #4, pages 1014-1018, 1977
They predicted a series of mechanical resonances, anti-resonances, and a mass-controlled region below these. These predictions are outlined in this chart:

The basic regions predicted by Lin and Garrelick are outlined above. Data from 50-5000 Hz is taken from IR-693, copyright NRC Canada, used with permission, and available at www.nrc.ca
There is a primary structural resonance at which performance is very poor. Then we see an anti-resonance at which performance is good, more resonances, and then coincidence (a different form of resonance). Well below the primary structural resonance, the performance of this type of wall is controlled by the mass and size of the partition in question. Data below 50 Hz is the graph above is hypothetical and included to illustrate the points.
Green Glue has large low frequency effect at this primary structural resonance due to its damping action. Below this primary resonance, the wall is mass controlled. The effect of damping is at a minima at the first anti-resonance and above this anti-resonance damping can deliver 10-30 dB of improvement.